前面說過 PortalStart明確執行策略后，要執行 ExecutorStart。

那么ExecutorStart 到底作了什么呢。以下是縮略：

/* ----------------------------------------------------------------* 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);elsestandard_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); }

核心一條就是，對 ?QueryDesc 型指針所指向的結構，進行了設置，以備后面的使用(ExecutorRun)。

上述代碼的注釋中已經說明：使用了CreateQueryDesc時產生的 QueryDesc，設置其tupDesc。

總結

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