/*
                 * Create QueryDesc in portal's context; for the moment, set
                 * the destination to DestNone.
                 */
               queryDesc = CreateQueryDesc((PlannedStmt *) linitial(portal->stmts),
                                            portal->sourceText,
                                            GetActiveSnapshot(),
                                            InvalidSnapshot,
                                            None_Receiver,
                                            params,
                                            0);

/*
 * CreateQueryDesc
 */
QueryDesc *
CreateQueryDesc(PlannedStmt *plannedstmt,
                const char *sourceText,
                Snapshot snapshot,
                Snapshot crosscheck_snapshot,
                DestReceiver *dest,
                ParamListInfo params,
                int instrument_options)
{
    QueryDesc  *qd = (QueryDesc *) palloc(sizeof(QueryDesc));

    qd->operation = plannedstmt->commandType;    /* operation */
    qd->plannedstmt = plannedstmt;        /* plan */
    qd->utilitystmt = plannedstmt->utilityStmt; /* in case DECLARE CURSOR */
    qd->sourceText = sourceText;    /* query text */
    qd->snapshot = RegisterSnapshot(snapshot);    /* snapshot */
    /* RI check snapshot */
    qd->crosscheck_snapshot = RegisterSnapshot(crosscheck_snapshot);
    qd->dest = dest;            /* output dest */
    qd->params = params;        /* parameter values passed into query */
    qd->instrument_options = instrument_options;        /* instrumentation
                                                         * wanted? */

    /* null these fields until set by ExecutorStart */
    qd->tupDesc = NULL;
    qd->estate = NULL;
    qd->planstate = NULL;
    qd->totaltime = NULL;

    return qd;
}

                /*
                 * Call ExecutorStart to prepare the plan for execution
                 */
                ExecutorStart(queryDesc, myeflags);

/* ----------------------------------------------------------------
 *        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)
    {
        case CMD_SELECT:

            /*
             * SELECT FOR UPDATE/SHARE and modifying CTEs need to mark tuples
             */
            if (queryDesc->plannedstmt->rowMarks != NIL ||
                queryDesc->plannedstmt->hasModifyingCTE)
                estate->es_output_cid = GetCurrentCommandId(true);

            /*
             * A SELECT without modifying CTEs can't possibly queue triggers,
             * so force skip-triggers mode. This is just a marginal efficiency
             * hack, since AfterTriggerBeginQuery/AfterTriggerEndQuery aren't
             * all that expensive, but we might as well do it.
             */
            if (!queryDesc->plannedstmt->hasModifyingCTE)
                eflags |= EXEC_FLAG_SKIP_TRIGGERS;
            break;

        case CMD_INSERT:
        case CMD_DELETE:
        case CMD_UPDATE:
            estate->es_output_cid = GetCurrentCommandId(true);
            break;

        default:
            elog(ERROR, "unrecognized operation code: %d",
                 (int) queryDesc->operation);
            break;
    }
    ...
}