Re: Autovacuum and OldestXmin

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...?

Seems like the thought experiment could occur frequently.

--
Simon Riggs
2ndQuadrant http://www.2ndQuadrant.com