From 931766aaec58b2ce09c82203456877e0b05e1271 Mon Sep 17 00:00:00 2001 From: Tom Lane Date: Thu, 3 Jul 2025 17:39:53 -0400 Subject: Simplify COALESCE() with one surviving argument. If, after removal of useless null-constant arguments, a CoalesceExpr has exactly one remaining argument, we can just take that argument as the result, without bothering to wrap a new CoalesceExpr around it. This isn't likely to produce any great improvement in runtime per se, but it can lead to better plans since the planner no longer has to treat the expression as non-strict. However, there were a few regression test cases that intentionally wrote COALESCE(x) as a shorthand way of creating a non-strict subexpression. To avoid ruining the intent of those tests, write COALESCE(x,x) instead. (If anyone ever proposes de-duplicating COALESCE arguments, we'll need another iteration of this arms race. But it seems pretty unlikely that such an optimization would be worthwhile.) Author: Maksim Milyutin Reviewed-by: Tom Lane Discussion: https://postgr.es/m/8e8573c3-1411-448d-877e-53258b7b2be0@tantorlabs.ru --- src/backend/optimizer/util/clauses.c | 7 +++++++ 1 file changed, 7 insertions(+) (limited to 'src/backend') diff --git a/src/backend/optimizer/util/clauses.c b/src/backend/optimizer/util/clauses.c index 26a3e050086..f45131c34c5 100644 --- a/src/backend/optimizer/util/clauses.c +++ b/src/backend/optimizer/util/clauses.c @@ -3333,6 +3333,13 @@ eval_const_expressions_mutator(Node *node, -1, coalesceexpr->coalescecollid); + /* + * If there's exactly one surviving argument, we no longer + * need COALESCE at all: the result is that argument + */ + if (list_length(newargs) == 1) + return (Node *) linitial(newargs); + newcoalesce = makeNode(CoalesceExpr); newcoalesce->coalescetype = coalesceexpr->coalescetype; newcoalesce->coalescecollid = coalesceexpr->coalescecollid; -- cgit v1.2.3