From: Tom Lane Date: Thu, 3 Jul 2025 21:39:53 +0000 (-0400) Subject: Simplify COALESCE() with one surviving argument. X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=931766aaec58b2ce09c82203456877e0b05e1271;p=thirdparty%2Fpostgresql.git 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 --- 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; diff --git a/src/test/regress/expected/join.out b/src/test/regress/expected/join.out index 390aabfb34b..46ddfa844c5 100644 --- a/src/test/regress/expected/join.out +++ b/src/test/regress/expected/join.out @@ -5626,14 +5626,14 @@ select * from (select 1 as id) as xx left join (tenk1 as a1 full join (select 1 as id) as yy on (a1.unique1 = yy.id)) - on (xx.id = coalesce(yy.id)); - QUERY PLAN ---------------------------------------- + on (xx.id = coalesce(yy.id, yy.id)); + QUERY PLAN +------------------------------------------ Nested Loop Left Join -> Result -> Hash Full Join Hash Cond: (a1.unique1 = (1)) - Filter: (1 = COALESCE((1))) + Filter: (1 = COALESCE((1), (1))) -> Seq Scan on tenk1 a1 -> Hash -> Result @@ -5643,7 +5643,7 @@ select * from (select 1 as id) as xx left join (tenk1 as a1 full join (select 1 as id) as yy on (a1.unique1 = yy.id)) - on (xx.id = coalesce(yy.id)); + on (xx.id = coalesce(yy.id, yy.id)); id | unique1 | unique2 | two | four | ten | twenty | hundred | thousand | twothousand | fivethous | tenthous | odd | even | stringu1 | stringu2 | string4 | id ----+---------+---------+-----+------+-----+--------+---------+----------+-------------+-----------+----------+-----+------+----------+----------+---------+---- 1 | 1 | 2838 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 2 | 3 | BAAAAA | EFEAAA | OOOOxx | 1 @@ -8411,20 +8411,20 @@ select * from int4_tbl i left join explain (verbose, costs off) select * from int4_tbl i left join - lateral (select coalesce(i) from int2_tbl j where i.f1 = j.f1) k on true; - QUERY PLAN -------------------------------------- + lateral (select coalesce(i, i) from int2_tbl j where i.f1 = j.f1) k on true; + QUERY PLAN +------------------------------------------ Nested Loop Left Join - Output: i.f1, (COALESCE(i.*)) + Output: i.f1, (COALESCE(i.*, i.*)) -> Seq Scan on public.int4_tbl i Output: i.f1, i.* -> Seq Scan on public.int2_tbl j - Output: j.f1, COALESCE(i.*) + Output: j.f1, COALESCE(i.*, i.*) Filter: (i.f1 = j.f1) (7 rows) select * from int4_tbl i left join - lateral (select coalesce(i) from int2_tbl j where i.f1 = j.f1) k on true; + lateral (select coalesce(i, i) from int2_tbl j where i.f1 = j.f1) k on true; f1 | coalesce -------------+---------- 0 | (0) @@ -9593,14 +9593,14 @@ CREATE STATISTICS group_tbl_stat (ndistinct) ON a, b FROM group_tbl; ANALYZE group_tbl; EXPLAIN (COSTS OFF) SELECT 1 FROM group_tbl t1 - LEFT JOIN (SELECT a c1, COALESCE(a) c2 FROM group_tbl t2) s ON TRUE + LEFT JOIN (SELECT a c1, COALESCE(a, a) c2 FROM group_tbl t2) s ON TRUE GROUP BY s.c1, s.c2; - QUERY PLAN --------------------------------------------- + QUERY PLAN +------------------------------------------------ Group - Group Key: t2.a, (COALESCE(t2.a)) + Group Key: t2.a, (COALESCE(t2.a, t2.a)) -> Sort - Sort Key: t2.a, (COALESCE(t2.a)) + Sort Key: t2.a, (COALESCE(t2.a, t2.a)) -> Nested Loop Left Join -> Seq Scan on group_tbl t1 -> Seq Scan on group_tbl t2 diff --git a/src/test/regress/expected/subselect.out b/src/test/regress/expected/subselect.out index 40d8056fcea..18fed63e738 100644 --- a/src/test/regress/expected/subselect.out +++ b/src/test/regress/expected/subselect.out @@ -2127,30 +2127,30 @@ explain (verbose, costs off) select ss2.