From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
---|---|
To: | John Brothers <johnbr(at)mindspring(dot)com> |
Cc: | pgsql-general(at)hub(dot)org, pgsql-sql(at)hub(dot)org |
Subject: | Re: [SQL] RE: [GENERAL] Problem with SELECT on large negative INT4 |
Date: | 2000-01-28 04:49:14 |
Message-ID: | 9485.949034954@sss.pgh.pa.us |
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Lists: | pgsql-general pgsql-hackers pgsql-sql |
John Brothers <johnbr(at)mindspring(dot)com> writes:
> I don't think that patch will work - Hiroshi whipped up that patch for
> me a week ago for a different problem - we have a table with duplicate
> primary keys, which seems to be an arithmetic overflow problem because
> the index key values can be both very large positive and very large
> negative numbers.
Actually, if Nicolas' table contains both very large positive and very
large negative integers, then his index could be messed up pretty badly.
What Hiroshi saw (and I missed :-() was that btint4cmp can fail and
return a result of the wrong sign if the difference between two integers
overflows. Since index sorting depends critically on the assumption
that the comparator always returns consistent results (a < b and b < c
must imply a < c, but this can fail if a - c overflows), you could have
an out-of-order index. And then probes into the index could fail to
find items they should find ... which is exactly the complained-of
symptom.
Hiroshi neglected to mention that you'd probably need to drop and
recreate the index after applying the patch; if it's indeed out of
order, just patching the comparator bug isn't enough to fix it.
regards, tom lane
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