Is a VACUUM or ANALYZE necessary after logical replication?

On 6/15/24 15:55, Koen De Groote wrote:

I've gone over all of
https://www.postgresql.org/docs/current/logical-replication.html
<https://www.postgresql.org/docs/current/logical-replication.html&gt; and
the only mentions of the word "index" I could find was in relation to
replica identity and examples of table definitions showing primary key
indexes.

Nothing is said about indexes. Maybe for good reason, maybe they are
fully functionality immediately after replication?

So the main question: Once a table is fully replicated, do I need to
vacuum(analyze) that table, or are the indexes on that table already
functional?

VACUUM/ANALYZE is not about making the index functional. The VACUUM
marks the space dead tuples occupy in the table and associated indexes
as available for recycling. The ANALYZE updates tables statistics to
help the planner make decisions on what query plan to use. On a fresh
table VACUUM will not be of much value, ANALYZE though will help by
creating up to date table statistics.

Regards,
Koen De Groote

--
Adrian Klaver
adrian.klaver@aklaver.com

Στις 16/6/24 02:13, ο/η Adrian Klaver έγραψε:

On 6/15/24 15:55, Koen De Groote wrote:

I've gone over all of
https://www.postgresql.org/docs/current/logical-replication.html
<https://www.postgresql.org/docs/current/logical-replication.html&gt;
and the only mentions of the word "index" I could find was in
relation to replica identity and examples of table definitions
showing primary key indexes.

Nothing is said about indexes. Maybe for good reason, maybe they are
fully functionality immediately after replication?

So the main question: Once a table is fully replicated, do I need to
vacuum(analyze) that table, or are the indexes on that table already
functional?

VACUUM/ANALYZE is not about making the index functional. The VACUUM
marks the space dead tuples occupy in the table and associated indexes
as available for recycling. The ANALYZE updates tables statistics to
help the planner make decisions on what query plan to use. On a fresh
table VACUUM will not be of much value, ANALYZE though will help by
creating up to date table statistics.

Hi Adrian, however in case the replication is problematic due to e.g. :

- wrong encoding to the new system, e.g. from SQL_ASCII to UTF-8

- ALWAYS triggers written without full schema qualification or other
problems throwing ERRORs, etc

- server restarts during the sync phase

- etc

those will produce rollbacks, hence bloating right from the start. In
those cases either VACUUM FULL will be needed, or even better correct
the errors in their source and repeat the whole process. Normally the
new DB (subscriber) should be a little smaller than the original
(publisher).

As you said, in any case, ANALYZE will be always needed afterwards, as
well as taking care of sequences. IMHO the sequence part would be nice
to be handled in a more elegant manner, e.g. by an option in pg_dump to
dump only sequences, for "nearly zero" downtime upgrades, this step
should happen rapidly at the switchover.

Regards,
Koen De Groote

--
Achilleas Mantzios
IT DEV - HEAD
IT DEPT
Dynacom Tankers Mgmt (as agents only)

On Saturday, June 15, 2024, Koen De Groote <kdg.dev@gmail.com> wrote:

I've gone over all of https://www.postgresql.org/docs/current/logical-
replication.html and the only mentions of the word "index" I could find
was in relation to replica identity and examples of table definitions
showing primary key indexes.

Nothing is said about indexes. Maybe for good reason, maybe they are fully
functionality immediately after replication?

So the main question: Once a table is fully replicated, do I need to
vacuum(analyze) that table, or are the indexes on that table already
functional?

The whole point of “logical” replication is that the
inserts/updates/deletes are reapplied on the secondary against the table
and the whatever triggers, indexes, or whatnot exist on that table in the
secondary behave just as if you connected to the server directly and issued
the corresponding SQL against it. As far as the replication system is
concerned none of those things on the primary matter nor does it have to
care about them on the secondary.

David J.