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Yet again on indices...

From: Jean-Paul ARGUDO <jean-paul(dot)argudo(at)idealx(dot)com>
To: pgsql-hackers(at)postgresql(dot)org
Cc: sdinot(at)idealx(dot)com, dbarth(at)idealx(dot)com, jean-paul(dot)argudo(at)idealx(dot)com
Subject: Yet again on indices...
Date: 2002-02-27 09:48:15
Message-ID: 20020227104815.A27363@singer.ird.idealx.com (view raw or flat)
Thread:
Lists: pgsql-hackers
Ok, 

I'm working on query analysis for a program in ecpg for business puposes. Look
at what I found on with PG 7.2: Please be cool with my french2english processor,
I got few bogomips in my brain dedicated to english (should have listen more in
class..):
----

line 962 (in the ecpg source..)

EXPLAIN SELECT t12_bskid, t12_pnb, t12_lne, t12_tck
FROM T12_20011231
WHERE t12_bskid >= 1   
ORDER BY t12_bskid, t12_pnb, t12_tck, t12_lne;

NOTICE:  QUERY PLAN:
 
Sort  (cost=3006.13..3006.13 rows=25693 width=46)
  ->  Seq Scan on t12_20011231  (cost=0.00..1124.20 rows=25693 width=46)

=> not good, table t12_20011231 as 26K tuples :-(

=> create index t12_idx_bskid_20011231 on t12_20011231 (t12_bskid);

Sort  (cost=3006.13..3006.13 rows=25693 width=46)
  ->  Seq Scan on t12_20011231  (cost=0.00..1124.20 rows=25693 width=46)

=> probably statistic refresh to be done: 
$ /usr/local/pgsql/bin/vacuumdb --analyze dbks

Sort  (cost=3006.13..3006.13 rows=25693 width=46)
  ->  Seq Scan on t12_20011231  (cost=0.00..1124.20 rows=25693 width=46)


=> Uh? Seq scan cheaper than index???  

=> let's disable seqscan to read cost of index:
postgresql.conf : enable_seqscan = false

Sort  (cost=3126.79..3126.79 rows=25693 width=46)
  ->  Index Scan using t12_idx_bskid_20011231 on t12_20011231
(cost=0.00..1244.86 rows=25693 width=46)

=> Uh? seq scan'cost is lower than index scan??  => mailto hackers

----

What's your opinion? 

I have to tell that this select opperates in a forloop statment . 
I hardly believe reading 26K tuples is cheaper thant index reading, but maybe
you'll ask me about buffers that should store de 26K tuples?...

But just after this query, there is another one that maybe will put data in
buffers, kicking t12_20011231 data blocks...

Well I feel a little stuck there. I'll continue with enable_scans=false, but
I feel bad beeing forced to do so... and still asking myself if this is good
idea.

Thanks for support, best regards.

-- 
Jean-Paul ARGUDO


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