From 6bb3abcfe33baab3ce53543be4e4d1151a047a46 Mon Sep 17 00:00:00 2001 From: Ziming Zhang Date: Sat, 19 Sep 2026 13:39:12 +0800 Subject: [PATCH v2] postgres_fdw: Fix cost estimation for semi join pushdown. In estimate_path_cost_size(), the number of rows on which the quals pushed down past a join are evaluated was estimated as the cross product of the two sides' row counts scaled by joinclause_sel. That is sensible as long as joinclause_sel is a probability per cross-product row pair, as it is for inner and outer joins. For a semi join, however, clauselist_selectivity() returns the fraction of the outer side's rows that have a match in the inner side, i.e. a probability per outer row, so the remote quals were charged on outer_rows * inner_rows * P(match) rows instead of the semi join's actual result size, outer_rows * P(match) inflating that cost term by roughly a factor of the inner side's row count and biasing the planner against pushing semi joins down (and against plans built on top of a pushed-down semi join, since the inflated total cost propagates to parent relations). Derive the join's output size from the outer side's row count for JOIN_SEMI instead. This mirrors how foreign_join_ok() already passes JOIN_INNER rather than the actual jointype when costing local_conds: conditions applied after a semi join are evaluated on the join's output rows, like quals in a WHERE clause. Add a regression test using a low-cardinality join key and a few pushed-down quals on the outer side: at default settings the planner chooses a local semi join without the fix and the pushed-down Foreign Scan with it. --- .../postgres_fdw/expected/postgres_fdw.out | 39 +++++++++++++++++++ contrib/postgres_fdw/postgres_fdw.c | 12 +++++- contrib/postgres_fdw/sql/postgres_fdw.sql | 17 ++++++++ 3 files changed, 67 insertions(+), 1 deletion(-) diff --git a/contrib/postgres_fdw/expected/postgres_fdw.out b/contrib/postgres_fdw/expected/postgres_fdw.out index a6295674daf..6bc10b56d18 100644 --- a/contrib/postgres_fdw/expected/postgres_fdw.out +++ b/contrib/postgres_fdw/expected/postgres_fdw.out @@ -2258,6 +2258,45 @@ SELECT t1.c1 FROM ft1 t1 WHERE NOT EXISTS (SELECT 1 FROM ft2 t2 WHERE t1.c1 = t2 119 (10 rows) +-- semi join pushdown must also win the cost comparison when there are +-- enough other pushed-down quals +CREATE TABLE "S 1".semi_cost_t1 (a int, b int); +INSERT INTO "S 1".semi_cost_t1 SELECT i, i % 100 FROM generate_series(1, 200) i; +CREATE TABLE "S 1".semi_cost_t2 (b int); +INSERT INTO "S 1".semi_cost_t2 SELECT i FROM generate_series(0, 99) i; +CREATE FOREIGN TABLE ft_semi_cost_t1 (a int, b int) SERVER loopback OPTIONS (schema_name 'S 1', table_name 'semi_cost_t1'); +CREATE FOREIGN TABLE ft_semi_cost_t2 (b int) SERVER loopback OPTIONS (schema_name 'S 1', table_name 'semi_cost_t2'); +ANALYZE ft_semi_cost_t1; +ANALYZE ft_semi_cost_t2; +EXPLAIN (VERBOSE, COSTS OFF) +SELECT t1.a FROM ft_semi_cost_t1 t1 WHERE t1.a >= 0 AND t1.a >= -1 AND t1.a >= -2 AND t1.a >= -3 AND t1.a >= -4 AND EXISTS (SELECT 1 FROM ft_semi_cost_t2 t2 WHERE t2.b = t1.b) ORDER BY t1.a OFFSET 190 LIMIT 10; + QUERY PLAN +--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- + Foreign Scan + Output: t1.a + Relations: (public.ft_semi_cost_t1 t1) SEMI JOIN (public.ft_semi_cost_t2 t2) + Remote SQL: SELECT r1.a FROM "S 1".semi_cost_t1 r1 WHERE ((r1.a >= 0)) AND ((r1.a >= (-1))) AND ((r1.a >= (-2))) AND ((r1.a >= (-3))) AND ((r1.a >= (-4))) AND EXISTS (SELECT NULL FROM "S 1".semi_cost_t2 r2 WHERE ((r1.b = r2.b))) ORDER BY r1.a ASC NULLS LAST LIMIT 10::bigint OFFSET 190::bigint +(4 rows) + +SELECT t1.a FROM