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Re: count(*) slow on large tables

From: Dror Matalon <dror(at)zapatec(dot)com>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: count(*) slow on large tables
Date: 2003-10-02 22:33:13
Message-ID: 20031002223313.GE87525@rlx11.zapatec.com (view raw or flat)
Thread:
Lists: pgsql-hackerspgsql-performance
I don't have an opinion on how hard it would be to implement the
tracking in the indexes, but "select count(*) from some table" is, in my
experience, a query that people tend to run quite often. 
One of the databases that I've used, I believe it was Informix, had that
info cached so that it always new how many rows there were in any
table. It was quite useful.


On Thu, Oct 02, 2003 at 05:57:30PM -0400, Christopher Browne wrote:
> jllachan(at)nsd(dot)ca (Jean-Luc Lachance) writes:
> > That's one of the draw back of MVCC.  
> > I once suggested that the transaction number and other house keeping
> > info be included in the index, but was told to forget it...
> > It would solve once and for all the issue of seq_scan vs index_scan.
> > It would simplify the aggregate problem.
> 
> It would only simplify _one_ case, namely the case where someone cares
> about the cardinality of a relation, and it would do that at
> _considerable_ cost.
> 
> A while back I outlined how this would have to be done, and for it to
> be done efficiently, it would be anything BUT simple.  
> 
> It would be very hairy to implement it correctly, and all this would
> cover is the single case of "SELECT COUNT(*) FROM SOME_TABLE;"
> 
> If you had a single WHERE clause attached, you would have to revert to
> walking through the tuples looking for the ones that are live and
> committed, which is true for any DBMS.
> 
> And it still begs the same question, of why the result of this query
> would be particularly meaningful to anyone.  I don't see the
> usefulness; I don't see the value of going to the considerable effort
> of "fixing" this purported problem.
> -- 
> let name="cbbrowne" and tld="libertyrms.info" in String.concat "@" [name;tld];;
> <http://dev6.int.libertyrms.com/>
> Christopher Browne
> (416) 646 3304 x124 (land)
> 
> ---------------------------(end of broadcast)---------------------------
> TIP 4: Don't 'kill -9' the postmaster

-- 
Dror Matalon, President
Zapatec Inc 
1700 MLK Way
Berkeley, CA 94709
http://www.zapatec.com

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