From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Simon Riggs <simon(at)2ndQuadrant(dot)com> |
Cc: | Gregory Stark <stark(at)enterprisedb(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: VACUUMs and WAL |
Date: | 2008-10-28 12:36:23 |
Message-ID: | 18585.1225197383@sss.pgh.pa.us |
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Lists: | pgsql-hackers |
Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
> I think what I am suggesting is two heap passes, but writing WAL and
> dirtying blocks on only one of the passes.
I think you've all forgotten about hint-bit setting. The assumption is
that the first VACUUM pass is going to update a lot of hint bits and we
might as well get some other work done with the same write.
Now of course that doesn't necessarily entail a WAL write too, but
it makes this less than a slam-dunk win.
Also, I think that the reason the code ended up this way is that there
were pretty severe difficulties in making the VACUUM code cope correctly
with un-pruned tuples. Pavan might remember more about that.
regards, tom lane
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