ft_semi_cost_t1 t1 WHERE t1.a >= 0 AND t1.a >= -1 AND t1.a >= -2 AND t1.a >= -3 AND t1.a >= -4 AND EXISTS (SELECT 1 FROM ft_semi_cost_t2 t2 WHERE t2.b = t1.b) ORDER BY t1.a OFFSET 190 LIMIT 10; + a +----- + 191 + 192 + 193 + 194 + 195 + 196 + 197 + 198 + 199 + 200 +(10 rows) + +DROP FOREIGN TABLE ft_semi_cost_t1; +DROP FOREIGN TABLE ft_semi_cost_t2; +DROP TABLE "S 1".semi_cost_t1; +DROP TABLE "S 1".semi_cost_t2; -- CROSS JOIN can be pushed down EXPLAIN (VERBOSE, COSTS OFF) SELECT t1.c1, t2.c1 FROM ft1 t1 CROSS JOIN ft2 t2 ORDER BY t1.c1, t2.c1 OFFSET 100 LIMIT 10; diff --git a/contrib/postgres_fdw/postgres_fdw.c b/contrib/postgres_fdw/postgres_fdw.c index c731ee199e2..dbde9f1a7b5 100644 --- a/contrib/postgres_fdw/postgres_fdw.c +++ b/contrib/postgres_fdw/postgres_fdw.c @@ -3640,7 +3640,17 @@ estimate_path_cost_size(PlannerInfo *root, run_cost = fpinfo_i->rel_total_cost - fpinfo_i->rel_startup_cost; run_cost += fpinfo_o->rel_total_cost - fpinfo_o->rel_startup_cost; run_cost += nrows * join_cost.per_tuple; - nrows = clamp_row_est(nrows * fpinfo->joinclause_sel); + /* + * For JOIN_SEMI, joinclause_sel is the fraction of the outer + * side's rows that have matches in the inner side (see + * foreign_join_ok), not a probability per cross-product row, so + * derive the join's output size from the outer row count. + */ + if (fpinfo->jointype == JOIN_SEMI) + nrows = fpinfo_o->rows * fpinfo->joinclause_sel; + else + nrows *= fpinfo->joinclause_sel; + nrows = clamp_row_est(nrows); run_cost += nrows * remote_conds_cost.per_tuple; run_cost += fpinfo->local_conds_cost.per_tuple * retrieved_rows; diff --git a/contrib/postgres_fdw/sql/postgres_fdw.sql b/contrib/postgres_fdw/sql/postgres_fdw.sql index eaeb90485e8..3ec0c5e9b03 100644 --- a/contrib/postgres_fdw/sql/postgres_fdw.sql +++ b/contrib/postgres_fdw/sql/postgres_fdw.sql @@ -711,6 +711,23 @@ SELECT t1.c1 FROM ft1 t1 WHERE EXISTS (SELECT 1 FROM ft2 t2 WHERE t1.c1 = t2.c1) EXPLAIN (VERBOSE, COSTS OFF) SELECT t1.c1 FROM ft1 t1 WHERE NOT EXISTS (SELECT 1 FROM ft2 t2 WHERE t1.c1 = t2.c2) ORDER BY t1.c1 OFFSET 100 LIMIT 10; SELECT t1.c1 FROM ft1 t1 WHERE NOT EXISTS (SELECT 1 FROM ft2 t2 WHERE t1.c1 = t2.c2) ORDER BY t1.c1 OFFSET 100 LIMIT 10; +-- semi join pushdown must also win the cost comparison when there are +-- enough other pushed-down quals +CREATE TABLE "S 1".semi_cost_t1 (a int, b int); +INSERT INTO "S 1".semi_cost_t1 SELECT i, i % 100 FROM generate_series(1, 200) i; +CREATE TABLE "S 1".semi_cost_t2 (b int); +INSERT INTO "S 1".semi_cost_t2 SELECT i FROM generate_series(0, 99) i; +CREATE FOREIGN TABLE ft_semi_cost_t1 (a int, b int) SERVER loopback OPTIONS (schema_name 'S 1', table_name 'semi_cost_t1'); +CREATE FOREIGN TABLE ft_semi_cost_t2 (b int) SERVER loopback OPTIONS (schema_name 'S 1', table_name 'semi_cost_t2'); +ANALYZE ft_semi_cost_t1; +ANALYZE ft_semi_cost_t2; +EXPLAIN (VERBOSE, COSTS OFF) +SELECT t1.a FROM ft_semi_cost_t1 t1 WHERE t1.a >= 0 AND t1.a >= -1 AND t1.a >= -2 AND t1.a >= -3 AND t1.a >= -4 AND EXISTS (SELECT 1 FROM ft_semi_cost_t2 t2 WHERE t2.b = t1.b) ORDER BY t1.a OFFSET 190 LIMIT 10; +SELECT t1.a FROM ft_semi_cost_t1 t1 WHERE t1.a >= 0 AND t1.a >= -1 AND t1.a >= -2 AND t1.a >= -3 AND t1.a >= -4 AND EXISTS (SELECT 1 FROM ft_semi_cost_t2 t2 WHERE t2.b = t1.b) ORDER BY t1.a OFFSET 190 LIMIT 10; +DROP FOREIGN TABLE ft_semi_cost_t1; +DROP FOREIGN TABLE ft_semi_cost_t2; +DROP TABLE "S 1".semi_cost_t1; +DROP TABLE "S 1".semi_cost_t2; -- CROSS JOIN can be pushed down EXPLAIN (VERBOSE, COSTS OFF) SELECT t1.c1, t2.c1 FROM ft1 t1 CROSS JOIN ft2 t2 ORDER BY t1.c1, t2.c1 OFFSET 100 LIMIT 10; -- 2.34.1