Date: Tue, 30 Apr 2024 21:05:31 -0500
From: Nathan Bossart <nathandbossart@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-hackers@postgresql.org
Subject: Re: pg_sequence_last_value() for unlogged sequences on standbys
Message-ID: <20240501020531.GA721953@nathanxps13>
References: <20240501005730.GA594666@nathanxps13>
 <1846474.1714525564@sss.pgh.pa.us>
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On Tue, Apr 30, 2024 at 08:13:17PM -0500, Nathan Bossart wrote:
> Good point.  I'll work on a patch along these lines, then.

This ended up being easier than I expected.  While unlogged sequences are
only supported on v15 and above, temporary sequences have been around since
v7.2, so this will probably need to be back-patched to all supported
versions.  The added test case won't work for v12-v14 since it uses an
unlogged sequence.  I'm not sure it's worth constructing a test case for
temporary sequences.

-- 
Nathan Bossart
Amazon Web Services: https://aws.amazon.com

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	filename="v1-0001-Fix-pg_sequence_last_value-for-non-permanent-sequ.patch"

From 71008f13da88f41a205e0643129162df9d2ebc81 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Tue, 30 Apr 2024 20:54:51 -0500
Subject: [PATCH v1 1/1] Fix pg_sequence_last_value() for non-permanent
 sequences on standbys.

---
 src/backend/commands/sequence.c       | 18 +++++++++++++-----
 src/test/recovery/t/001_stream_rep.pl |  8 ++++++++
 2 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/src/backend/commands/sequence.c b/src/backend/commands/sequence.c
index 46103561c3..659d2ad4fc 100644
--- a/src/backend/commands/sequence.c
+++ b/src/backend/commands/sequence.c
@@ -1780,7 +1780,7 @@ pg_sequence_last_value(PG_FUNCTION_ARGS)
 	Buffer		buf;
 	HeapTupleData seqtuple;
 	Form_pg_sequence_data seq;
-	bool		is_called;
+	bool		is_called = false;
 	int64		result;
 
 	/* open and lock sequence */
@@ -1792,12 +1792,20 @@ pg_sequence_last_value(PG_FUNCTION_ARGS)
 				 errmsg("permission denied for sequence %s",
 						RelationGetRelationName(seqrel))));
 
-	seq = read_seq_tuple(seqrel, &buf, &seqtuple);
+	/*
+	 * For the benefit of the pg_sequences system view, we return NULL for
+	 * temporary and unlogged sequences on standbys instead of throwing an
+	 * error.
+	 */
+	if (RelationIsPermanent(seqrel) || !RecoveryInProgress())
+	{
+		seq = read_seq_tuple(seqrel, &buf, &seqtuple);
 
-	is_called = seq->is_called;
-	result = seq->last_value;
+		is_called = seq->is_called;
+		result = seq->last_value;
 
-	UnlockReleaseBuffer(buf);
+		UnlockReleaseBuffer(buf);
+	}
 	sequence_close(seqrel, NoLock);
 
 	if (is_called)
diff --git a/src/test/recovery/t/001_stream_rep.pl b/src/test/recovery/t/001_stream_rep.pl
index 5311ade509..4c698b5ce1 100644
--- a/src/test/recovery/t/001_stream_rep.pl
+++ b/src/test/recovery/t/001_stream_rep.pl
@@ -95,6 +95,14 @@ $result = $node_standby_2->safe_psql('postgres', "SELECT * FROM seq1");
 print "standby 2: $result\n";
 is($result, qq(33|0|t), 'check streamed sequence content on standby 2');
 
+# Check pg_sequence_last_value() returns NULL for unlogged sequence on standby
+$node_primary->safe_psql('postgres',
+	"CREATE UNLOGGED SEQUENCE ulseq; SELECT nextval('ulseq')");
+$node_primary->wait_for_replay_catchup($node_standby_1);
+is($node_standby_1->safe_psql('postgres',
+	"SELECT pg_sequence_last_value('ulseq'::regclass) IS NULL"),
+	't', 'pg_sequence_last_value() on unlogged sequence on standby 1');
+
 # Check that only READ-only queries can run on standbys
 is($node_standby_1->psql('postgres', 'INSERT INTO tab_int VALUES (1)'),
 	3, 'read-only queries on standby 1');
-- 
2.25.1


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