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Commit8405d5a

Browse files
author
Etsuro Fujita
committed
postgres_fdw: Refuse to send FETCH FIRST WITH TIES to remote servers.
Previously, when considering LIMIT pushdown, postgres_fdw failed tocheck whether the query has this clause, which led to pushing falseLIMIT clauses, causing incorrect results.This clause has been supported since v13, so we need to do aremote-version check before deciding that it will be safe to push such aclause, but we do not currently have a way to do the check (withoutaccessing the remote server); disable pushing such a clause for now.Oversight in commit357889e. Back-patch to v13, where that commitadded the support.Per bug #18467 from Onder Kalaci.Patch by Japin Li, per a suggestion from Tom Lane, with some changes tothe comments by me. Review by Onder Kalaci, Alvaro Herrera, and me.Discussion:https://postgr.es/m/18467-7bb89084ff03a08d%40postgresql.org
1 parentd9ff92c commit8405d5a

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‎contrib/postgres_fdw/expected/postgres_fdw.out‎

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@@ -1047,6 +1047,27 @@ SELECT * FROM ft1 t1 WHERE t1.c1 === t1.c2 order by t1.c2 limit 1;
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1 | 1 | 00001 | Fri Jan 02 00:00:00 1970 PST | Fri Jan 02 00:00:00 1970 | 1 | 1 | foo
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(1 row)
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-- Ensure we don't ship FETCH FIRST .. WITH TIES
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EXPLAIN (VERBOSE, COSTS OFF)
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SELECT t1.c2 FROM ft1 t1 WHERE t1.c1 > 960 ORDER BY t1.c2 FETCH FIRST 2 ROWS WITH TIES;
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QUERY PLAN
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-------------------------------------------------------------------------------------------------
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Limit
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Output: c2
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-> Foreign Scan on public.ft1 t1
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Output: c2
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Remote SQL: SELECT c2 FROM "S 1"."T 1" WHERE (("C 1" > 960)) ORDER BY c2 ASC NULLS LAST
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(5 rows)
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SELECT t1.c2 FROM ft1 t1 WHERE t1.c1 > 960 ORDER BY t1.c2 FETCH FIRST 2 ROWS WITH TIES;
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c2
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----
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0
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0
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0
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0
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(4 rows)
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-- Test CASE pushdown
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EXPLAIN (VERBOSE, COSTS OFF)
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SELECT c1,c2,c3 FROM ft2 WHERE CASE WHEN c1 > 990 THEN c1 END < 1000 ORDER BY c1;

‎contrib/postgres_fdw/postgres_fdw.c‎

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Original file line numberDiff line numberDiff line change
@@ -7045,6 +7045,20 @@ add_foreign_final_paths(PlannerInfo *root, RelOptInfo *input_rel,
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if (ifpinfo->local_conds)
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return;
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/*
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* If the query has FETCH FIRST .. WITH TIES, 1) it must have ORDER BY as
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* well, which is used to determine which additional rows tie for the last
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* place in the result set, and 2) ORDER BY must already have been
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* determined to be safe to push down before we get here. So in that case
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* the FETCH clause is safe to push down with ORDER BY if the remote
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* server is v13 or later, but if not, the remote query will fail entirely
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* for lack of support for it. Since we do not currently have a way to do
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* a remote-version check (without accessing the remote server), disable
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* pushing the FETCH clause for now.
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*/
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if (parse->limitOption==LIMIT_OPTION_WITH_TIES)
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return;
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/*
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* Also, the LIMIT/OFFSET cannot be pushed down, if their expressions are
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* not safe to remote.

‎contrib/postgres_fdw/sql/postgres_fdw.sql‎

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Original file line numberDiff line numberDiff line change
@@ -408,6 +408,11 @@ EXPLAIN (VERBOSE, COSTS OFF)
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SELECT*FROM ft1 t1WHEREt1.c1===t1.c2order byt1.c2limit1;
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SELECT*FROM ft1 t1WHEREt1.c1===t1.c2order byt1.c2limit1;
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-- Ensure we don't ship FETCH FIRST .. WITH TIES
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EXPLAIN (VERBOSE, COSTS OFF)
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SELECTt1.c2FROM ft1 t1WHEREt1.c1>960ORDER BYt1.c2 FETCH FIRST2 ROWS WITH TIES;
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SELECTt1.c2FROM ft1 t1WHEREt1.c1>960ORDER BYt1.c2 FETCH FIRST2 ROWS WITH TIES;
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-- Test CASE pushdown
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EXPLAIN (VERBOSE, COSTS OFF)
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SELECT c1,c2,c3FROM ft2WHERE CASE WHEN c1>990 THEN c1 END<1000ORDER BY c1;

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