/*
 * PortalRunSelect
 *        Execute a portal's query in PORTAL_ONE_SELECT mode, and also
 *        when fetching from a completed holdStore in PORTAL_ONE_RETURNING,
 *        PORTAL_ONE_MOD_WITH, and PORTAL_UTIL_SELECT cases.
 *
 * This handles simple N-rows-forward-or-backward cases.  For more complex
 * nonsequential access to a portal, see PortalRunFetch.
 *
 * count <= 0 is interpreted as a no-op: the destination gets started up
 * and shut down, but nothing else happens.  Also, count == FETCH_ALL is
 * interpreted as "all rows".
 *
 * Caller must already have validated the Portal and done appropriate
 * setup (cf. PortalRun).
 *
 * Returns number of rows processed (suitable for use in result tag)
 */
static long
PortalRunSelect(Portal portal,
                bool forward,
                long count,
                DestReceiver *dest)
{
    ...
/*
     * Determine which direction to go in, and check to see if we're already
     * at the end of the available tuples in that direction.  If so, set the
     * direction to NoMovement to avoid trying to fetch any tuples.  (This
     * check exists because not all plan node types are robust about being
     * called again if they've already returned NULL once.)  Then call the
     * executor (we must not skip this, because the destination needs to see a
     * setup and shutdown even if no tuples are available).  Finally, update
     * the portal position state depending on the number of tuples that were
     * retrieved.
     */
    if (forward)
    {

        if (portal->atEnd || count <= 0)
            direction = NoMovementScanDirection;
        else
            direction = ForwardScanDirection;

        /* In the executor, zero count processes all rows */
        if (count == FETCH_ALL)
            count = 0;

        if (portal->holdStore)
            nprocessed = RunFromStore(portal, direction, count, dest);
        else
        {
            PushActiveSnapshot(queryDesc->snapshot);
            ExecutorRun(queryDesc, direction, count);
            nprocessed = queryDesc->estate->es_processed;
            PopActiveSnapshot();
        }

        if (!ScanDirectionIsNoMovement(direction))
        {
            ...
        }
    }
    else
    {
       ...
    }

    return nprocessed;
}