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Re: Index corruption

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: "Marko Kreen" <markokr(at)gmail(dot)com>
Cc: "Marc Munro" <marc(at)bloodnok(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Index corruption
Date: 2006-06-30 13:55:54
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Lists: pgsql-hackers
"Marko Kreen" <markokr(at)gmail(dot)com> writes:
> The sl_log_* tables are indexed on xid, where the relations between
> values are not exactly stable.  When having high enough activity on
> one node or having nodes with XIDs on different enough positions
> funny things happen.

Yeah, that was one of the first things I thought about, but the range of
XIDs involved in these test cases isn't large enough to involve a
wraparound, and anyway it's now looking like the problem is loss of heap
entries, not index corruption at all.

Slony's use of XID comparison semantics for indexes is definitely pretty
scary though.  If I were them I'd find a way to get rid of it.  In
theory, since the table is only supposed to contain "recent" XIDs,
as long as you keep it vacuumed the index should never contain any
inconsistently-comparable XIDs ... but in a big index, the boundary keys
for upper-level index pages might hang around an awful long time ...

			regards, tom lane

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