Re: [HACKERS] Bug? relpages, reltuples resets to zero

Bruce Momjian wrote:

> I have seen the optimizer stop using indexes, but could never reproduce
> it, and hoped my mega-patch would have fix it.
>
> My only guess is that vacuum has changed the buffer cache copy of the
> pg_class tuple, but did not mark it as dirty, so it was not written back
> out when removed from the buffer cache. When reloaded after the query,
> the buffer cache is loaded from the disk copy, and the disk copy has
> zeros, because the vacuum copy was not written to disk.
>
> The active code is in vacuum.c::vc_updstats:

Your guess was right, thanks. But your solution does not work
:-(

I found a way to easily reproduce the error.

Run VACUUM
Restart postmaster
-> relpages and reltuples gone

I think the simple way of modifying the tuple in the page
does not work. I found that catalog/index.c does the same for
relpages and reltuples in UpdateStats(). Calling
UpdateStats() after vc_updstats() as a quick hack solved the
problem.

I'm now cvsup'ing, then I'll modify vacuum.c to do it the
same way as index.c does it. I don't know if calling
UpdateStats() instead is really a good idea, because vacuum
potentially truncates files and UpdateStats() does
RelationGetNumberOfBlocks() instead of getting it by
argument. This might be wrong at that time.

Let's see what happens when vacuum does it harder.

Later, Jan

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