PostgreSQL在何处处理 sql查询之四

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

        /*
         * Create unnamed portal to run the query or queries in. If there
         * already is one, silently drop it.
         */
        portal = CreatePortal("", true, true);
        /* 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);

再来看PortalDefineQuery的定义：

/*
 * PortalDefineQuery
 *        A simple subroutine to establish a portal's query.
 *
 * Notes: as of PG 8.4, caller MUST supply a sourceText string; it is not
 * allowed anymore to pass NULL.  (If you really don't have source text,
 * you can pass a constant string, perhaps "(query not available)".)
 *
 * commandTag shall be NULL if and only if the original query string
 * (before rewriting) was an empty string.    Also, the passed commandTag must
 * be a pointer to a constant string, since it is not copied.
 *
 * If cplan is provided, then it is a cached plan containing the stmts, and
 * the caller must have done GetCachedPlan(), causing a refcount increment.
 * The refcount will be released when the portal is destroyed.
 *
 * If cplan is NULL, then it is the caller's responsibility to ensure that
 * the passed plan trees have adequate lifetime.  Typically this is done by
 * copying them into the portal's heap context.
 *
 * The caller is also responsible for ensuring that the passed prepStmtName
 * (if not NULL) and sourceText have adequate lifetime.
 *
 * NB: this function mustn't do much beyond storing the passed values; in
 * particular don't do anything that risks elog(ERROR).  If that were to
 * happen here before storing the cplan reference, we'd leak the plancache
 * refcount that the caller is trying to hand off to us.
 */
void
PortalDefineQuery(Portal portal,
                  const char *prepStmtName,
                  const char *sourceText,
                  const char *commandTag,
                  List *stmts,
                  CachedPlan *cplan)
{
    AssertArg(PortalIsValid(portal));
    AssertState(portal->status == PORTAL_NEW);

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

    portal->prepStmtName = prepStmtName;
    portal->sourceText = sourceText;
    portal->commandTag = commandTag;
    portal->stmts = stmts;
    portal->cplan = cplan;
    portal->status = PORTAL_DEFINED;
}

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。