Re: Broken index?

On 2001.09.07 15:53 Tom Lane wrote:
> Paul Green <traktion@webleicester.co.uk> writes:
> >> Hmm.  What PG version is this, on what platform?
>
> > Redhat 7.1, (Linux 2.4.2-2) on a K6-2 300 with 128 mb ram running PG
> 7.0
>
> > What *is* wierd is that if I do, say, "select *
> > from player where name='bob'" I get one result where as if I do a
> "select *
> > from player where name like '%bob', I get many bobs (about 10)
> returned.
>
> This is entirely consistent with the theory of a broken index.  The %bob
> query does a sequential scan and looks at every row, but the other will
> do an index lookup and return only the rows it finds via the index.

Ahh yes, that seems to make sense.

>
> Did you compile 7.0 with locale support?  If so, a plausible theory is
> that you've started the postmaster with different locale settings at
> different times --- a change in locale would alter the effective sort
> order, rendering what had been a valid index corrupt.  The btree search
> and update algorithms fail completely if the data in the index is not
> really in the correct order.
>
> Unfortunately it is *very* easy to start the postmaster with
> inconsistent locale environment, since what you have in your login
> profile is often different from the environment that system boot scripts
> run in.

I didn't actually compile it at all - I used the default RPMs provided by
Redhat. I'm unsure whether they have compiled in support or not, but I know
I always start/stop postgres using the init.d scripts, which always start
the database under the user postgres. As far as I'm aware, I've never
changed the locale of the system/db either. I just checked using 'locale'
to see how the system was set up under the usernames I use to log in to the
machine and they are all en_GB anyway. I also checked /etc/sysconfig/i18n
and that too was set to en_GB, so I think I'm safe on this front.

Any ideas?

>
> 7.1 contains fixes that force PG always to use the initdb-time locale
> setting, so as to eliminate this class of problems.  I'd suggest an
> upgrade.  Meanwhile, you need to take steps to eliminate the duplicate
> rows from your table.

If you are convinced that the above is not the problem, I will try to delay
the upgrade for now. If you think that it may still be a locale setting,
then I guess I will have to though :)

>
> > Also, if I do a "select count(id) from player" I get the
> > same number as "select distinct count(id) from player" even though they
> > should clearly be different.
>
> Eh?  Looks like the same thing to me.  Perhaps you are thinking of
> "select count(distinct id) from player"

Yes, sorry, that is what I meant.

>
> > Also, I've just tried to do a query to update all rows by "update
> player
> > set flatname=lower(name)" and that failed complaining about duplicates,
> yet
> > if you specify the id, the row is altered. Crazy huh?
>
> Once the index becomes corrupt, the behavior will be awfully
> unpredictable...

:)

>
>             regards, tom lane
>
--
Paul Green
Programmer

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