PostgreSQL在何处处理 sql查询之六

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。