PostgreSQL在何处处理 sql查询之二十八

simple_exec_query函数中，有如下一句：
plantree_list = pg_plan_queries(querytree_list, 0, NULL);
那么，plantree_list 里面，到底有什么，让我来给它大卸八块：
plantree_list 是List *类型(指向List 的指针)：

typedef struct List
{
    NodeTag        type;            /* T_List, T_IntList, or T_OidList */
    int            length;
    ListCell   *head;
    ListCell   *tail;
} List;

struct ListCell
{
    union
    {
        void       *ptr_value;
        int            int_value;
        Oid            oid_value;
    }            data;
    ListCell   *next;
};

由于 ListCell中有一个union 类型，这就比较麻烦了。所以还是先看看在Portal里如何处理的：

static void
exec_simple_query(const char *query_string)
{
    ...
    foreach(parsetree_item, parsetree_list)
    {
              ...
              plantree_list = pg_plan_queries(querytree_list, 0, NULL);

              ...
              PortalDefineQuery(portal,
                          NULL,
                          query_string,
                          commandTag,
                          plantree_list,
                          NULL);              

             ...
             PortalStart(portal, NULL, 0, snapshot_set);
             ...
        }
    ...

}

先看看 PortalDefineQuery 函数是如何使用 plantree_list 的：

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;
}

就是把plan_tree 赋值给 portal->stmts 了。

再看 PortalStart 是如何处理的：ChoosePortalStrategy 要用到  portal->stmts。

void
PortalStart(Portal portal, ParamListInfo params,
            int eflags, bool use_active_snapshot)
{
    ...
    PG_TRY();
    {
        ActivePortal = portal;
        CurrentResourceOwner = portal->resowner;
        PortalContext = PortalGetHeapMemory(portal);

        oldContext = MemoryContextSwitchTo(PortalGetHeapMemory(portal));

        /* Must remember portal param list, if any */
        portal->portalParams = params;

        /*
         * Determine the portal execution strategy
         */
        portal->strategy = ChoosePortalStrategy(portal->stmts);

        /*
         * Fire her up according to the strategy
         */
        switch (portal->strategy)
        {
           ...
        }
    }
    PG_CATCH();
    {
        /* Uncaught error while executing portal: mark it dead */
        MarkPortalFailed(portal);

        /* Restore global vars and propagate error */
        ActivePortal = saveActivePortal;
        CurrentResourceOwner = saveResourceOwner;
        PortalContext = savePortalContext;

        PG_RE_THROW();
    }
    PG_END_TRY();

    MemoryContextSwitchTo(oldContext);

    ActivePortal = saveActivePortal;
    CurrentResourceOwner = saveResourceOwner;
    PortalContext = savePortalContext;

    portal->status = PORTAL_READY;
}