Function gather_merge_init

src/backend/executor/nodeGatherMerge.c:443–520

* Initialize the Gather Merge. * * Reset data structures to ensure they're empty. Then pull at least one * tuple from leader + each worker (or set its "done" indicator), and set up * the heap. */

 * the heap.
 */
static void
gather_merge_init(GatherMergeState *gm_state)
{
	int			nreaders = gm_state->nreaders;
	bool		nowait = true;
	int			i;

	/* Assert that gather_merge_setup made enough space */
	Assert(nreaders <= castNode(GatherMerge, gm_state->ps.plan)->num_workers);

	/* Reset leader's tuple slot to empty */
	gm_state->gm_slots[0] = NULL;

	/* Reset the tuple slot and tuple array for each worker */
	for (i = 0; i < nreaders; i++)
	{
		/* Reset tuple array to empty */
		gm_state->gm_tuple_buffers[i].nTuples = 0;
		gm_state->gm_tuple_buffers[i].readCounter = 0;
		/* Reset done flag to not-done */
		gm_state->gm_tuple_buffers[i].done = false;
		/* Ensure output slot is empty */
		ExecClearTuple(gm_state->gm_slots[i + 1]);
	}

	/* Reset binary heap to empty */
	binaryheap_reset(gm_state->gm_heap);

	/*
	 * First, try to read a tuple from each worker (including leader) in
	 * nowait mode.  After this, if not all workers were able to produce a
	 * tuple (or a "done" indication), then re-read from remaining workers,
	 * this time using wait mode.  Add all live readers (those producing at
	 * least one tuple) to the heap.
	 */
reread:
	for (i = 0; i <= nreaders; i++)
	{
		CHECK_FOR_INTERRUPTS();

		/* skip this source if already known done */
		if ((i == 0) ? gm_state->need_to_scan_locally :
			!gm_state->gm_tuple_buffers[i - 1].done)
		{
			if (TupIsNull(gm_state->gm_slots[i]))
			{
				/* Don't have a tuple yet, try to get one */
				if (gather_merge_readnext(gm_state, i, nowait))
					binaryheap_add_unordered(gm_state->gm_heap,
											 Int32GetDatum(i));
			}
			else
			{
				/*
				 * We already got at least one tuple from this worker, but
				 * might as well see if it has any more ready by now.
				 */
				load_tuple_array(gm_state, i);