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Allow RTE_SUBQUERY rels to be considered parallel-safe.

There isn't really any reason not to; the original comments here werepartly confused about subplans versus subquery-in-FROM, and partlydependent on restrictions that no longer apply now that subqueries returnPaths not Plans. Depending on what's inside the subquery, it might failto produce any parallel_safe Paths, but that's fine.Tom Lane and Robert Haas

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src/backend/optimizer/path
- allpaths.c

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`‎src/backend/optimizer/path/allpaths.c`

Lines changed: 12 additions & 8 deletions

Original file line number	Diff line number	Diff line change
`@@ -575,15 +575,19 @@ set_rel_consider_parallel(PlannerInfo root, RelOptInfo rel,`
`575`	`575`	`caseRTE_SUBQUERY:`
`576`	`576`
`577`	`577`	`/*`
`578`		`- * Subplans currently aren't passed to workers. Even if they`
`579`		`- * were, the subplan might be using parallelism internally, and we`
`580`		`- * can't support nested Gather nodes at present. Finally, we`
`581`		`- * don't have a good way of knowing whether the subplan involves`
`582`		`- * any parallel-restricted operations. It would be nice to relax`
`583`		`- * this restriction some day, but it's going to take a fair amount`
`584`		`- * of work.`
	`578`	`+ * There's no intrinsic problem with scanning a subquery-in-FROM`
	`579`	`+ * (as distinct from a SubPlan or InitPlan) in a parallel worker.`
	`580`	`+ * If the subquery doesn't happen to have any parallel-safe paths,`
	`581`	`+ * then flagging it as consider_parallel won't change anything,`
	`582`	`+ * but that's true for plain tables, too. We must set`
	`583`	`+ * consider_parallel based on the rel's own quals and targetlist,`
	`584`	`+ * so that if a subquery path is parallel-safe but the quals and`
	`585`	`+ * projection we're sticking onto it are not, we correctly mark`
	`586`	`+ * the SubqueryScanPath as not parallel-safe. (Note that`
	`587`	`+ * set_subquery_pathlist() might push some of these quals down`
	`588`	`+ * into the subquery itself, but that doesn't change anything.)`
`585`	`589`	`*/`
`586`		`-return;`
	`590`	`+break;`
`587`	`591`
`588`	`592`	`caseRTE_JOIN:`
`589`	`593`	`/* Shouldn't happen; we're only considering baserels here. */`

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Commit110a6db

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`‎src/backend/optimizer/path/allpaths.c`

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