PostgreSQL在何处处理 sql查询之四

看看portal生成完毕后，干了什么(PortalDefineQuery)：

 1         /*

 2          * Create unnamed portal to run the query or queries in. If there

 3          * already is one, silently drop it.

 4          */

 5         portal = CreatePortal("", true, true);

 6         /* Don't display the portal in pg_cursors */

 7         portal->visible = false;

 8 

 9         /*

10          * We don't have to copy anything into the portal, because everything

11          * we are passing here is in MessageContext, which will outlive the

12          * portal anyway.

13          */

14         PortalDefineQuery(portal,

15                           NULL,

16                           query_string,

17                           commandTag,

18                           plantree_list,

19                           NULL);

再来看PortalDefineQuery的定义：

 1 /*

 2  * PortalDefineQuery

 3  *        A simple subroutine to establish a portal's query.

 4  *

 5  * Notes: as of PG 8.4, caller MUST supply a sourceText string; it is not

 6  * allowed anymore to pass NULL.  (If you really don't have source text,

 7  * you can pass a constant string, perhaps "(query not available)".)

 8  *

 9  * commandTag shall be NULL if and only if the original query string

10  * (before rewriting) was an empty string.    Also, the passed commandTag must

11  * be a pointer to a constant string, since it is not copied.

12  *

13  * If cplan is provided, then it is a cached plan containing the stmts, and

14  * the caller must have done GetCachedPlan(), causing a refcount increment.

15  * The refcount will be released when the portal is destroyed.

16  *

17  * If cplan is NULL, then it is the caller's responsibility to ensure that

18  * the passed plan trees have adequate lifetime.  Typically this is done by

19  * copying them into the portal's heap context.

20  *

21  * The caller is also responsible for ensuring that the passed prepStmtName

22  * (if not NULL) and sourceText have adequate lifetime.

23  *

24  * NB: this function mustn't do much beyond storing the passed values; in

25  * particular don't do anything that risks elog(ERROR).  If that were to

26  * happen here before storing the cplan reference, we'd leak the plancache

27  * refcount that the caller is trying to hand off to us.

28  */

29 void

30 PortalDefineQuery(Portal portal,

31                   const char *prepStmtName,

32                   const char *sourceText,

33                   const char *commandTag,

34                   List *stmts,

35                   CachedPlan *cplan)

36 {

37     AssertArg(PortalIsValid(portal));

38     AssertState(portal->status == PORTAL_NEW);

39 

40     AssertArg(sourceText != NULL);

41     AssertArg(commandTag != NULL || stmts == NIL);

42 

43     portal->prepStmtName = prepStmtName;

44     portal->sourceText = sourceText;

45     portal->commandTag = commandTag;

46     portal->stmts = stmts;

47     portal->cplan = cplan;

48     portal->status = PORTAL_DEFINED;

49 }

根据参数调用的情况:

*prepStmtName是NULL，sourceText是 query_string，commandTag传递了。
Lits *stmts 是 exec_simple_query中生成的 plantree_list，CachedPlan *cplan 是空值。

在我使用 select * from tab01 这种语句的时候，commandTag是 SELECT。