Re: When do vacuumed pages/tuples become available for reuse?

On 04/11/2019 05:48 PM, Tom Lane wrote:
> rihad <rihad@mail.ru> writes:
>> If an autovacuum job on a huge table runs for 5-6 hours, do its freed
>> pages/tuples become available for reuse immediately when they are marked
>> as free, or only at the end of the multi-hour vacuum operation?
> They'll be freed in batches, where the size of a batch depends on the
> autovacuum_work_mem or maintenance_work_mem setting.  The basic
> work cycle is
>
> * scan table to find dead tuples, save their TIDs in working memory;
>    continue until end of table or working memory full
> * scan indexes to find index entries matching those TIDs, remove 'em
> * go back to table and remove the previously-found tuples
> * if not end of table, repeat
>
> So a larger work-mem setting means fewer passes over the indexes,
> but a longer time until space is reclaimed.
>
>             regards, tom lane
> .
>
Thanks! Our autovacuum_work_mem = 1GB, so this probably means any space 
would be available for reuse only at the end of the vacuum? Are there 
any downsides in decreasing it to, say, 64MB? I see only pluses )