From ccd8f97922944566d26c7d90eb67ab7848ee9905 Mon Sep 17 00:00:00 2001 From: Robert Haas Date: Tue, 22 Dec 2015 13:46:40 -0500 Subject: postgres_fdw: Consider requesting sorted data so we can do a merge join. When use_remote_estimate is enabled, consider adding ORDER BY to the query we sending to the remote server so that we can use that ordered data for a merge join. Commit f18c944b6137329ac4a6b2dce5745c5dc21a8578 arranges to push down the query pathkeys, which seems like the case mostly likely to be a win, but testing shows this can sometimes win, too. For a regular table, we know which indexes are present and therefore test whether the ordering provided by each such index is useful. Here, we take the opposite approach: guess what orderings would be useful if they could be generated cheaply, and then ask the remote side what those will cost. Ashutosh Bapat, with very substantial cosmetic revisions by me. Also reviewed by Rushabh Lathia. --- contrib/postgres_fdw/sql/postgres_fdw.sql | 40 +++++++++++++++++++++++++++++++ 1 file changed, 40 insertions(+) (limited to 'contrib/postgres_fdw/sql') diff --git a/contrib/postgres_fdw/sql/postgres_fdw.sql b/contrib/postgres_fdw/sql/postgres_fdw.sql index 671e38ceaa3..73fa9f6de09 100644 --- a/contrib/postgres_fdw/sql/postgres_fdw.sql +++ b/contrib/postgres_fdw/sql/postgres_fdw.sql @@ -178,6 +178,20 @@ SELECT * FROM ft1 t1 WHERE t1.c3 = (SELECT MAX(c3) FROM ft2 t2) ORDER BY c1; WITH t1 AS (SELECT * FROM ft1 WHERE c1 <= 10) SELECT t2.c1, t2.c2, t2.c3, t2.c4 FROM t1, ft2 t2 WHERE t1.c1 = t2.c1 ORDER BY t1.c1; -- fixed values SELECT 'fixed', NULL FROM ft1 t1 WHERE c1 = 1; +-- Test forcing the remote server to produce sorted data for a merge join. +SET enable_hashjoin TO false; +SET enable_nestloop TO false; +-- inner join; expressions in the clauses appear in the equivalence class list +EXPLAIN (VERBOSE, COSTS false) + SELECT t1.c1, t2."C 1" FROM ft2 t1 JOIN "S 1"."T 1" t2 ON (t1.c1 = t2."C 1") OFFSET 100 LIMIT 10; +SELECT t1.c1, t2."C 1" FROM ft2 t1 JOIN "S 1"."T 1" t2 ON (t1.c1 = t2."C 1") OFFSET 100 LIMIT 10; +-- outer join; expressions in the clauses do not appear in equivalence class +-- list but no output change as compared to the previous query +EXPLAIN (VERBOSE, COSTS false) + SELECT t1.c1, t2."C 1" FROM ft2 t1 LEFT JOIN "S 1"."T 1" t2 ON (t1.c1 = t2."C 1") OFFSET 100 LIMIT 10; +SELECT t1.c1, t2."C 1" FROM ft2 t1 LEFT JOIN "S 1"."T 1" t2 ON (t1.c1 = t2."C 1") OFFSET 100 LIMIT 10; +RESET enable_hashjoin; +RESET enable_nestloop; -- =================================================================== -- WHERE with remotely-executable conditions @@ -812,6 +826,32 @@ where bar.f1 = ss.f1; select tableoid::regclass, * from bar order by 1,2; +-- Test forcing the remote server to produce sorted data for a merge join, +-- but the foreign table is an inheritance child. +truncate table loct1; +truncate table only foo; +\set num_rows_foo 2000 +insert into loct1 select generate_series(0, :num_rows_foo, 2), generate_series(0, :num_rows_foo, 2), generate_series(0, :num_rows_foo, 2); +insert into foo select generate_series(1, :num_rows_foo, 2), generate_series(1, :num_rows_foo, 2); +SET enable_hashjoin to false; +SET enable_nestloop to false; +alter foreign table foo2 options (use_remote_estimate 'true'); +create index i_loct1_f1 on loct1(f1); +create index i_foo_f1 on foo(f1); +analyze foo; +analyze loct1; +-- inner join; expressions in the clauses appear in the equivalence class list +explain (verbose, costs off) + select foo.f1, loct1.f1 from foo join loct1 on (foo.f1 = loct1.f1) order by foo.f2 offset 10 limit 10; +select foo.f1, loct1.f1 from foo join loct1 on (foo.f1 = loct1.f1) order by foo.f2 offset 10 limit 10; +-- outer join; expressions in the clauses do not appear in equivalence class +-- list but no output change as compared to the previous query +explain (verbose, costs off) + select foo.f1, loct1.f1 from foo left join loct1 on (foo.f1 = loct1.f1) order by foo.f2 offset 10 limit 10; +select foo.f1, loct1.f1 from foo left join loct1 on (foo.f1 = loct1.f1) order by foo.f2 offset 10 limit 10; +RESET enable_hashjoin; +RESET enable_nestloop; + -- Test that WHERE CURRENT OF is not supported begin; declare c cursor for select * from bar where f1 = 7; -- cgit v1.2.3