Function compute_parallel_worker

src/backend/optimizer/path/allpaths.c:4695–4813

* Compute the number of parallel workers that should be used to scan a * relation. We compute the parallel workers based on the size of the heap to * be scanned and the size of the index to be scanned, then choose a minimum * of those. * * "heap_pages" is the number of pages from the table that we expect to scan, or * -1 if we don't expect to scan any. * * "index_pages" is the number of p

 * comes from a GUC.
 */
int
compute_parallel_worker(PlannerInfo *root, RelOptInfo *rel, double heap_pages, double index_pages,
						int max_workers)
{
	int			parallel_workers = 0;

	/*
	 * If the user has set the parallel_workers reloption, use that; otherwise
	 * select a default number of workers.
	 */
	if (rel->rel_parallel_workers != -1)
		parallel_workers = rel->rel_parallel_workers;
	else
	{
		/*
		 * We need to reconsider parallel workers for AO/AOCO tables
		 * because page number in ao is quite hard to estimate.
		 * The parallel for AO/AOCO tables is based on segment file count in
		 * pg_appendonly which will be updated by analyze/vacuum/truncate processes.
		 */
		if (rel->reloptkind == RELOPT_BASEREL && (AMHandlerIsAO(rel->amhandler)))
		{
			Oid			aorelid = root->simple_rte_array[rel->relid]->relid;
			HeapTuple	aotup;
			Form_pg_appendonly aoform;
			aotup = SearchSysCache1(AORELID, ObjectIdGetDatum(aorelid));
			if (!HeapTupleIsValid(aotup))
				ereport(ERROR,
					(errcode(ERRCODE_UNDEFINED_OBJECT),
					errmsg("appendonly table relid %u does not exist in pg_appendonly", aorelid)));

			aoform = (Form_pg_appendonly) GETSTRUCT(aotup);
			Assert(aoform->segfilecount >= 0);
			parallel_workers = Min(aoform->segfilecount, max_workers);
			ReleaseSysCache(aotup);

			/*
			 * Disable parallel for AO/AOCO for:
			 * 1.We don't support parallel/non-parallel IndexScan/IndexOnlyScan.
			 * 2.If parallel_workers is 1, it is pointless in gp parallel mode.
			 */
			if (parallel_workers == 1 || index_pages >= 0)
				parallel_workers = 0;
		}
		else
		{
			/*
			 * If the number of pages being scanned is insufficient to justify a
			 * parallel scan, just return zero ... unless it's an inheritance
			 * child. In that case, we want to generate a parallel path here
			 * anyway.  It might not be worthwhile just for this relation, but
			 * when combined with all of its inheritance siblings it may well pay
			 * off.
			 */
			if (rel->reloptkind == RELOPT_BASEREL &&
				((heap_pages >= 0 && heap_pages < min_parallel_table_scan_size) ||
				 (index_pages >= 0 && index_pages < min_parallel_index_scan_size)))
				return 0;