BRIN autosummarization may need a snapshot

It's possible to define BRIN indexes on functions that require a
snapshot to run, but the autosummarization feature introduced by commit
7526e10224f0 fails to provide one.  This causes autovacuum to leave a
BRIN placeholder tuple behind after a failed work-item execution, making
such indexes less efficient.  Repair by obtaining a snapshot prior to
running the task, and add a test to verify this behavior.

Author: Álvaro Herrera <alvherre@kurilemu.de>
Reported-by: Giovanni Fabris <giovanni.fabris@icon.it>
Reported-by: Arthur Nascimento <tureba@gmail.com>
Backpatch-through: 13
Discussion: https://postgr.es/m/202511031106.h4fwyuyui6fz@alvherre.pgsql

diff --git a/src/backend/postmaster/autovacuum.c b/src/backend/postmaster/autovacuum.c
index 94c93c6e4b5864e38ffa045bacd8b1dae7355768..107cd82e2e19b7eba1d27bd9955d2c3dca5019f3 100644 (file)
--- a/src/backend/postmaster/autovacuum.c
+++ b/src/backend/postmaster/autovacuum.c
@@ -2615,7 +2615,9 @@ deleted:
                workitem->avw_active = true;
                LWLockRelease(AutovacuumLock);
 
+               PushActiveSnapshot(GetTransactionSnapshot());
                perform_work_item(workitem);
+               PopActiveSnapshot();
 
                /*
                 * Check for config changes before acquiring lock for further jobs.

diff --git a/src/test/modules/brin/t/01_workitems.pl b/src/test/modules/brin/t/01_workitems.pl
index 3108c02cf4df9eb968e266d84b3ce93de14fe97b..0495dac2f740ff767918e268dbc7680e0ab58cb4 100644 (file)
--- a/src/test/modules/brin/t/01_workitems.pl
+++ b/src/test/modules/brin/t/01_workitems.pl
@@ -24,13 +24,31 @@ $node->safe_psql(
         create index brin_wi_idx on brin_wi using brin (a) with (pages_per_range=1, autosummarize=on);
         '
 );
+# Another table with an index that requires a snapshot to run
+$node->safe_psql(
+       'postgres',
+       'create table journal (d timestamp) with (fillfactor = 10);
+        create function packdate(d timestamp) returns text language plpgsql
+          as $$ begin return to_char(d, \'yyyymm\'); end; $$
+          returns null on null input immutable;
+        create index brin_packdate_idx on journal using brin (packdate(d))
+          with (autosummarize = on, pages_per_range = 1);
+        '
+);
+
 my $count = $node->safe_psql('postgres',
        "select count(*) from brin_page_items(get_raw_page('brin_wi_idx', 2), 'brin_wi_idx'::regclass)"
 );
-is($count, '1', "initial index state is correct");
+is($count, '1', "initial brin_wi_index index state is correct");
+$count = $node->safe_psql('postgres',
+       "select count(*) from brin_page_items(get_raw_page('brin_packdate_idx', 2), 'brin_packdate_idx'::regclass)"
+);
+is($count, '1', "initial brin_packdate_idx index state is correct");
 
 $node->safe_psql('postgres',
        'insert into brin_wi select * from generate_series(1, 100)');
+$node->safe_psql('postgres',
+       "insert into journal select * from generate_series(timestamp '1976-08-01', '1976-10-28', '1 day')");
 
 $node->poll_query_until(
        'postgres',
@@ -38,9 +56,22 @@ $node->poll_query_until(
        't');
 
 $count = $node->safe_psql('postgres',
-       "select count(*) > 1 from brin_page_items(get_raw_page('brin_wi_idx', 2), 'brin_wi_idx'::regclass)"
+       "select count(*) from brin_page_items(get_raw_page('brin_wi_idx', 2), 'brin_wi_idx'::regclass)
+        where not placeholder;"
 );
-is($count, 't', "index got summarized");
+cmp_ok($count, '>', '1', "$count brin_wi_idx ranges got summarized");
+
+$node->poll_query_until(
+       'postgres',
+       "select count(*) > 1 from brin_page_items(get_raw_page('brin_packdate_idx', 2), 'brin_packdate_idx'::regclass)",
+       't');
+
+$count = $node->safe_psql('postgres',
+       "select count(*) from brin_page_items(get_raw_page('brin_packdate_idx', 2), 'brin_packdate_idx'::regclass)
+        where not placeholder;"
+);
+cmp_ok($count, '>', '1', "$count brin_packdate_idx ranges got summarized");
+
 $node->stop;
 
 done_testing();