* from int8_tbl t1 left join (int8_tbl t2 left join - (select coalesce(q1) as x, * from int8_tbl t3) ss1 on t2.q1 = ss1.q2 inner join + (select coalesce(q1, q1) as x, * from int8_tbl t3) ss1 on t2.q1 = ss1.q2 inner join lateral (select ss1.x as y, * from int8_tbl t4) ss2 on t2.q2 = ss2.q1) on t1.q2 = ss2.q1 order by 1, 2, 3; - QUERY PLAN ----------------------------------------------------------------- + QUERY PLAN +----------------------------------------------------------------------- Sort - Output: (COALESCE(t3.q1)), t4.q1, t4.q2 - Sort Key: (COALESCE(t3.q1)), t4.q1, t4.q2 + Output: (COALESCE(t3.q1, t3.q1)), t4.q1, t4.q2 + Sort Key: (COALESCE(t3.q1, t3.q1)), t4.q1, t4.q2 -> Hash Right Join - Output: (COALESCE(t3.q1)), t4.q1, t4.q2 + Output: (COALESCE(t3.q1, t3.q1)), t4.q1, t4.q2 Hash Cond: (t4.q1 = t1.q2) -> Hash Join - Output: (COALESCE(t3.q1)), t4.q1, t4.q2 + Output: (COALESCE(t3.q1, t3.q1)), t4.q1, t4.q2 Hash Cond: (t2.q2 = t4.q1) -> Hash Left Join - Output: t2.q2, (COALESCE(t3.q1)) + Output: t2.q2, (COALESCE(t3.q1, t3.q1)) Hash Cond: (t2.q1 = t3.q2) -> Seq Scan on public.int8_tbl t2 Output: t2.q1, t2.q2 -> Hash - Output: t3.q2, (COALESCE(t3.q1)) + Output: t3.q2, (COALESCE(t3.q1, t3.q1)) -> Seq Scan on public.int8_tbl t3 - Output: t3.q2, COALESCE(t3.q1) + Output: t3.q2, COALESCE(t3.q1, t3.q1) -> Hash Output: t4.q1, t4.q2 -> Seq Scan on public.int8_tbl t4 @@ -2164,7 +2164,7 @@ order by 1, 2, 3; select ss2.* from int8_tbl t1 left join (int8_tbl t2 left join - (select coalesce(q1) as x, * from int8_tbl t3) ss1 on t2.q1 = ss1.q2 inner join + (select coalesce(q1, q1) as x, * from int8_tbl t3) ss1 on t2.q1 = ss1.q2 inner join lateral (select ss1.x as y, * from int8_tbl t4) ss2 on t2.q2 = ss2.q1) on t1.q2 = ss2.q1 order by 1, 2, 3; @@ -2201,32 +2201,32 @@ explain (verbose, costs off) select ss2.* from int8_tbl t1 left join (int8_tbl t2 left join - (select coalesce(q1) as x, * from int8_tbl t3) ss1 on t2.q1 = ss1.q2 left join + (select coalesce(q1, q1) as x, * from int8_tbl t3) ss1 on t2.q1 = ss1.q2 left join lateral (select ss1.x as y, * from int8_tbl t4) ss2 on t2.q2 = ss2.q1) on t1.q2 = ss2.q1 order by 1, 2, 3; - QUERY PLAN ----------------------------------------------------------------- + QUERY PLAN +----------------------------------------------------------------------- Sort - Output: ((COALESCE(t3.q1))), t4.q1, t4.q2 - Sort Key: ((COALESCE(t3.q1))), t4.q1, t4.q2 + Output: ((COALESCE(t3.q1, t3.q1))), t4.q1, t4.q2 + Sort Key: ((COALESCE(t3.q1, t3.q1))), t4.q1, t4.q2 -> Hash Right Join - Output: ((COALESCE(t3.q1))), t4.q1, t4.q2 + Output: ((COALESCE(t3.q1, t3.q1))), t4.q1, t4.q2 Hash Cond: (t4.q1 = t1.q2) -> Nested Loop - Output: t4.q1, t4.q2, ((COALESCE(t3.q1))) + Output: t4.q1, t4.q2, ((COALESCE(t3.q1, t3.q1))) Join Filter: (t2.q2 = t4.q1) -> Hash Left Join - Output: t2.q2, (COALESCE(t3.q1)) + Output: t2.q2, (COALESCE(t3.q1, t3.q1)) Hash Cond: (t2.q1 = t3.q2) -> Seq Scan on public.int8_tbl t2 Output: t2.q1, t2.q2 -> Hash - Output: t3.q2, (COALESCE(t3.q1)) + Output: t3.q2, (COALESCE(t3.q1, t3.q1)) -> Seq Scan on public.int8_tbl t3 - Output: t3.q2, COALESCE(t3.q1) + Output: t3.q2, COALESCE(t3.q1, t3.q1) -> Seq Scan on public.int8_tbl t4 - Output: t4.q1, t4.q2, (COALESCE(t3.q1)) + Output: t4.q1, t4.q2, (COALESCE(t3.q1, t3.q1)) -> Hash Output: t1.q2 -> Seq Scan on public.int8_tbl t1 @@ -2236,7 +2236,7 @@ order by 1, 2, 3; select ss2.