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 everything11          * we are passing here is in MessageContext, which will outlive the12          * 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 string10  * (before rewriting) was an empty string.    Also, the passed commandTag must11  * 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, and14  * 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 that18  * the passed plan trees have adequate lifetime.  Typically this is done by19  * copying them into the portal's heap context.20  *21  * The caller is also responsible for ensuring that the passed prepStmtName22  * (if not NULL) and sourceText have adequate lifetime.23  *24  * NB: this function mustn't do much beyond storing the passed values; in25  * particular don't do anything that risks elog(ERROR).  If that were to26  * happen here before storing the cplan reference, we'd leak the plancache27  * refcount that the caller is trying to hand off to us.28  */29 void30 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。