PostgreSQL查询语句执行过程

PostgreSQL查询语句执行过程

一、背景

为了分析postgresql代码，了解其执行查询语句的过程，我采用eclipse + gdb集成调试环境，在客户端执行一个查询语句，观察分析其执行流程及重要数据结构变化中数据。

二、环境准备

参考文章：使用eclipse调试分析PostgreSQL11

查询执行流程

Postgresql 执行insert、delete、update、select都是通过postgres.c里面的exec_simple_query方法，其基本流程是

1. 启动事务 start_xact_command();
2. 进行语法分析，生成语法树

parsetree_list = pg_parse_query(query_string);

只是简单的产生raw parse tree，这个里面不涉及语义检查。只是做语法扫描。

1. 语义分析和查询重写。

querytree_list = pg_analyze_and_rewrite(parsetree, query_string, NULL, 0, NULL);

会进行语义分析，会访问数据库中的对像，需要持有锁。这个过程会将简单的一个select 语句拆分成多个部分，将parse tree转换成query tree。如将整个select语句转换成：from 部分，where条件部分，group by 部分，order by 部分以及having 部分等。是任何数据库都需要操作的，并且非常重要的一环。

1. 生成执行计划

plantree_list = pg_plan_queries(querytree_list,CURSOR_OPT_PARALLEL_OK, NULL);

根据上面的query tree产生执行计划。这部分核心代码在planner.c中，是PG的Query Optimizer。会根据表和索引的统计信息去计算不同路径的可能代价值，最后选出最优者。

1. 执行查询

        /** Run the portal to completion, and then drop it (and the receiver).*/(void) PortalRun(portal,FETCH_ALL,true,  /* always top level */true,receiver,receiver,completionTag);

执行plan，它会遍历每个节点，以致完成。最后将查询结果返回给客户端。

1. 结束事务 finish_xact_command();

三、分析过程

客户端执行查询的SQL

## 查询 IS系的学生名单，返回年龄最大的两位
select s.* from t_student s where s.sdept='IS' order by s.sno desc limit 2;

执行SQL命令，不能用jdbc客户端。
经过分析，jdbc客户端，会把insert、delete、update、select相关的SQL转为preparedStatement执行。把查询语句分开为多个命令执行(执行多轮，上面命令执行了4轮，暂时没有深入每轮执行的区别)：

SQL语法分析命令 parse
绑定参数命令 bind
描述类型 describe
执行 execute
同步、并提交事务sync

所以本次代码分析exec_simple_query执行过程，采用psql执行查询命令。

主要结构体分析

1. 链表

 /* src/include/nodes/pg_list.h */typedef struct List
{NodeTag     type;           /*节点类型： T_List, T_IntList, or T_OidList */int         length;     /*链表长度 */ListCell   *head;  /*链表头指针 */ListCell   *tail;  /*链表尾指针 */
} List;struct ListCell
{union{  /* 用联合体报错 */void       *ptr_value; /* 除了Oid、int 的节点值，全部用ptr_value指针 */int         int_value; /*int 类型值 */Oid         oid_value;  /*Oid类型值，对象id */}           data;ListCell   *next;   /*下一节点*/
};

1. PostgreSQL的 portal 定义
   在postgresql中，portal 作为查询语句的执行状态，可以使用在 查询游标和协议级别上

/*  src/include/utils/portal.h */typedef enum PortalStrategy
{PORTAL_ONE_SELECT,PORTAL_ONE_RETURNING,PORTAL_ONE_MOD_WITH,PORTAL_UTIL_SELECT,PORTAL_MULTI_QUERY
} PortalStrategy;/** A portal is always in one of these states.  It is possible to transit* from ACTIVE back to READY if the query is not run to completion;* otherwise we never back up in status.*/
typedef enum PortalStatus
{PORTAL_NEW,                 /* freshly created */PORTAL_DEFINED,             /* PortalDefineQuery done */PORTAL_READY,               /* PortalStart complete, can run it */PORTAL_ACTIVE,              /* portal is running (can't delete it) */PORTAL_DONE,                /* portal is finished (don't re-run it) */PORTAL_FAILED               /* portal got error (can't re-run it) */
} PortalStatus;typedef struct PortalData *Portal;typedef struct PortalData
{/* Bookkeeping data */const char *name;           /* portal's name */const char *prepStmtName;   /* source prepared statement (NULL if none) */MemoryContext portalContext;    /* subsidiary memory for portal */ResourceOwner resowner;     /* resources owned by portal */void        (*cleanup) (Portal portal); /* cleanup hook *//** State data for remembering which subtransaction(s) the portal was* created or used in.  If the portal is held over from a previous* transaction, both subxids are InvalidSubTransactionId.  Otherwise,* createSubid is the creating subxact and activeSubid is the last subxact* in which we ran the portal.*/SubTransactionId createSubid;   /* 创建的子事务号 the creating subxact */SubTransactionId activeSubid;   /* 当前活动的子事务号 the last subxact with activity *//* The query or queries the portal will execute */const char *sourceText;     /* text of query (as of 8.4, never NULL) */const char *commandTag;     /* command tag for original query */List       *stmts;          /* list of PlannedStmts */CachedPlan *cplan;          /* CachedPlan, if stmts are from one */ParamListInfo portalParams; /* params to pass to query */QueryEnvironment *queryEnv; /* environment for query *//* Features/options */PortalStrategy strategy;    /* 查询策略 see above */int         cursorOptions;  /* DECLARE CURSOR option bits */bool        run_once;       /* portal will only be run once *//* Status data */PortalStatus status;        /* 查询状态 see above */bool        portalPinned;   /* a pinned portal can't be dropped */bool        autoHeld;       /* was automatically converted from pinned to* held (see HoldPinnedPortals()) *//* If not NULL, Executor is active; call ExecutorEnd eventually: */QueryDesc  *queryDesc;      /* 查询描述 info needed for executor invocation *//* If portal returns tuples, this is their tupdesc: */TupleDesc   tupDesc;        /* descriptor for result tuples *//* and these are the format codes to use for the columns: */int16      *formats;        /* a format code for each column *//** Where we store tuples for a held cursor or a PORTAL_ONE_RETURNING or* PORTAL_UTIL_SELECT query.  (A cursor held past the end of its* transaction no longer has any active executor state.)*/Tuplestorestate *holdStore; /* store for holdable cursors */MemoryContext holdContext;  /* memory containing holdStore *//** Snapshot under which tuples in the holdStore were read.  We must keep a* reference to this snapshot if there is any possibility that the tuples* contain TOAST references, because releasing the snapshot could allow* recently-dead rows to be vacuumed away, along with any toast data* belonging to them.  In the case of a held cursor, we avoid needing to* keep such a snapshot by forcibly detoasting the data.*/Snapshot    holdSnapshot;   /* registered snapshot, or NULL if none *//** atStart, atEnd and portalPos indicate the current cursor position.* portalPos is zero before the first row, N after fetching N'th row of* query.  After we run off the end, portalPos = # of rows in query, and* atEnd is true.  Note that atStart implies portalPos == 0, but not the* reverse: we might have backed up only as far as the first row, not to* the start.  Also note that various code inspects atStart and atEnd, but* only the portal movement routines should touch portalPos.*/bool        atStart;bool        atEnd;uint64      portalPos;/* Presentation data, primarily used by the pg_cursors system view */TimestampTz creation_time;  /* time at which this portal was defined */bool        visible;        /* include this portal in pg_cursors? */
}           PortalData;

1. 语法分析结果的处理节点

/*  src/include/nodes/parsenodes.h *//**      RawStmt --- container for any one statement's raw parse tree** Parse analysis converts a raw parse tree headed by a RawStmt node into* an analyzed statement headed by a Query node.  For optimizable statements,* the conversion is complex.  For utility statements, the parser usually just* transfers the raw parse tree (sans RawStmt) into the utilityStmt field of* the Query node, and all the useful work happens at execution time.** stmt_location/stmt_len identify the portion of the source text string* containing this raw statement (useful for multi-statement strings).*/
typedef struct RawStmt
{NodeTag     type; /* 节点类型 raw parse tree */Node       *stmt;           /* 节点内容 raw parse tree */int         stmt_location;  /* start location, or -1 if unknown */int         stmt_len;       /* length in bytes; 0 means "rest of string" */
} RawStmt;