* from int8_tbl t1 left join (int8_tbl t2 left join - (select coalesce(q1) as x, * from int8_tbl t3) ss1 on t2.q1 = ss1.q2 left join + (select coalesce(q1, q1) as x, * from int8_tbl t3) ss1 on t2.q1 = ss1.q2 left join lateral (select ss1.x as y, * from int8_tbl t4) ss2 on t2.q2 = ss2.q1) on t1.q2 = ss2.q1 order by 1, 2, 3; diff --git a/src/test/regress/sql/join.sql b/src/test/regress/sql/join.sql index f6e7070db65..5f0a475894d 100644 --- a/src/test/regress/sql/join.sql +++ b/src/test/regress/sql/join.sql @@ -1977,13 +1977,13 @@ select * from (select 1 as id) as xx left join (tenk1 as a1 full join (select 1 as id) as yy on (a1.unique1 = yy.id)) - on (xx.id = coalesce(yy.id)); + on (xx.id = coalesce(yy.id, yy.id)); select * from (select 1 as id) as xx left join (tenk1 as a1 full join (select 1 as id) as yy on (a1.unique1 = yy.id)) - on (xx.id = coalesce(yy.id)); + on (xx.id = coalesce(yy.id, yy.id)); -- -- test ability to push constants through outer join clauses @@ -3169,9 +3169,9 @@ select * from int4_tbl i left join lateral (select * from int2_tbl j where i.f1 = j.f1) k on true; explain (verbose, costs off) select * from int4_tbl i left join - lateral (select coalesce(i) from int2_tbl j where i.f1 = j.f1) k on true; + lateral (select coalesce(i, i) from int2_tbl j where i.f1 = j.f1) k on true; select * from int4_tbl i left join - lateral (select coalesce(i) from int2_tbl j where i.f1 = j.f1) k on true; + lateral (select coalesce(i, i) from int2_tbl j where i.f1 = j.f1) k on true; explain (verbose, costs off) select * from int4_tbl a, lateral ( @@ -3637,7 +3637,7 @@ ANALYZE group_tbl; EXPLAIN (COSTS OFF) SELECT 1 FROM group_tbl t1 - LEFT JOIN (SELECT a c1, COALESCE(a) c2 FROM group_tbl t2) s ON TRUE + LEFT JOIN (SELECT a c1, COALESCE(a, a) c2 FROM group_tbl t2) s ON TRUE GROUP BY s.c1, s.c2; DROP TABLE group_tbl; diff --git a/src/test/regress/sql/subselect.sql b/src/test/regress/sql/subselect.sql index fec38ef85a6..d9a841fbc9f 100644 --- a/src/test/regress/sql/subselect.sql +++ b/src/test/regress/sql/subselect.sql @@ -1041,7 +1041,7 @@ explain (verbose, costs off) select ss2.* from int8_tbl t1 left join (int8_tbl t2 left join - (select coalesce(q1) as x, * from int8_tbl t3) ss1 on t2.q1 = ss1.q2 inner join + (select coalesce(q1, q1) as x, * from int8_tbl t3) ss1 on t2.q1 = ss1.q2 inner join lateral (select ss1.x as y, * from int8_tbl t4) ss2 on t2.q2 = ss2.q1) on t1.q2 = ss2.q1 order by 1, 2, 3; @@ -1049,7 +1049,7 @@ order by 1, 2, 3; select ss2.* from int8_tbl t1 left join (int8_tbl t2 left join - (select coalesce(q1) as x, * from int8_tbl t3) ss1 on t2.q1 = ss1.q2 inner join + (select coalesce(q1, q1) as x, * from int8_tbl t3) ss1 on t2.q1 = ss1.q2 inner join lateral (select ss1.x as y, * from int8_tbl t4) ss2 on t2.q2 = ss2.q1) on t1.q2 = ss2.q1 order by 1, 2, 3; @@ -1059,7 +1059,7 @@ explain (verbose, costs off) select ss2.* from int8_tbl t1 left join (int8_tbl t2 left join - (select coalesce(q1) as x, * from int8_tbl t3) ss1 on t2.q1 = ss1.q2 left join + (select coalesce(q1, q1) as x, * from int8_tbl t3) ss1 on t2.q1 = ss1.q2 left join lateral (select ss1.x as y, * from int8_tbl t4) ss2 on t2.q2 = ss2.q1) on t1.q2 = ss2.q1 order by 1, 2, 3; @@ -1067,7 +1067,7 @@ order by 1, 2, 3; select ss2.* from int8_tbl t1 left join (int8_tbl t2 left join - (select coalesce(q1) as x, * from int8_tbl t3) ss1 on t2.q1 = ss1.q2 left join + (select coalesce(q1, q1) as x, * from int8_tbl t3) ss1 on t2.q1 = ss1.q2 left join lateral (select ss1.x as y, * from int8_tbl t4) ss2 on t2.q2 = ss2.q1) on t1.q2 = ss2.q1 order by 1, 2, 3;