On 04/11/2019 05:48 PM, Tom Lane wrote:
rihad <rihad@xxxxxxx> 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 )