1. 查询树数据结构

/*  src/include/nodes/parsenodes.h *//******************************************************************************  Query Tree 查询树*****************************************************************************//** Query -*    Parse analysis turns all statements into a Query tree*    for further processing by the rewriter and planner.**    Utility statements (i.e. non-optimizable statements) have the*    utilityStmt field set, and the rest of the Query is mostly dummy.**    Planning converts a Query tree into a Plan tree headed by a PlannedStmt*    node --- the Query structure is not used by the executor.*/
typedef struct Query
{NodeTag     type; 节点类型CmdType     commandType;    /* 查询操作类型 select|insert|update|delete|utility */QuerySource querySource;    /* 查询来源，一般为输入的SQL。where did I come from? */uint64      queryId;        /* query identifier (can be set by plugins) */bool        canSetTag;      /* do I set the command result tag? */Node       *utilityStmt;    /* non-null if commandType == CMD_UTILITY */int         resultRelation; /* rtable index of target relation for* INSERT/UPDATE/DELETE; 0 for SELECT */bool        hasAggs;        /* has aggregates in tlist or havingQual */bool        hasWindowFuncs; /* has window functions in tlist */bool        hasTargetSRFs;  /* has set-returning functions in tlist */bool        hasSubLinks;    /* has subquery SubLink */bool        hasDistinctOn;  /* distinctClause is from DISTINCT ON */bool        hasRecursive;   /* WITH RECURSIVE was specified */bool        hasModifyingCTE;    /* has INSERT/UPDATE/DELETE in WITH */bool        hasForUpdate;   /* FOR [KEY] UPDATE/SHARE was specified */bool        hasRowSecurity; /* rewriter has applied some RLS policy */List       *cteList;        /* WITH list (of CommonTableExpr's) */List       *rtable;         /* 涉及的表范围 list of range table entries */FromExpr   *jointree;       /*表的关联树 table join tree (FROM and WHERE clauses) */List       *targetList;     /*返回列列表 target list (of TargetEntry) */OverridingKind override;    /* OVERRIDING clause */OnConflictExpr *onConflict; /* ON CONFLICT DO [NOTHING | UPDATE] */List       *returningList;  /* return-values list (of TargetEntry) */List       *groupClause;    /* a list of SortGroupClause's */List       *groupingSets;   /* a list of GroupingSet's if present */Node       *havingQual;     /* qualifications applied to groups */List       *windowClause;   /* a list of WindowClause's */List       *distinctClause; /* a list of SortGroupClause's */List       *sortClause;     /*排序树 a list of SortGroupClause's */Node       *limitOffset;    /* 跳过的记录数 # of result tuples to skip (int8 expr) */Node       *limitCount;     /*返回的记录数 # of result tuples to return (int8 expr) */List       *rowMarks;       /* a list of RowMarkClause's */Node       *setOperations;  /* set-operation tree if this is top level of* a UNION/INTERSECT/EXCEPT query */List       *constraintDeps; /* a list of pg_constraint OIDs that the query* depends on to be semantically valid */List       *withCheckOptions;   /* a list of WithCheckOption's, which are* only added during rewrite and therefore* are not written out as part of Query. *//** The following two fields identify the portion of the source text string* containing this query.  They are typically only populated in top-level* Queries, not in sub-queries.  When not set, they might both be zero, or* both be -1 meaning "unknown".*/int         stmt_location;  /* start location, or -1 if unknown */int         stmt_len;       /* length in bytes; 0 means "rest of string" */
} Query;

5.执行计划 数据结构

/*  src/include/nodes/plannodes.h *//* ----------------*      PlannedStmt node 执行计划处理节点** The output of the planner is a Plan tree headed by a PlannedStmt node.* PlannedStmt holds the "one time" information needed by the executor.** For simplicity in APIs, we also wrap utility statements in PlannedStmt* nodes; in such cases, commandType == CMD_UTILITY, the statement itself* is in the utilityStmt field, and the rest of the struct is mostly dummy.* (We do use canSetTag, stmt_location, stmt_len, and possibly queryId.)* ----------------*/
typedef struct PlannedStmt
{NodeTag     type;CmdType     commandType;    /* 操作类型 select|insert|update|delete|utility */uint64      queryId;        /* query identifier (copied from Query) */bool        hasReturning;   /* is it insert|update|delete RETURNING? */bool        hasModifyingCTE;    /* has insert|update|delete in WITH? */bool        canSetTag;      /* do I set the command result tag? */bool        transientPlan;  /* redo plan when TransactionXmin changes? */bool        dependsOnRole;  /* is plan specific to current role? */bool        parallelModeNeeded; /* 是否需要并行执行 parallel mode required to execute? */int         jitFlags;       /* which forms of JIT should be performed */struct Plan *planTree;      /*执行计划树 tree of Plan nodes */List       *rtable;         /*处理涉及到的表 list of RangeTblEntry nodes *//* rtable indexes of target relations for INSERT/UPDATE/DELETE */List       *resultRelations;    /* integer list of RT indexes, or NIL *//** rtable indexes of non-leaf target relations for UPDATE/DELETE on all* the partitioned tables mentioned in the query.*/List       *nonleafResultRelations;/** rtable indexes of root target relations for UPDATE/DELETE; this list* maintains a subset of the RT indexes in nonleafResultRelations,* indicating the roots of the respective partition hierarchies.*/List       *rootResultRelations;List       *subplans;       /*子计划列表 Plan trees for SubPlan expressions; note* that some could be NULL */Bitmapset  *rewindPlanIDs;  /* indices of subplans that require REWIND */List       *rowMarks;       /* a list of PlanRowMark's */List       *relationOids;   /* 依赖的对象ID，一般为查询设计的表或者试图的对象id， OIDs of relations the plan depends on */List       *invalItems;     /* other dependencies, as PlanInvalItems */List       *paramExecTypes; /* type OIDs for PARAM_EXEC Params */Node       *utilityStmt;    /* non-null if this is utility stmt *//* statement location in source string (copied from Query) */int         stmt_location;  /* start location, or -1 if unknown */int         stmt_len;       /* length in bytes; 0 means "rest of string" */
} PlannedStmt;/* ----------------*      Plan node 执行计划节点** All plan nodes "derive" from the Plan structure by having the* Plan structure as the first field.  This ensures that everything works* when nodes are cast to Plan's.  (node pointers are frequently cast to Plan** when passed around generically in the executor)** We never actually instantiate any Plan nodes; this is just the common* abstract superclass for all Plan-type nodes.* ----------------*/
typedef struct Plan
{NodeTag     type;/** estimated execution costs for plan (see costsize.c for more info)*/Cost        startup_cost;   /* 启动代价 cost expended before fetching any tuples */Cost        total_cost;     /* 总代价 total cost (assuming all tuples fetched) *//** planner's estimate of result size of this plan step*/double      plan_rows;      /* number of rows plan is expected to emit */int         plan_width;     /* average row width in bytes *//** information needed for parallel query*/bool        parallel_aware; /* engage parallel-aware logic? */bool        parallel_safe;  /* OK to use as part of parallel plan? *//** Common structural data for all Plan types.*/int         plan_node_id;   /* unique across entire final plan tree */List       *targetlist;     /*返回的结果信息 target list to be computed at this node */List       *qual;           /* implicitly-ANDed qual conditions */struct Plan *lefttree;      /*执行计划左子树 input plan tree(s) */struct Plan *righttree;      /*执行计划右子树 input plan tree(s) */List       *initPlan;       /* Init Plan nodes (un-correlated expr* subselects) *//** Information for management of parameter-change-driven rescanning** extParam includes the paramIDs of all external PARAM_EXEC params* affecting this plan node or its children.  setParam params from the* node's initPlans are not included, but their extParams are.** allParam includes all the extParam paramIDs, plus the IDs of local* params that affect the node (i.e., the setParams of its initplans).* These are _all_ the PARAM_EXEC params that affect this node.*/Bitmapset  *extParam;Bitmapset  *allParam;
} Plan;

