Re: Removing oids with pg_repack

On Wednesday, November 22, 2023 at 01:20:18 AM EST, Achilleas Mantzios <a.mantzios@cloud.gatewaynet.com> wrote:

Στις 21/11/23 20:41, ο/η CG έγραψε:

I have a very large PostgreSQL 9.5 database that still has very large tables with oids. I'm trying to get rid of the oids with as little downtime as possible so I can prep the database for upgrade past PostgreSQL 11. I had a wild idea to mod pg_repack to write a new table without oids. I think it almost works. 

To test out my idea I made a new table wipe_oid_test with oids. I filled it with a few rows of data.

........

But PostgreSQL still thinks that the table has oids:

mydata=# \d+ wipe_oid_test

                   Table "public.wipe_oid_test"

 Column | Type | Modifiers | Storage  | Stats target | Description 

--------+------+-----------+----------+--------------+-------------

 k      | text | not null  | extended |              | 

 v      | text |           | extended |              | 

Indexes:

    "wipe_oid_test_pkey" PRIMARY KEY, btree (k)

Has OIDs: yes

Except where does it mention in the pg_repack docs (or source) that it is meant to be used for NO OIDS conversion ?

It does not-- I was trying to leverage and tweak the base functionality of pg_repack which sets up triggers and migrates data. I figured if the target table was created without OIDs that when pg_repack did the "swap" operation that the new table would take over with the added bonus of not having oids.

I can modify pg_class and set relhasoids = false, but it isn't actually eliminating the oid column. `\d+` will report not report that it has oids, but the oid column is still present and returns the same result before updating pg_class.

Just Dont!

Noted. ;)

So I'm definitely missing something. I really need a point in the right direction.... Please help! ;)

There are a few of methods to get rid of OIDs :

- ALTER TABLE .. SET WITHOUT OIDS (just mentioning, you already checked that)

This makes the database unusable for hours and hours and hours because it locks the table entirely while it performs the operation. That's just something that we can't afford.

- Use table copy +  use of a trigger to log changes : https://dba.stackexchange.com/questions/259359/eliminating-oids-while-upgrading-postgresql-from-9-4-to-12

That SO is not quite the effect I'm going for. The poster of that SO was using OIDS in their application and needed a solution to maintain those values after conversion. I simply want to eliminate them without the extraordinary downtime the database would experience during ALTER operations.

- Use of Inheritance (the most neat solution I have seen, this is what I used for a 2TB table conversion) : https://www.percona.com/blog/performing-etl-using-inheritance-in-postgresql/

This is closest to the effect I was going for. pg_repack essentially creates a second table and fills it with the data from the first table while ensuring standard db operations against that table continue to function while the data is being moved from the old table to the new table. The process outlined in the Percona ETL strategy has to be repeated per-table, which is work I was hoping to avoid by leveraging 95% of the functionality of pg_repack while supplying my own 5% as the resulting table would not have oids regardless of the source table's configuration.

For my experiment, Table A did have oids. Table B (created by pg_repack) did not (at least at creation). When the "swap" operation happened in pg_repack, the metadata for Table A was assigned to Table B. I'm just trying to figure out what metadata I need to change in the system tables to reflect the actual table structure. 

I have the fallback position for the Percona ETL strategy. But I feel like I'm REALLY close with pg_repack and I just don't understand enough about the system internals to nudge it to correctness and need some expert assistance to tap it in the hole.

CG

-- 
Achilleas Mantzios IT DEV - HEAD IT DEPT Dynacom Tankers Mgmt

-- 
Achilleas Mantzios IT DEV - HEAD IT DEPT Dynacom Tankers Mgmt