Function gather_merge_setup

src/backend/executor/nodeGatherMerge.c:395–434

* Set up the data structures that we'll need for Gather Merge. * * We allocate these once on the basis of gm->num_workers, which is an * upper bound for the number of workers we'll actually have. During * a rescan, we reset the structures to empty. This approach simplifies * not leaking memory across rescans. * * In the gm_slots[] array, index 0 is for the leader, and indexes 1 to n * ar

 * 0 to n-1; it has no entry for the leader.
 */
static void
gather_merge_setup(GatherMergeState *gm_state)
{
	GatherMerge *gm = castNode(GatherMerge, gm_state->ps.plan);
	int			nreaders = gm->num_workers;
	int			i;

	/*
	 * Allocate gm_slots for the number of workers + one more slot for leader.
	 * Slot 0 is always for the leader.  Leader always calls ExecProcNode() to
	 * read the tuple, and then stores it directly into its gm_slots entry.
	 * For other slots, code below will call ExecInitExtraTupleSlot() to
	 * create a slot for the worker's results.  Note that during any single
	 * scan, we might have fewer than num_workers available workers, in which
	 * case the extra array entries go unused.
	 */
	gm_state->gm_slots = (TupleTableSlot **)
		palloc0((nreaders + 1) * sizeof(TupleTableSlot *));

	/* Allocate the tuple slot and tuple array for each worker */
	gm_state->gm_tuple_buffers = (GMReaderTupleBuffer *)
		palloc0(nreaders * sizeof(GMReaderTupleBuffer));

	for (i = 0; i < nreaders; i++)
	{
		/* Allocate the tuple array with length MAX_TUPLE_STORE */
		gm_state->gm_tuple_buffers[i].tuple =
			(MinimalTuple *) palloc0(sizeof(MinimalTuple) * MAX_TUPLE_STORE);

		/* Initialize tuple slot for worker */
		gm_state->gm_slots[i + 1] =
			ExecInitExtraTupleSlot(gm_state->ps.state, gm_state->tupDesc,
								   &TTSOpsMinimalTuple);
	}

	/* Allocate the resources for the merge */
	gm_state->gm_heap = binaryheap_allocate(nreaders + 1,
											heap_compare_slots,
											gm_state);
}

/*
 * Initialize the Gather Merge.