查询主要方法 exec_simple_query

/*  src/backend/tcop/postgres.c *//** exec_simple_query** Execute a "simple Query" protocol message.*/
static void
exec_simple_query(const char *query_string)
{CommandDest dest = whereToSendOutput;  /*执行结果输出目标，DestRemote */MemoryContext oldcontext; List       *parsetree_list; /*查询语法树 列表 */ListCell   *parsetree_item; /*查询语法树 */bool        save_log_statement_stats = log_statement_stats;bool        was_logged = false;bool        use_implicit_block;char        msec_str[32];/** Report query to various monitoring facilities.*/debug_query_string = query_string;pgstat_report_activity(STATE_RUNNING, query_string);TRACE_POSTGRESQL_QUERY_START(query_string);/** We use save_log_statement_stats so ShowUsage doesn't report incorrect* results because ResetUsage wasn't called.*/if (save_log_statement_stats)ResetUsage();/** Start up a transaction command.  All queries generated by the* query_string will be in this same command block, *unless* we find a* BEGIN/COMMIT/ABORT statement; we have to force a new xact command after* one of those, else bad things will happen in xact.c. (Note that this* will normally change current memory context.)*/start_xact_command(); /* 启动事务 *//** Zap any pre-existing unnamed statement.  (While not strictly necessary,* it seems best to define simple-Query mode as if it used the unnamed* statement and portal; this ensures we recover any storage used by prior* unnamed operations.)*/drop_unnamed_stmt();/** Switch to appropriate context for constructing parsetrees.*/oldcontext = MemoryContextSwitchTo(MessageContext);/** Do basic parsing of the query or queries (this should be safe even if* we are in aborted transaction state!)*/parsetree_list = pg_parse_query(query_string);  /* 对SQL命令进行语法分析 *//* Log immediately if dictated by log_statement */if (check_log_statement(parsetree_list)){ereport(LOG,(errmsg("statement: %s", query_string),errhidestmt(true),errdetail_execute(parsetree_list)));was_logged = true;}/** Switch back to transaction context to enter the loop.*/MemoryContextSwitchTo(oldcontext);/** For historical reasons, if multiple SQL statements are given in a* single "simple Query" message, we execute them as a single transaction,* unless explicit transaction control commands are included to make* portions of the list be separate transactions.  To represent this* behavior properly in the transaction machinery, we use an "implicit"* transaction block.*/use_implicit_block = (list_length(parsetree_list) > 1);/** Run through the raw parsetree(s) and process each one.*/foreach(parsetree_item, parsetree_list){/* 分析一个语法树 */RawStmt    *parsetree = lfirst_node(RawStmt, parsetree_item); /* 转换语法树为RawStmt类型 */bool        snapshot_set = false;const char *commandTag;char        completionTag[COMPLETION_TAG_BUFSIZE];List       *querytree_list,*plantree_list;Portal      portal;DestReceiver *receiver;int16       format;/** Get the command name for use in status display (it also becomes the* default completion tag, down inside PortalRun).  Set ps_status and* do any special start-of-SQL-command processing needed by the* destination.*/commandTag = CreateCommandTag(parsetree->stmt);set_ps_display(commandTag, false);BeginCommand(commandTag, dest);/** If we are in an aborted transaction, reject all commands except* COMMIT/ABORT.  It is important that this test occur before we try* to do parse analysis, rewrite, or planning, since all those phases* try to do database accesses, which may fail in abort state. (It* might be safe to allow some additional utility commands in this* state, but not many...)*/if (IsAbortedTransactionBlockState() &&!IsTransactionExitStmt(parsetree->stmt))ereport(ERROR,(errcode(ERRCODE_IN_FAILED_SQL_TRANSACTION),errmsg("current transaction is aborted, ""commands ignored until end of transaction block"),errdetail_abort()));/* Make sure we are in a transaction command */start_xact_command();  /* 事务开始 *//** If using an implicit transaction block, and we're not already in a* transaction block, start an implicit block to force this statement* to be grouped together with any following ones.  (We must do this* each time through the loop; otherwise, a COMMIT/ROLLBACK in the* list would cause later statements to not be grouped.)*/if (use_implicit_block)BeginImplicitTransactionBlock();/* If we got a cancel signal in parsing or prior command, quit */CHECK_FOR_INTERRUPTS();/** Set up a snapshot if parse analysis/planning will need one.* 对于insert、delete、update、select 操作，需要建立语法树快照*/if (analyze_requires_snapshot(parsetree)){PushActiveSnapshot(GetTransactionSnapshot());snapshot_set = true;}/** OK to analyze, rewrite, and plan this query.** Switch to appropriate context for constructing querytrees (again,* these must outlive the execution context).*/oldcontext = MemoryContextSwitchTo(MessageContext);querytree_list = pg_analyze_and_rewrite(parsetree, query_string,NULL, 0, NULL);  /* 语义分析和查询重写，返回查询树列表 */plantree_list = pg_plan_queries(querytree_list,CURSOR_OPT_PARALLEL_OK, NULL);  /* 生成执行计划，并优化 *//* Done with the snapshot used for parsing/planning */if (snapshot_set)PopActiveSnapshot();/* If we got a cancel signal in analysis or planning, quit */CHECK_FOR_INTERRUPTS();/** Create unnamed portal to run the query or queries in. If there* already is one, silently drop it.*/portal = CreatePortal("", true, true); /* 创建一个查询的portal *//* Don't display the portal in pg_cursors */portal->visible = false;/** We don't have to copy anything into the portal, because everything* we are passing here is in MessageContext, which will outlive the* portal anyway.*/PortalDefineQuery(portal,NULL,query_string,commandTag,plantree_list,NULL);  /* 设置的portal 相关的查询计划 *//** Start the portal.  No parameters here.*/PortalStart(portal, NULL, 0, InvalidSnapshot);/** Select the appropriate output format: text unless we are doing a* FETCH from a binary cursor.  (Pretty grotty to have to do this here* --- but it avoids grottiness in other places.  Ah, the joys of* backward compatibility...)*/format = 0;             /* TEXT is default */if (IsA(parsetree->stmt, FetchStmt)){FetchStmt  *stmt = (FetchStmt *) parsetree->stmt;if (!stmt->ismove){Portal      fportal = GetPortalByName(stmt->portalname);if (PortalIsValid(fportal) &&(fportal->cursorOptions & CURSOR_OPT_BINARY))format = 1; /* BINARY */}}PortalSetResultFormat(portal, 1, &format);  /* 设置的portal 相关的返回格式 *//** Now we can create the destination receiver object.*/receiver = CreateDestReceiver(dest);  /* 创建结果接收者 */if (dest == DestRemote)SetRemoteDestReceiverParams(receiver, portal);/** Switch back to transaction context for execution.*/MemoryContextSwitchTo(oldcontext);/** Run the portal to completion, and then drop it (and the receiver).*/(void) PortalRun(portal,FETCH_ALL,true,  /* always top level */true,receiver,receiver,completionTag);  /* 执行 portal 查询操作 */receiver->rDestroy(receiver);PortalDrop(portal, false);if (lnext(parsetree_item) == NULL){/** If this is the last parsetree of the query string, close down* transaction statement before reporting command-complete.  This* is so that any end-of-transaction errors are reported before* the command-complete message is issued, to avoid confusing* clients who will expect either a command-complete message or an* error, not one and then the other.  Also, if we're using an* implicit transaction block, we must close that out first.*/if (use_implicit_block)EndImplicitTransactionBlock();finish_xact_command();}else if (IsA(parsetree->stmt, TransactionStmt)){/** If this was a transaction control statement, commit it. We will* start a new xact command for the next command.*/finish_xact_command();}else{/** We need a CommandCounterIncrement after every query, except* those that start or end a transaction block.*/CommandCounterIncrement();}/** Tell client that we're done with this query.  Note we emit exactly* one EndCommand report for each raw parsetree, thus one for each SQL* command the client sent, regardless of rewriting. (But a command* aborted by error will not send an EndCommand report at all.)*/EndCommand(completionTag, dest);}                           /* end loop over parsetrees *//** Close down transaction statement, if one is open.  (This will only do* something if the parsetree list was empty; otherwise the last loop* iteration already did it.)*/finish_xact_command(); /*  结束事务  *//** If there were no parsetrees, return EmptyQueryResponse message.*/if (!parsetree_list)NullCommand(dest);/** Emit duration logging if appropriate.*/switch (check_log_duration(msec_str, was_logged)){case 1:ereport(LOG,(errmsg("duration: %s ms", msec_str),errhidestmt(true)));break;case 2:ereport(LOG,(errmsg("duration: %s ms  statement: %s",msec_str, query_string),errhidestmt(true),errdetail_execute(parsetree_list)));break;}if (save_log_statement_stats)ShowUsage("QUERY STATISTICS");TRACE_POSTGRESQL_QUERY_DONE(query_string);debug_query_string = NULL;
}

执行

// psql中获取当前连接的进程id
postgres=# select pg_backend_pid();pg_backend_pid
----------------14329
(1 row)// 切换到gdb 绑定进程
(gdb) attach 14329
Attaching to process 14329
...
// 设置断点
(gdb) b exec_simple_query
Breakpoint 1 at 0x8c35db: file postgres.c, line 893.
(gdb) c
Continuing.// 切换到psql，执行查询
postgres=# select s.* from t_student s where s.sdept='IS' order by s.sno desc imit 2;// 切换gdb 绑定进程

函数调用栈信息

函数栈

query_string的值

select s.*
from t_student s
where s.sdept='IS'
order by s.sno desc 
limit 2;

## 执行SQL
2018-08-07 19:57:58.718 CST,"appusr","postgres",38496,"127.0.0.1:48658",5b698942.9660,3,"idle",2018-08-07 19:57:54 CST,3/2,0,LOG,00000,"statement: select s.*
from t_student s
where s.sdept='IS'
order by s.sno desc 
limit 2;",,,,,,,,,"psql"

查询语法树

在exec_simple_query中调用了pg_parse_query进行语法分析校验

语法分析函数调用栈

1. 输入的query_string的值

select s.*
from t_student s
where s.sdept='IS'
order by s.sno desc 
limit 2;

