前面说过 PortalStart明确执行策略后，要执行 ExecutorStart。
那么ExecutorStart 到底作了什么呢。以下是缩略：
/* ---------------------------------------------------------------- * ExecutorStart * * This routine must be called at the beginning of any execution of any * query plan * * Takes a QueryDesc previously created by CreateQueryDesc (which is separate * only because some places use QueryDescs for utility commands). The tupDesc * field of the QueryDesc is filled in to describe the tuples that will be * returned, and the internal fields (estate and planstate) are set up. * * eflags contains flag bits as described in executor.h. * * NB: the CurrentMemoryContext when this is called will become the parent * of the per-query context used for this Executor invocation. * * We provide a function hook variable that lets loadable plugins * get control when ExecutorStart is called. Such a plugin would * normally call standard_ExecutorStart(). * * ---------------------------------------------------------------- */ void ExecutorStart(QueryDesc *queryDesc, int eflags) { if (ExecutorStart_hook) (*ExecutorStart_hook) (queryDesc, eflags); else standard_ExecutorStart(queryDesc, eflags); } void standard_ExecutorStart(QueryDesc *queryDesc, int eflags) { ... /* * If non-read-only query, set the command ID to mark output tuples with */ switch (queryDesc->operation) { ... } /* * Copy other important information into the EState */ estate->es_snapshot = RegisterSnapshot(queryDesc->snapshot); estate->es_crosscheck_snapshot = RegisterSnapshot(queryDesc->crosscheck_snapshot); estate->es_top_eflags = eflags; estate->es_instrument = queryDesc->instrument_options; /* * Initialize the plan state tree */ InitPlan(queryDesc, eflags); /* * Set up an AFTER-trigger statement context, unless told not to, or * unless it's EXPLAIN-only mode (when ExecutorFinish won't be called). */ if (!(eflags & (EXEC_FLAG_SKIP_TRIGGERS | EXEC_FLAG_EXPLAIN_ONLY))) AfterTriggerBeginQuery(); MemoryContextSwitchTo(oldcontext); }
核心一条就是，对 QueryDesc 型指针所指向的结构，进行了设置，以备后面的使用(ExecutorRun)。
上述代码的注释中已经说明：使用了CreateQueryDesc时产生的 QueryDesc，设置其tupDesc。