Re: Yet again on indices...

On Wed, 2002-02-27 at 14:48, Jean-Paul ARGUDO wrote:
> 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;
> 
...

> 
> 
> => Uh? Seq scan cheaper than index???  
> 
> => let's disable seqscan to read cost of index:
> postgresql.conf : enable_seqscan = false

You could just do 

set enable_seqscan to 'off'

in sql

> 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

It often is. Really.

> ----
> 
> What's your opinion? 

What are the real performance numbers ?

If they are other than what postgresql optimiser thinks you can change
them in system table.

----------------
Hannu