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From: Dimitrios Apostolou <[email protected]>
To: Adrian Klaver <[email protected]>
Cc: Laurenz Albe <[email protected]>
Cc: [email protected]
Subject: Re: Experience and feedback on pg_restore --data-only
Date: Mon, 24 Mar 2025 16:51:30 +0100 (CET)
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
	<[email protected]>
	<[email protected]>
	<[email protected]>

On Mon, 24 Mar 2025, Adrian Klaver wrote:

> On 3/24/25 07:24, Dimitrios Apostolou wrote:
>>  On Sun, 23 Mar 2025, Laurenz Albe wrote:
>>
>>>  On Thu, 2025-03-20 at 23:48 +0100, Dimitrios Apostolou wrote:
>>>>  Performance issues: (important as my db size is >5TB)
>>>>
>>>>  * WAL writes: I didn't manage to avoid writing to the WAL, despite
>>>>  having
>>>>     setting wal_level=minimal. I even wrote my own function to ALTER all
>>>>     tables to UNLOGGED, but failed with "could not change table T to
>>>>     unlogged because it references logged table".  I'm out of ideas on
>>>>  this
>>>>     one.
>>>
>>>  You'd have to create an load the table in the same transaction, that is,
>>>  you'd have to run pg_restore with --single-transaction.
>>
>>  That would restore the schema from the dump, while I want to create the
>>  schema from the SQL code in version control.
>
>
> I am not following, from your original post:
>
> "
> ... create a
> clean database by running the SQL schema definition from version control, and
> then copy the data for only the tables created.
>
> For this case, I choose to run pg_restore --data-only, and run it as the user
> who owns the database (dbowner), not as a superuser, in order to avoid
> changes being introduced under the radar.
> "
>
> You are running the process in two steps, where the first does not involve
> pg_restore. Not sure why doing the pg_restore --data-only portion in single
> transaction is not possible?

Laurenz informed me that I could avoid writing to the WAL if I "create and
load the table in a single transaction".
I haven't tried, but here is what I would do to try --single-transaction:

Transaction 1: manually issuing all of CREATE TABLE etc.

Transaction 2: pg_restore --single-transaction --data-only

The COPY command in transaction 2 would still need to write to WAL, since
it's separate from the CREATE TABLE.

Am I wrong somewhere?

>>  Something that might work, would be for pg_restore to issue a TRUNCATE
>>  before the COPY. I believe this would require superuser privelege though,
>>  that I would prefer to avoid. Currently I issue TRUNCATE for all tables
>>  manually before running pg_restore, but of course this is in a different
>>  transaction so it doesn't help.
>>
>>  By the way do you see potential problems with using --single-transaction
>>  to restore billion-rows tables?
>
> COPY is all or none(version 17+ caveat(see
> https://www.postgresql.org/docs/current/sql-copy.html  ON_ERROR)), so if the
> data dump fails in --single-transaction everything rolls back.

So if I restore all tables, then an error about a "table not found" would
not roll back already copied tables, since it's not part of a COPY?


Thank you for the feedback,
Dimitris

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