From: Christoph Haller <ch@rodos.fzk.de>
Message-Id: <200402091502.QAA19246@rodos>
Subject: Re: Index not used - now me
To: paul@tmsl.demon.co.uk (Paul Thomas)
Date: Mon, 09 Feb 2004 16:02:40 MET
Cc: pgsql-sql@postgresql.org
In-Reply-To: <20040209142819.A12693@bacon>;
	from "Paul Thomas" at Feb 9, 104 3:40 pm

> 
> 
> On 09/02/2004 12:50 Christoph Haller wrote:
> 
> > 
> > \d Transfer_ModRange
> >    Table "public.transfer_modrange"
> >      Column     |   Type   | Modifiers
> > ----------------+----------+-----------
> >  module_pointer | smallint | not null
> >  from_module    | smallint | not null
> >  to_module      | smallint | not null
> >  primary_key    | integer  | not null
> > Indexes: tmr_primkey_index btree (primary_key)
> > 
> > explain analyze SELECT Module_Pointer FROM Transfer_ModRange
> >  WHERE Primary_Key  = 13 AND From_Module <= 2 AND To_Module   >= 2 ;
> >                                                               QUERY PLAN
> > --------------------------------------------------------------------------------------------------------------------------------------
> >  Index Scan using tmr_primkey_index on transfer_modrange
> > (cost=0.00..115.09 rows=14 width=2) (actual time=2.11..2.11 rows=0
> > loops=1)
> >    Index Cond: (primary_key = 13)
> >    Filter: ((from_module <= 2) AND (to_module >= 2))
> >  Total runtime: 2.46 msec
> > 
> > Now
> > set enable_seqscan to on ;
> > explain analyze SELECT Module_Pointer FROM Transfer_ModRange
> >  WHERE Primary_Key  = 13 AND From_Module <= 2 AND To_Module   >= 2 ;
> >                                                   QUERY PLAN
> > --------------------------------------------------------------------------------------------------------------
> >  Seq Scan on transfer_modrange  (cost=0.00..104.93 rows=14 width=2)
> > (actual time=45.91..45.91 rows=0 loops=1)
> >    Filter: ((primary_key = 13) AND (from_module <= 2) AND (to_module >=
> > 2))
> >  Total runtime: 46.19 msec
> > 
> > That's odd. May I please have an explanation for this.
> > Probably I should mention both tables have far less than 10.000 tuples.
> > VACUUM and ANALYZE was done just before.
> 
> That's because it's acually more efficent to do a seqscan on your small 
> table. When you have only a small table (like many of us do when testing), 
> the whole table will probably fit on one 8K page so the lowest cost 
> operation (= quickest) is to get that page.  It was disabling seqscan that 
> was forcing an index scan to appear to be the least costly operation. BTW, 
> you can't actually prevent PG doing a seqscan if there's no alternative 
> plan. All set enable_seqscan = false does is make a seqscan appear very 
> expensive so that the planner is less likely to pick it.
> 
> HTH
> 
> -- 
> Paul Thomas
> 
Thanks for your reply so far, but there is one thing I still don't understand. 
You wrote 
It was disabling seqscan that 
was forcing an index scan to appear to be the least costly operation. 

Why appear? If the Index Scan has a Total runtime: 2.46 msec and the Seq Scan 
a Total runtime: 46.19 msec, then the Index Scan is much faster. 
Or am I completely off the track reading the explain analyze output? 

Again, thanks for your time. 

Regards, Christoph