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

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

diff --git a/contrib/pg_plan_advice/Makefile b/contrib/pg_plan_advice/Makefile
index d016723794dae2a77d96698b040304fbd793b14c..c844846dd54d80ab2e6e6dead2bf6db3e7c9f1b3 100644 (file)
--- a/contrib/pg_plan_advice/Makefile
+++ b/contrib/pg_plan_advice/Makefile
@@ -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
 

diff --git a/contrib/pg_plan_advice/meson.build b/contrib/pg_plan_advice/meson.build
index f2098947b644bf80b0f1f3427f22f5473127a8ea..bbab676be31bd5b1614e0958342e488da9ad78c5 100644 (file)
--- a/contrib/pg_plan_advice/meson.build
+++ b/contrib/pg_plan_advice/meson.build
@@ -64,4 +64,9 @@ tests += {
       'syntax',
     ],
   },
+  'tap': {
+    'tests': [
+      't/001_foreign_scan.pl',
+    ],
+  },
 }

diff --git a/contrib/pg_plan_advice/pgpa_scan.c b/contrib/pg_plan_advice/pgpa_scan.c
index 21b58a0ac42745453029c1c8cb6a8b65e7cfed3d..a7ee09335fd6837e6a8381a9502ae2eaa96b8502 100644 (file)
--- a/contrib/pg_plan_advice/pgpa_scan.c
+++ b/contrib/pg_plan_advice/pgpa_scan.c
@@ -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:
 

diff --git a/contrib/pg_plan_advice/t/001_foreign_scan.pl b/contrib/pg_plan_advice/t/001_foreign_scan.pl
new file mode 100644 (file)
index 0000000..96c7385
--- /dev/null
+++ b/contrib/pg_plan_advice/t/001_foreign_scan.pl
@@ -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();