1. 返回语法树信息如下：

## 语法树
2018-08-07 19:57:58.720 CST,"appusr","postgres",38496,"127.0.0.1:48658",5b698942.9660,4,"SELECT",2018-08-07 19:57:54 CST,3/2,0,LOG,00000,"parse tree:","   {QUERY :commandType 1 :querySource 0 :canSetTag true :utilityStmt <> :resultRelation 0 :hasAggs false :hasWindowFuncs false :hasTargetSRFs false :hasSubLinks false :hasDistinctOn false :hasRecursive false :hasModifyingCTE false :hasForUpdate false :hasRowSecurity false :cteList <> :rtable ({RTE :alias {ALIAS :aliasname s :colnames <>}:eref {ALIAS :aliasname s :colnames (""sno"" ""sname"" ""ssex"" ""sage"" ""sdept"")}:rtekind 0 :relid 16408 :relkind r :tablesample <> :lateral false :inh true :inFromCl true :requiredPerms 2 :checkAsUser 0 :selectedCols (b 9 10 11 12 13):insertedCols (b):updatedCols (b):securityQuals <>}):jointree {FROMEXPR :fromlist ({RANGETBLREF :rtindex 1}):quals {OPEXPR :opno 1054 :opfuncid 1048 :opresulttype 16 :opretset false :opcollid 0 :inputcollid 100 :args ({VAR :varno 1 :varattno 5 :vartype 1042 :vartypmod 24 :varcollid 100 :varlevelsup 0 :varnoold 1 :varoattno 5 :location 34}{CONST :consttype 1042 :consttypmod -1 :constcollid 100 :constlen -1 :constbyval false :constisnull false :location 42 :constvalue 6 [ 24 0 0 0 73 83 ]}):location 41}}:targetList ({TARGETENTRY :expr {VAR :varno 1 :varattno 1 :vartype 1042 :vartypmod 13 :varcollid 100 :varlevelsup 0 :varnoold 1 :varoattno 1 :location 7}:resno 1 :resname sno :ressortgroupref 1 :resorigtbl 16408 :resorigcol 1 :resjunk false}{TARGETENTRY :expr {VAR :varno 1 :varattno 2 :vartype 1042 :vartypmod 24 :varcollid 100 :varlevelsup 0 :varnoold 1 :varoattno 2 :location 7}:resno 2 :resname sname :ressortgroupref 0 :resorigtbl 16408 :resorigcol 2 :resjunk false}{TARGETENTRY :expr {VAR :varno 1 :varattno 3 :vartype 1042 :vartypmod 6 :varcollid 100 :varlevelsup 0 :varnoold 1 :varoattno 3 :location 7}:resno 3 :resname ssex :ressortgroupref 0 :resorigtbl 16408 :resorigcol 3 :resjunk false}{TARGETENTRY :expr {VAR :varno 1 :varattno 4 :vartype 21 :vartypmod -1 :varcollid 0 :varlevelsup 0 :varnoold 1 :varoattno 4 :location 7}:resno 4 :resname sage :ressortgroupref 0 :resorigtbl 16408 :resorigcol 4 :resjunk false}{TARGETENTRY :expr {VAR :varno 1 :varattno 5 :vartype 1042 :vartypmod 24 :varcollid 100 :varlevelsup 0 :varnoold 1 :varoattno 5 :location 7}:resno 5 :resname sdept :ressortgroupref 0 :resorigtbl 16408 :resorigcol 5 :resjunk false}):override 0 :onConflict <> :returningList <> :groupClause <> :groupingSets <> :havingQual <> :windowClause <> :distinctClause <> :sortClause ({SORTGROUPCLAUSE :tleSortGroupRef 1 :eqop 1054 :sortop 1060 :nulls_first true :hashable true}):limitOffset <> :limitCount {FUNCEXPR :funcid 481 :funcresulttype 20 :funcretset false :funcvariadic false :funcformat 2 :funccollid 0 :inputcollid 0 :args ({CONST :consttype 23 :consttypmod -1 :constcollid 0 :constlen 4 :constbyval true :constisnull false :location 74 :constvalue 4 [ 2 0 0 0 0 0 0 0 ]}):location -1}:rowMarks <> :setOperations <> :constraintDeps <> :stmt_location 0 :stmt_len 75}
",,,,,"select s.*
from t_student s
where s.sdept='IS'
order by s.sno desc 
limit 2;",,,"psql"

#语法分析树
(gdb) p *(parsetree_list)
$5 = {type = T_List, length = 1, head = 0x1f985d8, tail = 0x1f985d8}
(gdb) p *(parsetree_list->head)
$6 = {data = {ptr_value = 0x1f985a0, int_value = 33129888,oid_value = 33129888}, next = 0x0}
(gdb) p *(parsetree_list->head->data->ptr_value)
Attempt to dereference a generic pointer.
(gdb) p *((Node*)parsetree_list->head->data->ptr_value)
$7 = {type = T_RawStmt}
(gdb) p *((RawStmt*)parsetree_list->head->data->ptr_value)
$8 = {type = T_RawStmt, stmt = 0x1f981c0, stmt_location = 0, stmt_len = 75}
(gdb) p *((RawStmt*)parsetree_list->head->data->ptr_value)->stmt
$9 = {type = T_SelectStmt}
(gdb) p *((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)
$10 = {type = T_SelectStmt, distinctClause = 0x0, intoClause = 0x0,targetList = 0x1f97dc8, fromClause = 0x1f97f00, whereClause = 0x1f980e0,groupClause = 0x0, havingClause = 0x0, windowClause = 0x0,valuesLists = 0x0, sortClause = 0x1f984a8, limitOffset = 0x0,limitCount = 0x1f984e0, lockingClause = 0x0, withClause = 0x0,op = SETOP_NONE, all = false, larg = 0x0, rarg = 0x0}
(gdb) p *((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->targetList
$11 = {type = T_List, length = 1, head = 0x1f97da0, tail = 0x1f97da0}
(gdb) p *((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->targetList->head
$12 = {data = {ptr_value = 0x1f97d48, int_value = 33127752,oid_value = 33127752}, next = 0x0}
(gdb) p *((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->targetList->head->data->ptr_value
Attempt to dereference a generic pointer.
(gdb) p *((Node*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->targetList->head->data->ptr_value)
$13 = {type = T_ResTarget}
(gdb) p *((ResTarget*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->targetList->head->data->ptr_value)
$14 = {type = T_ResTarget, name = 0x0, indirection = 0x0, val = 0x1f97cc0,location = 7}
(gdb) p *((ResTarget*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->targetList->head->data->ptr_value)->val
$15 = {type = T_ColumnRef}
(gdb) p *((ColumnRef)((ResTarget*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->targetList->head->data->ptr_value)->val)
Structure has no component named operator*.
(gdb) p *((ColumnRef*)((ResTarget*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->targetList->head->data->ptr_value)->val)
$16 = {type = T_ColumnRef, fields = 0x1f97c88, location = 7}
(gdb) p *((ColumnRef*)((ResTarget*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->targetList->head->data->ptr_value)->val)->fields
$17 = {type = T_List, length = 2, head = 0x1f97d20, tail = 0x1f97c60}
(gdb) p *((ColumnRef*)((ResTarget*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->targetList->head->data->ptr_value)->val)->fields->head
$18 = {data = {ptr_value = 0x1f97cf8, int_value = 33127672,oid_value = 33127672}, next = 0x1f97c60}
(gdb) p *((ColumnRef*)((ResTarget*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->targetList->head->data->ptr_value)->val)->fields->head->data->ptr_value
Attempt to dereference a generic pointer.
(gdb) p *((Node*)((ColumnRef*)((ResTarget*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->targetList->head->data->ptr_value)->val)->fields->head->data->ptr_value)
$19 = {type = T_String}
(gdb) p *((String*)((ColumnRef*)((ResTarget*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->targetList->head->data->ptr_value)->val)->fields->head->data->ptr_value)
No symbol "String" in current context.
(gdb) p *((Value*)((ColumnRef*)((ResTarget*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->targetList->head->data->ptr_value)->val)->fields->head->data->ptr_value)
$20 = {type = T_String, val = {ival = 33127456, str = 0x1f97c20 "s"}}
(gdb) p *((Value*)((ColumnRef*)((ResTarget*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->targetList->head->data->ptr_value)->val)->fields->head->data->ptr_value)->val->str
$21 = 115 's'
(gdb) p ((Value*)((ColumnRef*)((ResTarget*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->targetList->head->data->ptr_value)->val)->fields->head->data->ptr_value)->val->str
$22 = 0x1f97c20 "s"
(gdb) p ((char*)((Value*)((ColumnRef*)((ResTarget*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->targetList->head->data->ptr_value)->val)->fields->head->data->ptr_value)->val->str)
$23 = 0x1f97c20 "s"
(gdb) p *((char*)((Value*)((ColumnRef*)((ResTarget*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->targetList->head->data->ptr_value)->val)->fields->head->data->ptr_value)->val->str)
$24 = 115 's'
(gdb) p *((ColumnRef*)((ResTarget*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->targetList->head->data->ptr_value)->val)->fields->head->next
$25 = {data = {ptr_value = 0x1f97c40, int_value = 33127488,oid_value = 33127488}, next = 0x0}
(gdb) p *((Node*)((ColumnRef*)((ResTarget*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->targetList->head->data->ptr_value)->val)->fields->head->next->data->ptr_value)
$26 = {type = T_A_Star}
(gdb) p *((S_Star*)((ColumnRef*)((ResTarget*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->targetList->head->data->ptr_value)->val)->fields->head->next->data->ptr_value)
No symbol "S_Star" in current context.
(gdb) p *((A_Star*)((ColumnRef*)((ResTarget*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->targetList->head->data->ptr_value)->val)->fields->head->next->data->ptr_value)
$27 = {type = T_A_Star}
(gdb) p *((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->fromClause
$28 = {type = T_List, length = 1, head = 0x1f97ed8, tail = 0x1f97ed8}
(gdb) p *((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->fromClause->head->data->ptr_value
Attempt to dereference a generic pointer.
(gdb)  p *((Node*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->fromClause->head->data->ptr_value)
$29 = {type = T_RangeVar}
(gdb)  p *((RangeVar*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->fromClause->head->data->ptr_value)
$30 = {type = T_RangeVar, catalogname = 0x0, schemaname = 0x0,relname = 0x1f97e00 "t_student", inh = true, relpersistence = 112 'p',alias = 0x1f97ea0, location = 16}
(gdb)  p *((RangeVar*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->fromClause->head->data->ptr_value)->alias
$31 = {type = T_Alias, aliasname = 0x1f97e28 "s", colnames = 0x0}
(gdb) p *((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->whereClause
$32 = {type = T_A_Expr}
(gdb) p *((A_Expr*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->whereClause)
$33 = {type = T_A_Expr, kind = AEXPR_OP, name = 0x1f98188, lexpr = 0x1f98000,rexpr = 0x1f980a8, location = 41}
(gdb) p *((A_Expr*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->whereClause)->lexpr
$34 = {type = T_ColumnRef}
(gdb) p *((ColumnRef*)((A_Expr*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->whereClause)->lexpr)
$35 = {type = T_ColumnRef, fields = 0x1f97fc8, location = 34}
(gdb) p *((ColumnRef*)((A_Expr*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->whereClause)->lexpr)->fields
$36 = {type = T_List, length = 2, head = 0x1f98060, tail = 0x1f97fa0}
(gdb) p *((ColumnRef*)((A_Expr*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->whereClause)->lexpr)->fields->head
$37 = {data = {ptr_value = 0x1f98038, int_value = 33128504,oid_value = 33128504}, next = 0x1f97fa0}
(gdb) p *((ColumnRef*)((A_Expr*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->whereClause)->lexpr)->fields->head->data->ptr_value
Attempt to dereference a generic pointer.
(gdb) p *((Node*)((ColumnRef*)((A_Expr*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->whereClause)->lexpr)->fields->head->data->ptr_value)
$38 = {type = T_String}
(gdb) p ((char*)((ColumnRef*)((A_Expr*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->whereClause)->lexpr)->fields->head->data->ptr_value)
$39 = 0x1f98038 
(gdb) p ((char*)((ColumnRef*)((A_Expr*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->whereClause)->lexpr)->fields->head->next->data->ptr_value)
$40 = 0x1f97f78 
(gdb)  p *((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->sortClause
$41 = {type = T_List, length = 1, head = 0x1f98480, tail = 0x1f98480}
(gdb)  p *((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->sortClause->head->data->ptr_value
Attempt to dereference a generic pointer.
(gdb)  p *((Node*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->sortClause->head->data->ptr_value)
$42 = {type = T_SortBy}
(gdb)  p *((SortBy*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->sortClause->head->data->ptr_value)
$43 = {type = T_SortBy, node = 0x1f983a0, sortby_dir = SORTBY_DESC,sortby_nulls = SORTBY_NULLS_DEFAULT, useOp = 0x0, location = -1}
(gdb)  p *((SortBy*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->sortClause->head->data->ptr_value)->node
$44 = {type = T_ColumnRef}
(gdb)  p *((ColumnRef*)((SortBy*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->sortClause->head->data->ptr_value)->node)
$45 = {type = T_ColumnRef, fields = 0x1f98368, location = 56}
(gdb)  p *((ColumnRef*)((SortBy*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->sortClause->head->data->ptr_value)->node)->fields
$46 = {type = T_List, length = 2, head = 0x1f98400, tail = 0x1f98340}
(gdb)  p *((ColumnRef*)((SortBy*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->sortClause->head->data->ptr_value)->node)->fields->head->data->ptr_value
Attempt to dereference a generic pointer.
(gdb)  p *((Node*)((ColumnRef*)((SortBy*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->sortClause->head->data->ptr_value)->node)->fields->head->data->ptr_value)
$47 = {type = T_String}
(gdb)  p *((Value*)((ColumnRef*)((SortBy*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->sortClause->head->data->ptr_value)->node)->fields->head->data->ptr_value)
$48 = {type = T_String, val = {ival = 33129176, str = 0x1f982d8 "s"}}
(gdb) p *((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->limitCount
$49 = {type = T_A_Const}
(gdb) p *((A_Const*)((SelectStmt*)((RawStmt*)parsetree_list->head->data->ptr_value)->stmt)->limitCount)
$50 = {type = T_A_Const, val = {type = T_Integer, val = {ival = 2,str = 0x2 
}}, location = 74}

