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postgres_fdw: Add ORDER BY to some remote SQL queries.

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If the join problem's entire ORDER BY clause can be pushed to the
remote server, consider a path that adds this ORDER BY clause.  If
use_remote_estimate is on, we cost this path using an additional
remote EXPLAIN.  If not, we just estimate that the path costs 20%
more, which is intended to be large enough that we won't request a
remote sort when it's not helpful, but small enough that we'll have
the remote side do the sort when in doubt.  In some cases, the remote
sort might actually be free, because the remote query plan might
happen to produce output that is ordered the way we need, but without
remote estimates we have no way of knowing that.

It might also be useful to request sorted output from the remote side
if it enables an efficient merge join, but this patch doesn't attempt
to handle that case.

Ashutosh Bapat with revisions by me.  Also reviewed by Fabrízio de Royes
Mello and Jeevan Chalke.
This commit is contained in:
Robert Haas
2015-11-03 12:46:06 -05:00
parent fc0b893521
commit f18c944b61
5 changed files with 222 additions and 30 deletions
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13
contrib/postgres_fdw/sql/postgres_fdw.sql
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@ -138,11 +138,12 @@ ALTER FOREIGN TABLE ft2 OPTIONS (use_remote_estimate 'true');
-- ===================================================================
-- simple queries
-- ===================================================================
-- single table, with/without alias
-- single table without alias
EXPLAIN (COSTS false) SELECT * FROM ft1 ORDER BY c3, c1 OFFSET 100 LIMIT 10;
SELECT * FROM ft1 ORDER BY c3, c1 OFFSET 100 LIMIT 10;
EXPLAIN (VERBOSE, COSTS false) SELECT * FROM ft1 t1 ORDER BY t1.c3, t1.c1 OFFSET 100 LIMIT 10;
SELECT * FROM ft1 t1 ORDER BY t1.c3, t1.c1 OFFSET 100 LIMIT 10;
-- single table with alias - also test that tableoid sort is not pushed to remote side
EXPLAIN (VERBOSE, COSTS false) SELECT * FROM ft1 t1 ORDER BY t1.c3, t1.c1, t1.tableoid OFFSET 100 LIMIT 10;
SELECT * FROM ft1 t1 ORDER BY t1.c3, t1.c1, t1.tableoid OFFSET 100 LIMIT 10;
-- whole-row reference
EXPLAIN (VERBOSE, COSTS false) SELECT t1 FROM ft1 t1 ORDER BY t1.c3, t1.c1 OFFSET 100 LIMIT 10;
SELECT t1 FROM ft1 t1 ORDER BY t1.c3, t1.c1 OFFSET 100 LIMIT 10;
@ -214,6 +215,12 @@ WHERE a.c2 = 6 AND b.c1 = a.c1 AND a.c8 = 'foo' AND b.c7 = upper(a.c7);
-- bug before 9.3.5 due to sloppy handling of remote-estimate parameters
SELECT * FROM ft1 WHERE c1 = ANY (ARRAY(SELECT c1 FROM ft2 WHERE c1 < 5));
SELECT * FROM ft2 WHERE c1 = ANY (ARRAY(SELECT c1 FROM ft1 WHERE c1 < 5));
-- we should not push order by clause with volatile expressions or unsafe
-- collations
EXPLAIN (VERBOSE, COSTS false)
SELECT * FROM ft2 ORDER BY ft2.c1, random();
EXPLAIN (VERBOSE, COSTS false)
SELECT * FROM ft2 ORDER BY ft2.c1, ft2.c3 collate "C";
-- ===================================================================
-- parameterized queries