看到了其源代码中的一段注释，似乎认识又提高了一层：

/* *     INTERFACE ROUTINES *        ExecInitNode    -        initialize a plan node and its subplans *        ExecProcNode    -        get a tuple by executing the plan node *        ExecEndNode        -        shut down a plan node and its subplans * *     NOTES *        This used to be three files.  It is now all combined into *        one file so that it is easier to keep ExecInitNode, ExecProcNode, *        and ExecEndNode in sync when new nodes are added. * *     EXAMPLE *        Suppose we want the age of the manager of the shoe department and *        the number of employees in that department.  So we have the query: * *                select DEPT.no_emps, EMP.age *                where EMP.name = DEPT.mgr and *                      DEPT.name = "shoe" * *        Suppose the planner gives us the following plan: * *                        Nest Loop (DEPT.mgr = EMP.name) *                        /        \ *                       /         \ *                   Seq Scan        Seq Scan *                    DEPT          EMP *                (name = "shoe") * *        ExecutorStart() is called first. *        It calls InitPlan() which calls ExecInitNode() on *        the root of the plan -- the nest loop node. * *      * ExecInitNode() notices that it is looking at a nest loop and *        as the code below demonstrates, it calls ExecInitNestLoop(). *        Eventually this calls ExecInitNode() on the right and left subplans *        and so forth until the entire plan is initialized.    The result *        of ExecInitNode() is a plan state tree built with the same structure *        as the underlying plan tree. * *      * Then when ExecutorRun() is called, it calls ExecutePlan() which calls *        ExecProcNode() repeatedly on the top node of the plan state tree. *        Each time this happens, ExecProcNode() will end up calling *        ExecNestLoop(), which calls ExecProcNode() on its subplans. *        Each of these subplans is a sequential scan so ExecSeqScan() is *        called.  The slots returned by ExecSeqScan() may contain *        tuples which contain the attributes ExecNestLoop() uses to *        form the tuples it returns. * *      * Eventually ExecSeqScan() stops returning tuples and the nest *        loop join ends.  Lastly, ExecutorEnd() calls ExecEndNode() which *        calls ExecEndNestLoop() which in turn calls ExecEndNode() on *        its subplans which result in ExecEndSeqScan(). * *        This should show how the executor works by having *        ExecInitNode(), ExecProcNode() and ExecEndNode() dispatch *        their work to the appopriate node support routines which may *        in turn call these routines themselves on their subplans.

那么，我能否一定制造出 Nest Loop效果呢