Document bug regarding read only transactions

Started by Tatsuo Ishiiover 8 years ago5 messages

ishii@sraoss.co.jp

over 8 years ago

1 attachment(s)

In https://www.postgresql.org/docs/10/static/hot-standby.html#hot-standby-users

It is explained that read only transactions (not in standby) allow to
update sequences.

In normal operation, <quote>read-only</> transactions are allowed to
update sequences and to use <command>LISTEN</>, <command>UNLISTEN</>, and
<command>NOTIFY</>, so Hot Standby sessions operate under slightly tighter
restrictions than ordinary read-only sessions. It is possible that some
of these restrictions might be loosened in a future release.

This is plain wrong.

BEGIN;
BEGIN
test=# SET transaction_read_only TO on;
SET
test=# SELECT nextval('t1_i_seq');
ERROR: cannot execute nextval() in a read-only transaction
test=# \q

Attached is the patch against master branch.

Best regards,
--
Tatsuo Ishii
SRA OSS, Inc. Japan
English: http://www.sraoss.co.jp/index_en.php
Japanese:http://www.sraoss.co.jp

Attachments:

high-availability.difftext/x-patch; charset=us-asciiDownload

diff --git a/doc/src/sgml/high-availability.sgml b/doc/src/sgml/high-availability.sgml
index 92e3b45..91cbabd 100644
--- a/doc/src/sgml/high-availability.sgml
+++ b/doc/src/sgml/high-availability.sgml
@@ -1824,7 +1824,7 @@ if (!triggered)
 
    <para>
     In normal operation, <quote>read-only</> transactions are allowed to
-    update sequences and to use <command>LISTEN</>, <command>UNLISTEN</>, and
+    use <command>LISTEN</>, <command>UNLISTEN</>, and
     <command>NOTIFY</>, so Hot Standby sessions operate under slightly tighter
     restrictions than ordinary read-only sessions.  It is possible that some
     of these restrictions might be loosened in a future release.

Michael Paquier

michael.paquier@gmail.com

over 8 years ago

In reply to: Tatsuo Ishii (#1)

Re: Document bug regarding read only transactions

On Wed, Jun 14, 2017 at 11:08 AM, Tatsuo Ishii <ishii@sraoss.co.jp> wrote:

<para>
In normal operation, <quote>read-only</> transactions are allowed to
-    update sequences and to use <command>LISTEN</>, <command>UNLISTEN</>, and
+    use <command>LISTEN</>, <command>UNLISTEN</>, and
<command>NOTIFY</>, so Hot Standby sessions operate under slightly tighter
restrictions than ordinary read-only sessions.  It is possible that some
of these restrictions might be loosened in a future release.

Your modification does not look completely correct to me either.
Temporary sequences can be updated in read-only transactions. I think
that the page listing the sequence-related functions should as well
mention those restrictions for nextval() and setval().
--
Michael

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Tatsuo Ishii

ishii@sraoss.co.jp

over 8 years ago

In reply to: Michael Paquier (#2)

Re: Document bug regarding read only transactions

Your modification does not look completely correct to me either.
Temporary sequences can be updated in read-only transactions.

Not sure. Temporary sequences are meaningless on standby because
"create temporary sequence" command cannot be executed on standby
anyway (and temporary sequence are not replicated to standby of
course).

I think
that the page listing the sequence-related functions should as well
mention those restrictions for nextval() and setval().

If we do so, ANALYZE, VACUUM, LISTEN and NOTIFY man pages should also
be updated to mention that they can be executed in read only
transaction but not in standby servers. I'm not sure it's worth the
trouble. Moreover, that will create maintenance headache once we
decide to remove some of the restrictions, because we need to update
multiple places in the doc.
--
Tatsuo Ishii
SRA OSS, Inc. Japan
English: http://www.sraoss.co.jp/index_en.php
Japanese:http://www.sraoss.co.jp

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Robert Haas

robertmhaas@gmail.com

over 8 years ago

In reply to: Tatsuo Ishii (#1)

Re: Document bug regarding read only transactions

On Tue, Jun 13, 2017 at 10:08 PM, Tatsuo Ishii <ishii@sraoss.co.jp> wrote:

In https://www.postgresql.org/docs/10/static/hot-standby.html#hot-standby-users

It is explained that read only transactions (not in standby) allow to
update sequences.

In normal operation, <quote>read-only</> transactions are allowed to
update sequences and to use <command>LISTEN</>, <command>UNLISTEN</>, and
<command>NOTIFY</>, so Hot Standby sessions operate under slightly tighter
restrictions than ordinary read-only sessions. It is possible that some
of these restrictions might be loosened in a future release.

This is plain wrong.

It used to be true. Tom changed it in commit
05d8a561ff85db1545f5768fe8d8dc9d99ad2ef7, back in 2010.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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Tatsuo Ishii

ishii@sraoss.co.jp

over 8 years ago

In reply to: Robert Haas (#4)

Re: Document bug regarding read only transactions

It used to be true. Tom changed it in commit
05d8a561ff85db1545f5768fe8d8dc9d99ad2ef7, back in 2010.

Thank you for the info. For a record, I will add it to the commit
message.

Best regards,
--
Tatsuo Ishii
SRA OSS, Inc. Japan
English: http://www.sraoss.co.jp/index_en.php
Japanese:http://www.sraoss.co.jp

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