Re: Autovacuum and OldestXmin

Simon Riggs wrote:
> On Thu, 2007-11-22 at 13:21 -0500, Tom Lane wrote:
>> Simon Riggs <simon(at)2ndquadrant(dot)com> writes:
>>> Why isn't VACUUM optimised the same way HOT is?
>> It doesn't do the same things HOT does.
>
> Thanks for the enlightenment :-)
>
> Clearly much of the code in heap_page_prune_opt() differs, yet the test
> for if (!PageIsPrunable(...)) could be repeated inside the main block
> scan loop in lazy_scan_heap().
>
> My thought-experiment:
>
> - a long running transaction is in progress
> - HOT cleans a block and then the block is not touched for a while, the
> total of all uncleanable updates cause a VACUUM to be triggered, which
> then scans the table, sees the block and scans the block again
> because...
>
> a) it could have checked !PageIsPrunable(), but didn't
>
> b) it is important that it attempt to clean the block again for
> reason...?

There might be dead tuples left over by aborted INSERTs, for example,
which don't set the Prunable-flag.

Even if we could use PageIsPrunable, it would be a bad thing from a
robustness point of view. If we ever failed to set the Prunable-flag on
a page for some reason, VACUUM would never remove the dead tuples.

Besides, I don't remember anyone complaining about VACUUM's CPU usage,
so it doesn't really matter.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com