On 31 January 2012 23:04, Tom Lane <t...@sss.pgh.pa.us> wrote: > Dean Rasheed <dean.a.rash...@gmail.com> writes: >> The thing I'm unsure about is whether sending sinval >> messages when the visibility map is extended is a good idea. > > Seems perfectly reasonable to me. They'd occur so seldom as to be > more than repaid if we can scrape some cost out of the mainline paths. >
OK, thanks. That's good. > The real objection to this probably is that if it only saves anything > for tables that don't have a VM yet, it's dubious whether it's worth > doing. But if we can avoid wasted checks for VM extension as well, > then I think it's probably a no-brainer. > > regards, tom lane Yes it applies in the same way to VM extension - if the table has grown and the VM has not yet been extended, but I don't see why that is any worse than the case of not having a VM yet. Actually I think that this is not such an uncommon case - for a table which has only had data inserted - no deletes or updates - it is tempting to think that ANALYSE is sufficient, and that there is no need to VACUUM. If it were simply the case that this caused an index-only scan to have no real benefit, you might be willing to live with normal index scan performance. But actually it causes a very significant performance regression beyond that, to well below 9.1 performance. Regards, Dean -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
