Re: synchronized scans for VACUUM

"Tom Lane" <tgl@sss.pgh.pa.us> writes:

> Jeff Davis <pgsql@j-davis.com> writes:
>> The objections to synchronized scans for VACUUM as listed in that thread
>> (summary):
>
>> 2. vacuum takes breaks from the scan to clean up the indexes when it
>> runs out of maintenance_work_mem.
>
>> 2. There have been suggestions about a more compact representation for
>> the tuple id list. If this works, it will solve this problem.
>
> It will certainly not "solve" the problem.  What it will do is mean that
> the breaks are further apart and longer, which seems to me to make the
> conflict with syncscan behavior worse not better.

How would it make them longer? They still have the same amount of i/o to do
scanning the indexes. I suppose they would dirty more pages which might slow
them down?

In any case I think the representation you proposed back when this idea last
came up was so compact that pretty much any size table ought to be
representable in a reasonable work_mem -- at least for the kind of machine
which would normally be dealing with that size table.

> It still seems to me that vacuum is unlikely to be a productive member
> of a syncscan herd --- it just isn't going to have similar scan-speed
> behavior to typical queries.

That's my thinking too. Our general direction has been toward reducing
vacuum's i/o bandwidth requirements, not worrying about making it run as fast
as possible.

That said if it happened to latch on to a sync scan herd it would have very
few cache misses which would cause it to rack up very few vacuum cost delay
points. Perhaps the vacuum cost delay for a cache hit ought to be 0?

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