对返回的语法树进行语义分析及查询重写 pg_analyze_and_rewrite

image.png

1. 输入信息

    

   

   parsetree

    

   query_string：

select s.*
from t_student s
where s.sdept='IS'
order by s.sno desc 
limit 2;

paramTypes =NULL；
numParams=0；
queryEnv =NULL；

1. 返回的查询重写结果为

#语义分析和查询重写后的查询树信息
1047                    querytree_list = pg_analyze_and_rewrite(parsetree, query_string,
(gdb)
1050                    plantree_list = pg_plan_queries(querytree_list,
(gdb) p *(querytree_list)
$51 = {type = T_List, length = 1, head = 0x2066ed0, tail = 0x2066ed0}
(gdb) p *((Node*)querytree_list->head->data->ptr_value)
$52 = {type = T_Query}
(gdb) p *((Query*)querytree_list->head->data->ptr_value)
$53 = {type = T_Query, commandType = CMD_SELECT, querySource = QSRC_ORIGINAL,queryId = 0, canSetTag = true, utilityStmt = 0x0, resultRelation = 0,hasAggs = false, hasWindowFuncs = false, hasTargetSRFs = false,hasSubLinks = false, hasDistinctOn = false, hasRecursive = false,hasModifyingCTE = false, hasForUpdate = false, hasRowSecurity = false,cteList = 0x0, rtable = 0x1f98c68, jointree = 0x207a0b8,targetList = 0x2079a58, override = OVERRIDING_NOT_SET, onConflict = 0x0,returningList = 0x0, groupClause = 0x0, groupingSets = 0x0,havingQual = 0x0, windowClause = 0x0, distinctClause = 0x0,sortClause = 0x2079f70, limitOffset = 0x0, limitCount = 0x207a060,rowMarks = 0x0, setOperations = 0x0, constraintDeps = 0x0,withCheckOptions = 0x0, stmt_location = 0, stmt_len = 75}
(gdb) p *((Query*)querytree_list->head->data->ptr_value)->rtable
$54 = {type = T_List, length = 1, head = 0x1f98c40, tail = 0x1f98c40}
(gdb) p *((Node*)((Query*)querytree_list->head->data->ptr_value)->rtable->head->data->ptr_value)
$55 = {type = T_RangeTblEntry}
(gdb) p *((RangeTblEntry*)((Query*)querytree_list->head->data->ptr_value)->rtable->head->data->ptr_value)
$56 = {type = T_RangeTblEntry, rtekind = RTE_RELATION, relid = 16408,relkind = 114 'r', tablesample = 0x0, subquery = 0x0,security_barrier = false, jointype = JOIN_INNER, joinaliasvars = 0x0,functions = 0x0, funcordinality = false, tablefunc = 0x0,values_lists = 0x0, ctename = 0x0, ctelevelsup = 0, self_reference = false,coltypes = 0x0, coltypmods = 0x0, colcollations = 0x0, enrname = 0x0,enrtuples = 0, alias = 0x1f97ea0, eref = 0x1f98980, lateral = false,inh = true, inFromCl = true, requiredPerms = 2, checkAsUser = 0,selectedCols = 0x2079a90, insertedCols = 0x0, updatedCols = 0x0,securityQuals = 0x0}
(gdb) p *((Query*)querytree_list->head->data->ptr_value)->jointree
$57 = {type = T_FromExpr, fromlist = 0x1f98d70, quals = 0x2079e60}
(gdb) p *((Query*)querytree_list->head->data->ptr_value)->jointree->fromlist
$58 = {type = T_List, length = 1, head = 0x1f98d48, tail = 0x1f98d48}
(gdb) p *((Node*)((Query*)querytree_list->head->data->ptr_value)->jointree->fromlist->head->data->ptr_value)
$59 = {type = T_RangeTblRef}
(gdb) p *((RangeTblRef*)((Query*)querytree_list->head->data->ptr_value)->jointree->fromlist->head->data->ptr_value)
$60 = {type = T_RangeTblRef, rtindex = 1}
(gdb) p *((Query*)querytree_list->head->data->ptr_value)->jointree->quals
$61 = {type = T_OpExpr}
(gdb) p *((OpExpr*)((Query*)querytree_list->head->data->ptr_value)->jointree->quals)
$62 = {xpr = {type = T_OpExpr}, opno = 1054, opfuncid = 1048,opresulttype = 16, opretset = false, opcollid = 0, inputcollid = 100,args = 0x2079d88, location = 41}
(gdb) p *((OpExpr*)((Query*)querytree_list->head->data->ptr_value)->jointree->quals)->args
$63 = {type = T_List, length = 2, head = 0x2079dc0, tail = 0x2079d60}
(gdb) p *((Node*)((OpExpr*)((Query*)querytree_list->head->data->ptr_value)->jointree->quals)->args->head->data->ptr_value)
$64 = {type = T_Var}
(gdb) p *((Var*)((OpExpr*)((Query*)querytree_list->head->data->ptr_value)->jointree->quals)->args->head->data->ptr_value)
$65 = {xpr = {type = T_Var}, varno = 1, varattno = 5, vartype = 1042,vartypmod = 24, varcollid = 100, varlevelsup = 0, varnoold = 1,varoattno = 5, location = 34}
(gdb) p *((Node*)((OpExpr*)((Query*)querytree_list->head->data->ptr_value)->jointree->quals)->args->head->next->data->ptr_value)
$66 = {type = T_Const}
(gdb) p *((Const*)((OpExpr*)((Query*)querytree_list->head->data->ptr_value)->jointree->quals)->args->head->next->data->ptr_value)
$67 = {xpr = {type = T_Const}, consttype = 1042, consttypmod = -1,constcollid = 100, constlen = -1, constvalue = 34053696,constisnull = false, constbyval = false, location = 42}
(gdb) p *((Node*)((Const*)((OpExpr*)((Query*)querytree_list->head->data->ptr_value)->jointree->quals)->args->head->next->data->ptr_value)->constvalue)
$75 = {type = T_BitmapHeapScan}
(gdb) p *((BitmapHeapScan*)((Const*)((OpExpr*)((Query*)querytree_list->head->data->ptr_value)->jointree->quals)->args->head->next->data->ptr_value)->constvalue)
$76 = {scan = {plan = {type = T_BitmapHeapScan,startup_cost = 3.1620201333839779e-322,total_cost = 2.3715151000379834e-322,plan_rows = 1.6365691319506455e-316, plan_width = 112,parallel_aware = 30, parallel_safe = 4, plan_node_id = 1048,targetlist = 0x0, qual = 0x64, lefttree = 0x2079d88, righttree = 0x29,initPlan = 0x7f7f7f7f7f7f7f7e, extParam = 0x7f7f7f7f7f7f7f7f,allParam = 0x40}, scanrelid = 40}, bitmapqualorig = 0x1f970b0}
(gdb) p *((Query*)querytree_list->head->data->ptr_value)->targetList
$77 = {type = T_List, length = 5, head = 0x2079a30, tail = 0x2079c88}
(gdb) p *((Node*)((Query*)querytree_list->head->data->ptr_value)->targetList->head->data->ptr_value)
$78 = {type = T_TargetEntry}
(gdb) p *((TargetEntry*)((Query*)querytree_list->head->data->ptr_value)->targetList->head->data->ptr_value)
$79 = {xpr = {type = T_TargetEntry}, expr = 0x1f98e50, resno = 1,resname = 0x1f98da8 "sno", ressortgroupref = 1, resorigtbl = 16408,resorigcol = 1, resjunk = false}
