PostgreSQL在何处处理 sql查询之十

回到 exec_simple_query函数上来。

/*
 * exec_simple_query
 *
 * Execute a "simple Query" protocol message.
 */
static void
exec_simple_query(const char *query_string)
{
    ...
    start_xact_command();
    
    ...
    parsetree_list = pg_parse_query(query_string);

    ...
    /*
     * Run through the raw parsetree(s) and process each one.
     */
    foreach(parsetree_item, parsetree_list)
    {
        Node       *parsetree = (Node *) lfirst(parsetree_item);

        ...

        querytree_list = pg_analyze_and_rewrite(parsetree, query_string,
                                                NULL, 0);

        plantree_list = pg_plan_queries(querytree_list, 0, NULL);

        /* If we got a cancel signal in analysis or planning, quit */
        CHECK_FOR_INTERRUPTS();
/*
         * 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);

        /*
         * Start the portal.
         *
         * If we took a snapshot for parsing/planning, the portal may be able
         * to reuse it for the execution phase.  Currently, this will only
         * happen in PORTAL_ONE_SELECT mode.  But even if PortalStart doesn't
         * end up being able to do this, keeping the parse/plan snapshot
         * around until after we start the portal doesn't cost much.
         */
        PortalStart(portal, NULL, 0, snapshot_set);

        .../*
         * Run the portal to completion, and then drop it (and the receiver).
         */
        (void) PortalRun(portal,
                         FETCH_ALL,
                         isTopLevel,
                         receiver,
                         receiver,
                         completionTag);

        (*receiver->rDestroy) (receiver);

        PortalDrop(portal, false);

        ...
        EndCommand(completionTag, dest);
    }                            /* end loop over parsetrees */

    /*
     * Close down transaction statement, if one is open.
     */
    finish_xact_command();

    ...

}

通过给表文件强制改名进行调试，可以看到对数据库表文件进行实际的物理访问，发生在 pg_plan_queries  函数执行的时候。

这个动作，早于 PortalStart函数。

如果继续追下去看，是这样的：

/*
 * Generate plans for a list of already-rewritten queries.
 *
 * Normal optimizable statements generate PlannedStmt entries in the result
 * list.  Utility statements are simply represented by their statement nodes.
 */
List *
pg_plan_queries(List *querytrees, int cursorOptions, ParamListInfo boundParams)
{
    List       *stmt_list = NIL;
    ListCell   *query_list;

    foreach(query_list, querytrees)
    {
        Query       *query = (Query *) lfirst(query_list);
        Node       *stmt;

        if (query->commandType == CMD_UTILITY)
        {
            /* Utility commands have no plans. */
            stmt = query->utilityStmt;
        }
        else
        {
            stmt = (Node *) pg_plan_query(query, cursorOptions, boundParams);
        }

        stmt_list = lappend(stmt_list, stmt);
    }

    return stmt_list;
}

上面 的 pg_plan_queries，会调用  pg_plan_query。pg_plan_query的主要执行如下：

/*
 * Generate a plan for a single already-rewritten query.
 * This is a thin wrapper around planner() and takes the same parameters.
 */
PlannedStmt *
pg_plan_query(Query *querytree, int cursorOptions, ParamListInfo boundParams)
{

    ...

/* call the optimizer */
    plan = planner(querytree, cursorOptions, boundParams);

    ...
    return plan;
}

如果发生对应表名的物理文件发生错误，就会在 planner函数调用时报错。