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pg_plan_advice: Don't generate FOREIGN_JOIN advice for a single relation.

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A foreign scan can target a single relation while still reaching the
fs_relids branch of pgpa_build_scan() -- for example, when postgres_fdw
pushes an aggregate down over one foreign table. In that case, no
advice should be emitted.

Author: Mahendra Singh Thalor <mahi6run@gmail.com>
Co-authored-by: Robert Haas <rhaas@postgresql.org>
Discussion: http://postgr.es/m/CAKYtNAofuAJBz6++SeikpCb=Y=MO1QgEuZNJ+KZOP2johF1r4Q@mail.gmail.com
This commit is contained in:
Robert Haas 2026-07-02 15:45:22 -04:00
parent 9ef89fdb61
commit 53e6f51eef
4 changed files with 97 additions and 3 deletions
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4
contrib/pg_plan_advice/Makefile
View file

@ -22,7 +22,9 @@ PGFILEDESC = "pg_plan_advice - help the planner get the right plan"
REGRESS = alternatives gather join_order join_strategy partitionwise \
prepared scan semijoin syntax
EXTRA_INSTALL = contrib/tsm_system_time
TAP_TESTS = 1
EXTRA_INSTALL = contrib/tsm_system_time contrib/postgres_fdw
EXTRA_CLEAN = pgpa_parser.h pgpa_parser.c pgpa_scanner.c

5
contrib/pg_plan_advice/meson.build
View file

@ -64,4 +64,9 @@ tests += {
'syntax',
],
},
'tap': {
'tests': [
't/001_foreign_scan.pl',
],
},
}

8
contrib/pg_plan_advice/pgpa_scan.c
View file

@ -141,9 +141,13 @@ pgpa_build_scan(pgpa_plan_walker_context *walker, Plan *plan,
* If multiple relations are being targeted by a single
* foreign scan, then the foreign join has been pushed to the
* remote side, and we want that to be reflected in the
* generated advice.
* generated advice. We can't emit FOREIGN_JOIN() advice for
* a single relation, so treat that case as an ordinary scan.
*/
strategy = PGPA_SCAN_FOREIGN;
if (bms_membership(relids) == BMS_MULTIPLE)
strategy = PGPA_SCAN_FOREIGN;
else
strategy = PGPA_SCAN_ORDINARY;
break;
case T_Append:

83
contrib/pg_plan_advice/t/001_foreign_scan.pl Normal file
View file

@ -0,0 +1,83 @@
# Copyright (c) 2021-2026, PostgreSQL Global Development Group
# Verify plan advice for foreign scans.
use strict;
use warnings FATAL => 'all';
use PostgreSQL::Test::Cluster;
use PostgreSQL::Test::Utils;
use Test::More;
my $node = PostgreSQL::Test::Cluster->new('node');
$node->init;
$node->append_conf('postgresql.conf',
"session_preload_libraries = 'pg_plan_advice'");
$node->start;
my $host = $node->host;
my $port = $node->port;
$node->safe_psql(
'postgres', qq{
CREATE EXTENSION postgres_fdw;
CREATE SERVER loopback FOREIGN DATA WRAPPER postgres_fdw
OPTIONS (host '$host', port '$port', dbname 'postgres');
CREATE USER MAPPING FOR CURRENT_USER SERVER loopback;
CREATE TABLE base_tab (a int);
CREATE TABLE base_tab2 (a int);
CREATE FOREIGN TABLE ftab (a int)
SERVER loopback OPTIONS (table_name 'base_tab');
CREATE FOREIGN TABLE ftab2 (a int)
SERVER loopback OPTIONS (table_name 'base_tab2');
});
# Get the generated advice from EXPLAIN output.
sub extract_generated_advice
{
my ($explain) = @_;
my $generated_advice = '';
my $collecting = 0;
foreach my $line (split /\n/, $explain)
{
if ($line =~ /Generated Plan Advice:/)
{
$collecting = 1;
next;
}
if ($collecting)
{
$line =~ s/^\s+//;
$line =~ s/\s+$//;
$generated_advice .= ' ' if $generated_advice ne '';
$generated_advice .= $line;
}
}
return $generated_advice;
}
# A pushed-down aggregate over a single foreign table yields a ForeignScan
# that names exactly one relation. No FOREIGN_JOIN advice should be generated.
my $agg_explain = $node->safe_psql('postgres',
"EXPLAIN (COSTS OFF, PLAN_ADVICE) SELECT count(*) FROM ftab;");
my $agg_pat = qr/\QRelations: Aggregate on (ftab)\E/;
like($agg_explain, $agg_pat, 'single-table aggregate is pushed down');
my $agg_advice = extract_generated_advice($agg_explain);
is($agg_advice, 'NO_GATHER(ftab)', 'advice for single-table aggregate');
# A foreign join should generate FOREIGN_JOIN advice. Here we force this by
# disabling local join methods.
my $join_explain = $node->safe_psql(
'postgres', q{
SET enable_mergejoin = off;
SET enable_hashjoin = off;
SET enable_nestloop = off;
EXPLAIN (COSTS OFF, PLAN_ADVICE) SELECT * FROM ftab JOIN ftab2 USING (a);
});
my $ja_expected = 'FOREIGN_JOIN((ftab ftab2)) NO_GATHER(ftab ftab2)';
my $ja_actual = extract_generated_advice($join_explain);
is($ja_actual, $ja_expected, 'advice for foreign join');
$node->stop;
done_testing();