From 2b461efc5195e9b359dcf0ac98487bdfcbd1347f Mon Sep 17 00:00:00 2001 From: Etsuro Fujita Date: Fri, 7 Jun 2024 17:45:08 +0900 Subject: [PATCH] postgres_fdw: Refuse to send FETCH FIRST WITH TIES to remote servers. Previously, when considering LIMIT pushdown, postgres_fdw failed to check whether the query has this clause, which led to pushing false LIMIT clauses, causing incorrect results. This clause has been supported since v13, so we need to do a remote-version check before deciding that it will be safe to push such a clause, but we do not currently have a way to do the check (without accessing the remote server); disable pushing such a clause for now. Oversight in commit 357889eb1. Back-patch to v13, where that commit added the support. Per bug #18467 from Onder Kalaci. Patch by Japin Li, per a suggestion from Tom Lane, with some changes to the comments by me. Review by Onder Kalaci, Alvaro Herrera, and me. Discussion: https://postgr.es/m/18467-7bb89084ff03a08d%40postgresql.org --- .../postgres_fdw/expected/postgres_fdw.out | 21 +++++++++++++++++++ contrib/postgres_fdw/postgres_fdw.c | 14 +++++++++++++ contrib/postgres_fdw/sql/postgres_fdw.sql | 5 +++++ 3 files changed, 40 insertions(+) diff --git a/contrib/postgres_fdw/expected/postgres_fdw.out b/contrib/postgres_fdw/expected/postgres_fdw.out index cb3e066590..9f7d3441cf 100644 --- a/contrib/postgres_fdw/expected/postgres_fdw.out +++ b/contrib/postgres_fdw/expected/postgres_fdw.out @@ -1038,6 +1038,27 @@ SELECT * FROM ft1 t1 WHERE t1.c1 === t1.c2 order by t1.c2 limit 1; 1 | 1 | 00001 | Fri Jan 02 00:00:00 1970 PST | Fri Jan 02 00:00:00 1970 | 1 | 1 | foo (1 row) +-- Ensure we don't ship FETCH FIRST .. WITH TIES +EXPLAIN (VERBOSE, COSTS OFF) +SELECT t1.c2 FROM ft1 t1 WHERE t1.c1 > 960 ORDER BY t1.c2 FETCH FIRST 2 ROWS WITH TIES; + QUERY PLAN +------------------------------------------------------------------------------------------------- + Limit + Output: c2 + -> Foreign Scan on public.ft1 t1 + Output: c2 + Remote SQL: SELECT c2 FROM "S 1"."T 1" WHERE (("C 1" > 960)) ORDER BY c2 ASC NULLS LAST +(5 rows) + +SELECT t1.c2 FROM ft1 t1 WHERE t1.c1 > 960 ORDER BY t1.c2 FETCH FIRST 2 ROWS WITH TIES; + c2 +---- + 0 + 0 + 0 + 0 +(4 rows) + -- check schema-qualification of regconfig constant CREATE TEXT SEARCH CONFIGURATION public.custom_search (COPY = pg_catalog.english); diff --git a/contrib/postgres_fdw/postgres_fdw.c b/contrib/postgres_fdw/postgres_fdw.c index e8bcfec22b..192b42c261 100644 --- a/contrib/postgres_fdw/postgres_fdw.c +++ b/contrib/postgres_fdw/postgres_fdw.c @@ -6338,6 +6338,20 @@ add_foreign_final_paths(PlannerInfo *root, RelOptInfo *input_rel, if (ifpinfo->local_conds) return; + /* + * If the query has FETCH FIRST .. WITH TIES, 1) it must have ORDER BY as + * well, which is used to determine which additional rows tie for the last + * place in the result set, and 2) ORDER BY must already have been + * determined to be safe to push down before we get here. So in that case + * the FETCH clause is safe to push down with ORDER BY if the remote + * server is v13 or later, but if not, the remote query will fail entirely + * for lack of support for it. Since we do not currently have a way to do + * a remote-version check (without accessing the remote server), disable + * pushing the FETCH clause for now. + */ + if (parse->limitOption == LIMIT_OPTION_WITH_TIES) + return; + /* * Also, the LIMIT/OFFSET cannot be pushed down, if their expressions are * not safe to remote. diff --git a/contrib/postgres_fdw/sql/postgres_fdw.sql b/contrib/postgres_fdw/sql/postgres_fdw.sql index 0fcf67483d..0e9a558a90 100644 --- a/contrib/postgres_fdw/sql/postgres_fdw.sql +++ b/contrib/postgres_fdw/sql/postgres_fdw.sql @@ -392,6 +392,11 @@ EXPLAIN (VERBOSE, COSTS OFF) SELECT * FROM ft1 t1 WHERE t1.c1 === t1.c2 order by t1.c2 limit 1; SELECT * FROM ft1 t1 WHERE t1.c1 === t1.c2 order by t1.c2 limit 1; +-- Ensure we don't ship FETCH FIRST .. WITH TIES +EXPLAIN (VERBOSE, COSTS OFF) +SELECT t1.c2 FROM ft1 t1 WHERE t1.c1 > 960 ORDER BY t1.c2 FETCH FIRST 2 ROWS WITH TIES; +SELECT t1.c2 FROM ft1 t1 WHERE t1.c1 > 960 ORDER BY t1.c2 FETCH FIRST 2 ROWS WITH TIES; + -- check schema-qualification of regconfig constant CREATE TEXT SEARCH CONFIGURATION public.custom_search (COPY = pg_catalog.english);