(gdb) p *((TargetEntry*)((Query*)querytree_list->head->data->ptr_value)->targetList->head->next->data->ptr_value)
$80 = {xpr = {type = T_TargetEntry}, expr = 0x1f98f78, resno = 2,resname = 0x1f98f08 "sname", ressortgroupref = 0, resorigtbl = 16408,resorigcol = 2, resjunk = false}
(gdb) p *((TargetEntry*)((Query*)querytree_list->head->data->ptr_value)->targetList->head->next->next->data->ptr_value)
$81 = {xpr = {type = T_TargetEntry}, expr = 0x1f99068, resno = 3,resname = 0x1f98ff8 "ssex", ressortgroupref = 0, resorigtbl = 16408,resorigcol = 3, resjunk = false}
(gdb) p *((TargetEntry*)((Query*)querytree_list->head->data->ptr_value)->targetList->head->next->next->next->data->ptr_value)
$82 = {xpr = {type = T_TargetEntry}, expr = 0x2079868, resno = 4,resname = 0x20797f8 "sage", ressortgroupref = 0, resorigtbl = 16408,resorigcol = 4, resjunk = false}
(gdb) p *((TargetEntry*)((Query*)querytree_list->head->data->ptr_value)->targetList->head->next->next->next->next->data->ptr_value)
$83 = {xpr = {type = T_TargetEntry}, expr = 0x2079958, resno = 5,resname = 0x20798e8 "sdept", ressortgroupref = 0, resorigtbl = 16408,resorigcol = 5, resjunk = false}
(gdb) p *((Query*)querytree_list->head->data->ptr_value)->sortClause
$84 = {type = T_List, length = 1, head = 0x2079f48, tail = 0x2079f48}
(gdb) p *((Node*)((Query*)querytree_list->head->data->ptr_value)->sortClause->head->data->ptr_value)
$85 = {type = T_SortGroupClause}
(gdb) p *((SortGroupClause*)((Query*)querytree_list->head->data->ptr_value)->sortClause->head->data->ptr_value)
$86 = {type = T_SortGroupClause, tleSortGroupRef = 1, eqop = 1054,sortop = 1060, nulls_first = true, hashable = true}
(gdb) p *((Query*)querytree_list->head->data->ptr_value)->limitCount
$87 = {type = T_FuncExpr}
(gdb) p *((FuncExpr*)((Query*)querytree_list->head->data->ptr_value)->limitCount)
$88 = {xpr = {type = T_FuncExpr}, funcid = 481, funcresulttype = 20,funcretset = false, funcvariadic = false, funcformat = COERCE_IMPLICIT_CAST,funccollid = 0, inputcollid = 0, args = 0x207a028, location = -1}
(gdb) p *((FuncExpr*)((Query*)querytree_list->head->data->ptr_value)->limitCount)->args
$89 = {type = T_List, length = 1, head = 0x207a000, tail = 0x207a000}
(gdb) p *((Node*)((FuncExpr*)((Query*)querytree_list->head->data->ptr_value)->limitCount)->args->head->data->ptr_value)
$90 = {type = T_Const}
(gdb) p *((Const*)((FuncExpr*)((Query*)querytree_list->head->data->ptr_value)->limitCount)->args->head->data->ptr_value)
$91 = {xpr = {type = T_Const}, consttype = 23, consttypmod = -1,constcollid = 0, constlen = 4, constvalue = 2, constisnull = false,constbyval = true, location = 74}
(gdb)

## 语义分析与查询重写
2018-08-07 19:57:58.720 CST,"appusr","postgres",38496,"127.0.0.1:48658",5b698942.9660,5,"SELECT",2018-08-07 19:57:54 CST,3/2,0,LOG,00000,"rewritten parse tree:","({QUERY :commandType 1 :querySource 0 :canSetTag true :utilityStmt <> :resultRelation 0 :hasAggs false :hasWindowFuncs false :hasTargetSRFs false :hasSubLinks false :hasDistinctOn false :hasRecursive false :hasModifyingCTE false :hasForUpdate false :hasRowSecurity false :cteList <> :rtable ({RTE :alias {ALIAS :aliasname s :colnames <>}:eref {ALIAS :aliasname s :colnames (""sno"" ""sname"" ""ssex"" ""sage"" ""sdept"")}:rtekind 0 :relid 16408 :relkind r :tablesample <> :lateral false :inh true :inFromCl true :requiredPerms 2 :checkAsUser 0 :selectedCols (b 9 10 11 12 13):insertedCols (b):updatedCols (b):securityQuals <>}):jointree {FROMEXPR :fromlist ({RANGETBLREF :rtindex 1}):quals {OPEXPR :opno 1054 :opfuncid 1048 :opresulttype 16 :opretset false :opcollid 0 :inputcollid 100 :args ({VAR :varno 1 :varattno 5 :vartype 1042 :vartypmod 24 :varcollid 100 :varlevelsup 0 :varnoold 1 :varoattno 5 :location 34}{CONST :consttype 1042 :consttypmod -1 :constcollid 100 :constlen -1 :constbyval false :constisnull false :location 42 :constvalue 6 [ 24 0 0 0 73 83 ]}):location 41}}:targetList ({TARGETENTRY :expr {VAR :varno 1 :varattno 1 :vartype 1042 :vartypmod 13 :varcollid 100 :varlevelsup 0 :varnoold 1 :varoattno 1 :location 7}:resno 1 :resname sno :ressortgroupref 1 :resorigtbl 16408 :resorigcol 1 :resjunk false}{TARGETENTRY :expr {VAR :varno 1 :varattno 2 :vartype 1042 :vartypmod 24 :varcollid 100 :varlevelsup 0 :varnoold 1 :varoattno 2 :location 7}:resno 2 :resname sname :ressortgroupref 0 :resorigtbl 16408 :resorigcol 2 :resjunk false}{TARGETENTRY :expr {VAR :varno 1 :varattno 3 :vartype 1042 :vartypmod 6 :varcollid 100 :varlevelsup 0 :varnoold 1 :varoattno 3 :location 7}:resno 3 :resname ssex :ressortgroupref 0 :resorigtbl 16408 :resorigcol 3 :resjunk false}{TARGETENTRY :expr {VAR :varno 1 :varattno 4 :vartype 21 :vartypmod -1 :varcollid 0 :varlevelsup 0 :varnoold 1 :varoattno 4 :location 7}:resno 4 :resname sage :ressortgroupref 0 :resorigtbl 16408 :resorigcol 4 :resjunk false}{TARGETENTRY :expr {VAR :varno 1 :varattno 5 :vartype 1042 :vartypmod 24 :varcollid 100 :varlevelsup 0 :varnoold 1 :varoattno 5 :location 7}:resno 5 :resname sdept :ressortgroupref 0 :resorigtbl 16408 :resorigcol 5 :resjunk false}):override 0 :onConflict <> :returningList <> :groupClause <> :groupingSets <> :havingQual <> :windowClause <> :distinctClause <> :sortClause ({SORTGROUPCLAUSE :tleSortGroupRef 1 :eqop 1054 :sortop 1060 :nulls_first true :hashable true}):limitOffset <> :limitCount {FUNCEXPR :funcid 481 :funcresulttype 20 :funcretset false :funcvariadic false :funcformat 2 :funccollid 0 :inputcollid 0 :args ({CONST :consttype 23 :consttypmod -1 :constcollid 0 :constlen 4 :constbyval true :constisnull false :location 74 :constvalue 4 [ 2 0 0 0 0 0 0 0 ]}):location -1}:rowMarks <> :setOperations <> :constraintDeps <> :stmt_location 0 :stmt_len 75}
)
",,,,,"select s.*
from t_student s
where s.sdept='IS'
order by s.sno desc 
limit 2;",,,"psql"

