Re: Odd out of memory problem.

Andrew Dunstan <andrew(at)dunslane(dot)net> writes:
> On 03/26/2012 01:34 PM, Tom Lane wrote:
>> Hm. The test case is just a straight pg_restore of lots and lots of LOs?
>> What pg_dump version was the dump made with?

> 8.4.8, same as the target. We get the same issue whether we restore
> direct to the database from pg_restore or via a text dump.

I believe I see the issue: when creating/loading LOs, we first do a
lo_create (which in 8.4 makes a "page zero" tuple in pg_largeobject
containing zero bytes of data) and then lo_write, which will do a
heap_update to overwrite that tuple with data. This is at the next
command in the same transaction, so the original tuple has to receive a
combo CID. Net result: we accumulate one new combo CID per large object
loaded in the same transaction. You can reproduce this without any
pg_dump involvement at all, using something like

create table mylos (id oid);
insert into mylos select lo_import('/tmp/junk') from generate_series(1,1000000);

The problem is gone in 9.0 and up because now we use a
pg_largeobject_metadata entry instead of a pg_largeobject row to flag
the existence of an empty large object. I don't see any very practical
backend fix for the problem in 8.x.

regards, tom lane