Re: Bug #769: Slow vacuuming due to error in optimization

Stephen Marshall <smarshall(at)wsicorp(dot)com> writes:
> Tom Lane wrote:
>> How large is "large", and what FSM parameters are you using? Do you
>> know how many pages were getting passed into MultiRecordFreeSpace?

> vacuum_mem is 16384
> max_fsm_relations is 100
> max_fsm_pages is 300000

> pg_largeobject data size = 4211617.693 K
> pg_largeobject file size = 7932208.000 K
> number of large objects = 870
> number of lo pages = 2106287
> number of messages = 469458

> Upon vacuuming, I found 300000 pages were passed to MultiRecordFreeSpace,
> with the minPage specified as 0 and the maxPage as -2. The pages passed
> exactly equaled the max_fsm_pages parameter.

Yeah; if you look at vacuumlazy.c you'll see it doesn't bother keeping
track of more than max_fsm_pages, since it knows the FSM will not
remember more than that either.

> This vacuum took over 30 minutes, with more than 25 minutes spent in
> MultiRecordFreeSpace.

Yow. Definitely need to fix that code ...

regards, tom lane