Function ExecGatherMerge

src/backend/executor/nodeGatherMerge.c:187–280

---------------------------------------------------------------- * ExecGatherMerge(node) * * Scans the relation via multiple workers and returns * the next qualifying tuple. * ---------------------------------------------------------------- */

 * ----------------------------------------------------------------
 */
static TupleTableSlot *
ExecGatherMerge(PlanState *pstate)
{
	GatherMergeState *node = castNode(GatherMergeState, pstate);
	TupleTableSlot *slot;
	ExprContext *econtext;

	CHECK_FOR_INTERRUPTS();

	/*
	 * As with Gather, we don't launch workers until this node is actually
	 * executed.
	 */
	if (!node->initialized)
	{
		EState	   *estate = node->ps.state;
		GatherMerge *gm = castNode(GatherMerge, node->ps.plan);

		/*
		 * Sometimes we might have to run without parallelism; but if parallel
		 * mode is active then we can try to fire up some workers.
		 */
		if (gm->num_workers > 0 && estate->es_use_parallel_mode)
		{
			ParallelContext *pcxt;

			/* Initialize, or re-initialize, shared state needed by workers. */
			if (!node->pei)
				node->pei = ExecInitParallelPlan(node->ps.lefttree,
												 estate,
												 gm->initParam,
												 gm->num_workers,
												 node->tuples_needed);
			else
				ExecParallelReinitialize(node->ps.lefttree,
										 node->pei,
										 gm->initParam);

			/* Try to launch workers. */
			pcxt = node->pei->pcxt;
			LaunchParallelWorkers(pcxt);
			/* We save # workers launched for the benefit of EXPLAIN */
			node->nworkers_launched = pcxt->nworkers_launched;

			/* Set up tuple queue readers to read the results. */
			if (pcxt->nworkers_launched > 0)
			{
				ExecParallelCreateReaders(node->pei);
				/* Make a working array showing the active readers */
				node->nreaders = pcxt->nworkers_launched;
				node->reader = (TupleQueueReader **)
					palloc(node->nreaders * sizeof(TupleQueueReader *));
				memcpy(node->reader, node->pei->reader,
					   node->nreaders * sizeof(TupleQueueReader *));
			}
			else
			{
				/* No workers?	Then never mind. */