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postgres_fdw: Inherit the local transaction's access/deferrable modes.

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Previously, postgres_fdw always 1) opened a remote transaction in READ
WRITE mode even when the local transaction was READ ONLY, causing a READ
ONLY transaction using it that references a foreign table mapped to a
remote view executing a volatile function to write in the remote side,
and 2) opened the remote transaction in NOT DEFERRABLE mode even when
the local transaction was DEFERRABLE, causing a SERIALIZABLE READ ONLY
DEFERRABLE transaction using it to abort due to a serialization failure
in the remote side.

To avoid these, modify postgres_fdw to open a remote transaction in the
same access/deferrable modes as the local transaction.  This commit also
modifies it to open a remote subtransaction in the same access mode as
the local subtransaction.

Although these issues exist since the introduction of postgres_fdw,
there have been no reports from the field.  So it seems fine to just fix
them in master only.

Author: Etsuro Fujita <etsuro.fujita@gmail.com>
Reviewed-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAPmGK16n_hcUUWuOdmeUS%2Bw4Q6dZvTEDHb%3DOP%3D5JBzo-M3QmpQ%40mail.gmail.com
This commit is contained in:
Etsuro Fujita
2025-06-01 17:30:00 +09:00
parent b006bcd531
commit e5a3c9d9b5
6 changed files with 347 additions and 8 deletions
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15
doc/src/sgml/postgres-fdw.sgml
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@@ -1077,6 +1077,21 @@ postgres=# SELECT postgres_fdw_disconnect_all();
<productname>PostgreSQL</productname> release might modify these rules.
</para>
<para>
The remote transaction is opened in the same read/write mode as the local
transaction: if the local transaction is <literal>READ ONLY</literal>,
the remote transaction is opened in <literal>READ ONLY</literal> mode,
otherwise it is opened in <literal>READ WRITE</literal> mode.
(This rule is also applied to remote and local subtransactions.)
</para>
<para>
The remote transaction is also opened in the same deferrable mode as the
local transaction: if the local transaction is <literal>DEFERRABLE</literal>,
the remote transaction is opened in <literal>DEFERRABLE</literal> mode,
otherwise it is opened in <literal>NOT DEFERRABLE</literal> mode.
</para>
<para>
Note that it is currently not supported by
<filename>postgres_fdw</filename> to prepare the remote transaction for