重写后的查询树

调用pg_plan_queries进行查询计划和优化

/** Generate plans for a list of already-rewritten queries.** For normal optimizable statements, invoke the planner.  For utility* statements, just make a wrapper PlannedStmt node.** The result is a list of PlannedStmt nodes.*/
List *
pg_plan_queries(List *querytrees, int cursorOptions, ParamListInfo boundParams)
{....
}

根据上面的query tree产生执行计划。这部分核心代码在planner.c中，是PG的Query Optimizer。会根据表和索引的统计信息去计算不同路径的可能代价值，最后选出最优者。

生成执行计划函数调用栈

1. 输入值：
   querytrees: 语义分析和查询重写后的查询树
   cursorOptions: CURSOR_OPT_PARALLEL_OK
   boundParams: NULL

2. 返回查询执行计划：

# 查询执行计划树
(gdb) p *(plantree_list)
$93 = {type = T_List, length = 1, head = 0x2083828, tail = 0x2083828}
(gdb) p *(plantree_list->head->data->ptr_value)
Attempt to dereference a generic pointer.
(gdb) p *((Node*)plantree_list->head->data->ptr_value)
$94 = {type = T_PlannedStmt}
(gdb) p *((PlannedStmt*)plantree_list->head->data->ptr_value)
$95 = {type = T_PlannedStmt, commandType = CMD_SELECT, queryId = 0,hasReturning = false, hasModifyingCTE = false, canSetTag = true,transientPlan = false, dependsOnRole = false, parallelModeNeeded = false,jitFlags = 0, planTree = 0x206a588, rtable = 0x206a760,resultRelations = 0x0, nonleafResultRelations = 0x0,rootResultRelations = 0x0, subplans = 0x0, rewindPlanIDs = 0x0,rowMarks = 0x0, relationOids = 0x206a7c0, invalItems = 0x0,paramExecTypes = 0x0, utilityStmt = 0x0, stmt_location = 0, stmt_len = 75}
(gdb) p *((PlannedStmt*)plantree_list->head->data->ptr_value)->planTree
$96 = {type = T_Limit, startup_cost = 14.01, total_cost = 14.015000000000001,plan_rows = 2, plan_width = 222, parallel_aware = false,parallel_safe = true, plan_node_id = 0, targetlist = 0x206a8d0, qual = 0x0,lefttree = 0x2069218, righttree = 0x0, initPlan = 0x0, extParam = 0x0,allParam = 0x0}
(gdb) p *((PlannedStmt*)plantree_list->head->data->ptr_value)->planTree->lefttree
$97 = {type = T_Sort, startup_cost = 14.01, total_cost = 14.015000000000001,plan_rows = 2, plan_width = 222, parallel_aware = false,parallel_safe = true, plan_node_id = 1, targetlist = 0x206ad40, qual = 0x0,lefttree = 0x206a1b8, righttree = 0x0, initPlan = 0x0, extParam = 0x0,allParam = 0x0}
(gdb) p *((PlannedStmt*)plantree_list->head->data->ptr_value)->planTree->lefttree->lefttree
$98 = {type = T_SeqScan, startup_cost = 0, total_cost = 14, plan_rows = 2,plan_width = 222, parallel_aware = false, parallel_safe = true,plan_node_id = 2, targetlist = 0x2069f20, qual = 0x206a180, lefttree = 0x0,righttree = 0x0, initPlan = 0x0, extParam = 0x0, allParam = 0x0}
(gdb) p *((PlannedStmt*)plantree_list->head->data->ptr_value)->planTree->lefttree->lefttree->targetlist
$99 = {type = T_List, length = 5, head = 0x2069ef8, tail = 0x206a130}
(gdb) p *((Node*)((PlannedStmt*)plantree_list->head->data->ptr_value)->planTree->lefttree->lefttree->targetlist->head->data->ptr_value)
$100 = {type = T_TargetEntry}
(gdb) p *((TargetEntry*)((PlannedStmt*)plantree_list->head->data->ptr_value)->planTree->lefttree->lefttree->targetlist->head->data->ptr_value)
$101 = {xpr = {type = T_TargetEntry}, expr = 0x2067ed0, resno = 1,resname = 0x0, ressortgroupref = 1, resorigtbl = 0, resorigcol = 0,resjunk = false}
(gdb) p *((TargetEntry*)((PlannedStmt*)plantree_list->head->data->ptr_value)->planTree->lefttree->lefttree->targetlist->head->data->ptr_value)->expr
$102 = {type = T_Var}
(gdb) p *((Var*)((TargetEntry*)((PlannedStmt*)plantree_list->head->data->ptr_value)->planTree->lefttree->lefttree->targetlist->head->data->ptr_value)->expr)
$103 = {xpr = {type = T_Var}, varno = 1, varattno = 1, vartype = 1042,vartypmod = 13, varcollid = 100, varlevelsup = 0, varnoold = 1,varoattno = 1, location = 7}
(gdb) p *((PlannedStmt*)plantree_list->head->data->ptr_value)->planTree->lefttree->lefttree->qual
$104 = {type = T_List, length = 1, head = 0x206a158, tail = 0x206a158}
(gdb) p *((Node*)((PlannedStmt*)plantree_list->head->data->ptr_value)->planTree->lefttree->lefttree->qual->head->data->ptr_value)
$105 = {type = T_OpExpr}
(gdb) p *((OpExpr*)((PlannedStmt*)plantree_list->head->data->ptr_value)->planTree->lefttree->lefttree->qual->head->data->ptr_value)
$106 = {xpr = {type = T_OpExpr}, opno = 1054, opfuncid = 1048,opresulttype = 16, opretset = false, opcollid = 0, inputcollid = 100,args = 0x2067460, location = 41}
(gdb) p *((OpExpr*)((PlannedStmt*)plantree_list->head->data->ptr_value)->planTree->lefttree->lefttree->qual->head->data->ptr_value)->args
$107 = {type = T_List, length = 2, head = 0x2067438, tail = 0x20674f0}
(gdb) p *((Node*)((OpExpr*)((PlannedStmt*)plantree_list->head->data->ptr_value)->planTree->lefttree->lefttree->qual->head->data->ptr_value)->args->head->data->ptr_value)
$108 = {type = T_Var}
(gdb) p *((Var*)((OpExpr*)((PlannedStmt*)plantree_list->head->data->ptr_value)->planTree->lefttree->lefttree->qual->head->data->ptr_value)->args->head->data->ptr_value)
$109 = {xpr = {type = T_Var}, varno = 1, varattno = 5, vartype = 1042,vartypmod = 24, varcollid = 100, varlevelsup = 0, varnoold = 1,varoattno = 5, location = 34}
(gdb) p *((Node*)((OpExpr*)((PlannedStmt*)plantree_list->head->data->ptr_value)->planTree->lefttree->lefttree->qual->head->data->ptr_value)->args->head->next->data->ptr_value)
$110 = {type = T_Const}
(gdb) p *((Var*)((OpExpr*)((PlannedStmt*)plantree_list->head->data->ptr_value)->planTree->lefttree->lefttree->qual->head->data->ptr_value)->args->head->next->data->ptr_value)
$111 = {xpr = {type = T_Const}, varno = 1042, varattno = -1, vartype = 100,vartypmod = -1, varcollid = 0, varlevelsup = 34053696, varnoold = 0,varoattno = 0, location = 42}
(gdb) p *((Node*)((Var*)((OpExpr*)((PlannedStmt*)plantree_list->head->data->ptr_value)->planTree->lefttree->lefttree->qual->head->data->ptr_value)->args->head->next->data->ptr_value))
$112 = {type = T_Const}
(gdb) p *((Node*)((Var*)((OpExpr*)((PlannedStmt*)plantree_list->head->data->ptr_value)->planTree->lefttree->lefttree->qual->head->data->ptr_value)->args->head->next->data->ptr_value)->varlevelsup)
$113 = {type = T_BitmapHeapScan}
(gdb) p *((BitmapHeapScan*)((Var*)((OpExpr*)((PlannedStmt*)plantree_list->head->data->ptr_value)->planTree->lefttree->lefttree->qual->head->data->ptr_value)->args->head->next->data->ptr_value)->varlevelsup)
$114 = {scan = {plan = {type = T_BitmapHeapScan,startup_cost = 3.1620201333839779e-322,total_cost = 2.3715151000379834e-322,plan_rows = 1.6365691319506455e-316, plan_width = 112,parallel_aware = 30, parallel_safe = 4, plan_node_id = 1048,targetlist = 0x0, qual = 0x64, lefttree = 0x2079d88, righttree = 0x29,initPlan = 0x7f7f7f7f7f7f7f7e, extParam = 0x7f7f7f7f7f7f7f7f,allParam = 0x40}, scanrelid = 40}, bitmapqualorig = 0x1f970b0}
(gdb) 

