"Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:
> Jeff Davis <pgsql(at)j-davis(dot)com> writes:
>> The objections to synchronized scans for VACUUM as listed in that thread
>> 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
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
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?
Get trained by Bruce Momjian - ask me about EnterpriseDB's PostgreSQL training!
In response to
pgsql-hackers by date
|Next:||From: Dirk Riehle||Date: 2008-06-01 13:31:44|
|Subject: Re: Feedback on blog post about Replication Feature
decision and its impact|
|Previous:||From: Pavel Stehule||Date: 2008-06-01 09:30:19|
|Subject: explain doesn't work with execute using|