Re: pg_toast oid limits

Natalie Wenz <nataliewenz(at)ebureau(dot)com> writes:
> I am seeing some performance issues that I'm trying to track down on a large database. One of the things I'm beginning to suspect is a particularly large table with many columns, about 200 of which (type text) contain large chunks of data. Now, for a given row, maybe 10-30 of those columns contain data, so not all 200 for each row, but the data can still be pretty sizable. There are currently around 750 million records in this table (and is about 22TB in size). I was trying to learn more about toast, and I see some references in the wiki and the hackers list to performance issues when you approach the 4 billion oids for a single table (which, I gather, are used when the data is toasted). Given my rudimentary understanding of how the whole toast thing works, I was wondering if there is a way to see how many oids are used for a table, or another way to know if we're running into toast limits for a single table.

You could do

select reltoastrelid::regclass from pg_class where relname = 'problem_table';

which will give you something like

reltoastrelid
-------------------------
pg_toast.pg_toast_78004

and then poke into the contents of that table. Probably

select count(distinct chunk_id) from pg_toast.pg_toast_78004;

would answer your immediate question.

regards, tom lane