## 执行计划
2018-08-07 19:57:58.722 CST,"appusr","postgres",38496,"127.0.0.1:48658",5b698942.9660,6,"SELECT",2018-08-07 19:57:54 CST,3/2,0,LOG,00000,"plan:","   {PLANNEDSTMT :commandType 1 :queryId 0 :hasReturning false :hasModifyingCTE false :canSetTag true :transientPlan false :dependsOnRole false :parallelModeNeeded false :jitFlags 0 :planTree {LIMIT :startup_cost 14.01 :total_cost 14.02 :plan_rows 2 :plan_width 222 :parallel_aware false :parallel_safe true :plan_node_id 0 :targetlist ({TARGETENTRY :expr {VAR :varno 65001 :varattno 1 :vartype 1042 :vartypmod 13 :varcollid 100 :varlevelsup 0 :varnoold 1 :varoattno 1 :location -1}:resno 1 :resname sno :ressortgroupref 1 :resorigtbl 16408 :resorigcol 1 :resjunk false}{TARGETENTRY :expr {VAR :varno 65001 :varattno 2 :vartype 1042 :vartypmod 24 :varcollid 100 :varlevelsup 0 :varnoold 1 :varoattno 2 :location -1}:resno 2 :resname sname :ressortgroupref 0 :resorigtbl 16408 :resorigcol 2 :resjunk false}{TARGETENTRY :expr {VAR :varno 65001 :varattno 3 :vartype 1042 :vartypmod 6 :varcollid 100 :varlevelsup 0 :varnoold 1 :varoattno 3 :location -1}:resno 3 :resname ssex :ressortgroupref 0 :resorigtbl 16408 :resorigcol 3 :resjunk false}{TARGETENTRY :expr {VAR :varno 65001 :varattno 4 :vartype 21 :vartypmod -1 :varcollid 0 :varlevelsup 0 :varnoold 1 :varoattno 4 :location -1}:resno 4 :resname sage :ressortgroupref 0 :resorigtbl 16408 :resorigcol 4 :resjunk false}{TARGETENTRY :expr {VAR :varno 65001 :varattno 5 :vartype 1042 :vartypmod 24 :varcollid 100 :varlevelsup 0 :varnoold 1 :varoattno 5 :location -1}:resno 5 :resname sdept :ressortgroupref 0 :resorigtbl 16408 :resorigcol 5 :resjunk false}):qual <> :lefttree {SORT :startup_cost 14.01 :total_cost 14.02 :plan_rows 2 :plan_width 222 :parallel_aware false :parallel_safe true :plan_node_id 1 :targetlist ({TARGETENTRY :expr {VAR :varno 65001 :varattno 1 :vartype 1042 :vartypmod 13 :varcollid 100 :varlevelsup 0 :varnoold 1 :varoattno 1 :location -1}:resno 1 :resname sno :ressortgroupref 1 :resorigtbl 16408 :resorigcol 1 :resjunk false}{TARGETENTRY :expr {VAR :varno 65001 :varattno 2 :vartype 1042 :vartypmod 24 :varcollid 100 :varlevelsup 0 :varnoold 1 :varoattno 2 :location -1}:resno 2 :resname sname :ressortgroupref 0 :resorigtbl 16408 :resorigcol 2 :resjunk false}{TARGETENTRY :expr {VAR :varno 65001 :varattno 3 :vartype 1042 :vartypmod 6 :varcollid 100 :varlevelsup 0 :varnoold 1 :varoattno 3 :location -1}:resno 3 :resname ssex :ressortgroupref 0 :resorigtbl 16408 :resorigcol 3 :resjunk false}{TARGETENTRY :expr {VAR :varno 65001 :varattno 4 :vartype 21 :vartypmod -1 :varcollid 0 :varlevelsup 0 :varnoold 1 :varoattno 4 :location -1}:resno 4 :resname sage :ressortgroupref 0 :resorigtbl 16408 :resorigcol 4 :resjunk false}{TARGETENTRY :expr {VAR :varno 65001 :varattno 5 :vartype 1042 :vartypmod 24 :varcollid 100 :varlevelsup 0 :varnoold 1 :varoattno 5 :location -1}:resno 5 :resname sdept :ressortgroupref 0 :resorigtbl 16408 :resorigcol 5 :resjunk false}):qual <> :lefttree {SEQSCAN :startup_cost 0.00 :total_cost 14.00 :plan_rows 2 :plan_width 222 :parallel_aware false :parallel_safe true :plan_node_id 2 :targetlist ({TARGETENTRY :expr {VAR :varno 1 :varattno 1 :vartype 1042 :vartypmod 13 :varcollid 100 :varlevelsup 0 :varnoold 1 :varoattno 1 :location 7}:resno 1 :resname <> :ressortgroupref 1 :resorigtbl 0 :resorigcol 0 :resjunk false}{TARGETENTRY :expr {VAR :varno 1 :varattno 2 :vartype 1042 :vartypmod 24 :varcollid 100 :varlevelsup 0 :varnoold 1 :varoattno 2 :location 7}:resno 2 :resname <> :ressortgroupref 0 :resorigtbl 0 :resorigcol 0 :resjunk false}{TARGETENTRY :expr {VAR :varno 1 :varattno 3 :vartype 1042 :vartypmod 6 :varcollid 100 :varlevelsup 0 :varnoold 1 :varoattno 3 :location 7}:resno 3 :resname <> :ressortgroupref 0 :resorigtbl 0 :resorigcol 0 :resjunk false}{TARGETENTRY :expr {VAR :varno 1 :varattno 4 :vartype 21 :vartypmod -1 :varcollid 0 :varlevelsup 0 :varnoold 1 :varoattno 4 :location 7}:resno 4 :resname <> :ressortgroupref 0 :resorigtbl 0 :resorigcol 0 :resjunk false}{TARGETENTRY :expr {VAR :varno 1 :varattno 5 :vartype 1042 :vartypmod 24 :varcollid 100 :varlevelsup 0 :varnoold 1 :varoattno 5 :location 7}:resno 5 :resname <> :ressortgroupref 0 :resorigtbl 0 :resorigcol 0 :resjunk false}):qual ({OPEXPR :opno 1054 :opfuncid 1048 :opresulttype 16 :opretset false :opcollid 0 :inputcollid 100 :args ({VAR :varno 1 :varattno 5 :vartype 1042 :vartypmod 24 :varcollid 100 :varlevelsup 0 :varnoold 1 :varoattno 5 :location 34}{CONST :consttype 1042 :consttypmod -1 :constcollid 100 :constlen -1 :constbyval false :constisnull false :location 42 :constvalue 6 [ 24 0 0 0 73 83 ]}):location 41}):lefttree <> :righttree <> :initPlan <> :extParam (b):allParam (b):scanrelid 1}:righttree <> :initPlan <> :extParam (b):allParam (b):numCols 1 :sortColIdx 1 :sortOperators 1060 :collations 100 :nullsFirst true}:righttree <> :initPlan <> :extParam (b):allParam (b):limitOffset <> :limitCount {CONST :consttype 20 :consttypmod -1 :constcollid 0 :constlen 8 :constbyval true :constisnull false :location -1 :constvalue 8 [ 2 0 0 0 0 0 0 0 ]}}:rtable ({RTE :alias {ALIAS :aliasname s :colnames <>}:eref {ALIAS :aliasname s :colnames (""sno"" ""sname"" ""ssex"" ""sage"" ""sdept"")}:rtekind 0 :relid 16408 :relkind r :tablesample <> :lateral false :inh false :inFromCl true :requiredPerms 2 :checkAsUser 0 :selectedCols (b 9 10 11 12 13):insertedCols (b):updatedCols (b):securityQuals <>}):resultRelations <> :nonleafResultRelations <> :rootResultRelations <> :subplans <> :rewindPlanIDs (b):rowMarks <> :relationOids (o 16408):invalItems <> :paramExecTypes <> :utilityStmt <> :stmt_location 0 :stmt_len 75}
",,,,,"select s.*
from t_student s
where s.sdept='IS'
order by s.sno desc 
limit 2;",,,"psql"

查询执行计划树

LOG:  duration: 4.676 mssno    |          sname          | ssex | sage |        sdept         
-----------+-------------------------+------+------+----------------------201215129 | 黄林林                  | 男   |   21 | IS                  201215126 | 李一平                  | 男   |   18 | IS                  
(2 rows)postgres=# 

总结：

本文以一个简单的查询为例子，大致说明了exec_simple_query的查询函数执行流程，及相关的查询树等数据结构分析。
重要说明了查询语法树、计划执行树的结构说明。

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