Date: Tue, 31 Oct 2006 19:20:26 -0500
From: mark@mark.mielke.cc
To: Ivan Voras <ivoras@fer.hr>
Cc: pgsql-performance@postgresql.org
Subject: Re: MVCC & indexes?
Message-ID: <20061101002025.GA21873@mark.mielke.cc>
References: <4547C65C.6090208@fer.hr>
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On Tue, Oct 31, 2006 at 10:55:40PM +0100, Ivan Voras wrote:
> Ok, so MVCC is the best thing since a guy put a round stone on a stick
> and called it "the wheel", but I've seen several references on this list
> about "indexes not being under MVCC" - at least that's how I read it,
> the original posts were explaining why indexes can't be used for solving
> MIN()/MAX()/COUNT() aggregates. Is this correct?

> In particular, I'm trying to find out is there (b)locking involved when
> concurrently updating and/or inserting records in an indexed table. My
> guess is that, since PG does copy+delete on updating, even updating a
> non-indexed field will require fixups in the index tree (to point to the
> new record) and thus (b)locking.

Short bits of blocking. The PostgreSQL index 'problem', is that indexes
are conservative. They only guarantee to return at least as much data as
you should see. They cannot be used to limit what you see to only as much
as you should see.

Cheers,
mark

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