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Re: Seqscan rather than Index

From: Richard Huxton <dev(at)archonet(dot)com>
To: David Brown <time(at)bigpond(dot)net(dot)au>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: Seqscan rather than Index
Date: 2004-12-17 13:03:50
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
David Brown wrote:
>> You might want to reduce random_page_cost a little.
>> Keep in mind that your test case is small enough to fit in RAM and
>> is probably not reflective of what will happen with larger tables.
> I am also running 8.0 rc1 for Windows. Despite many hours spent
> tweaking various planner cost constants, I found little effect on
> cost estimates. Even reducing random_page_cost from 4.0 to 0.1 had
> negligible impact and failed to significantly influence the planner.

I'm not sure setting random_page_cost below 1.0 makes much sense.

> Increasing the statistics target for the last_name column to 250 or
> so *may* help, at least if you're only selecting one name at a time.

Not going to do anything in this case. The planner is roughly right 
about how many rows will be returned, it's just not expecting everything 
to be in RAM.

> That's the standard advice around here and the only thing I've found
> useful. Half the threads in this forum are about under-utilized
> indexes. It would be great if someone could admit the planner is
> broken and talk about actually fixing it!

Not sure I agree here - when the stats are accurate, you can get the 
planner to make near-optimal choices most of the time. Is there any 
particular pattern you've seen?

> I'm unconvinced that the planner only favours sequential scans as
> table size decreases. In my experience so far, larger tables have the
> same problem only it's more noticeable.

Hmm - assuming your statistics are good, this would suggest the other 
cost settings just aren't right for your hardware.

> The issue hits PostgreSQL harder than others because of its awful
> sequential scan speed, which is two to five times slower than other
> DBMS. The archives show there has been talk for years about this, but
> it seems, no solution. The obvious thing to consider is the block
> size, but people have tried increasing this in the past with only
> marginal success.

Must admit this puzzles me. Are you saying you can't saturate your disk 
I/O? Or are you saying other DBMS store records in 0.5 to 0.2 times less 
space than PG?

   Richard Huxton
   Archonet Ltd

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