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。

posted @ 2013-05-22 10:43  健哥的数据花园  阅读(547)  评论(0)  编辑  收藏  举报