Re: Index ignored with "is not distinct from", 8.2 beta2

One issue is that I'm not sure think you've got your sugar quite right.
Have you tested with:

(col IS NOT NULL AND 123 IS NOT NULL AND col = 123) OR   (col IS NULL and 123 IS NULL) ?

It's possible that the planner doesn't know about using an index for
DISTINCT; or it might just want an index that's defined WHERE col IS NOT
NULL.

On Wed, Nov 01, 2006 at 03:29:33PM -0500, JEAN-PIERRE PELLETIER wrote:
> I've reposted this from pgsql-performance where I got no response.
> 
> ==========================================
> 
> Hi,
> 
> I wanted to use "exp1 is not distinct from exp2" which I tough was syntaxic 
> sugar for
> exp1 is not null and exp2 is not null and exp1 = exp2 or exp1 is null and 
> exp2 is null
> but my index is ignored with "is not distinct from".
> 
> Is this the expected behavior ?
> 
> create temporary table t as select * from generate_series(1,1000000) t(col);
> create unique index i on t(col);
> analyze t;
> 
> -- These queries don't use the index
> select count(*) from t where col is not distinct from 123;
> select count(*) from t where not col is distinct from 123;
> 
> -- This query use the index
> select count(*) from t where col is not null and 123 is not null and col = 
> 123 or col is null and 123 is null;
> 
> explain analyze select count(*) from t where col is not distinct from 123;
>                                                 QUERY PLAN
> ------------------------------------------------------------------------------------------------------------
> Aggregate  (cost=19154.79..19154.80 rows=1 width=0) (actual 
> time=228.200..228.202 rows=1 loops=1)
>   ->  Seq Scan on t  (cost=0.00..17904.90 rows=499956 width=0) (actual 
> time=0.042..228.133 rows=1 loops=1)
>         Filter: (NOT (col IS DISTINCT FROM 123))
> Total runtime: 228.290 ms
> (4 rows)
> Time: 219.000 ms
> 
> explain analyze select count(*) from t where not col is distinct from 123;
>                                                 QUERY PLAN
> ------------------------------------------------------------------------------------------------------------
> Aggregate  (cost=19154.79..19154.80 rows=1 width=0) (actual 
> time=235.950..235.952 rows=1 loops=1)
>   ->  Seq Scan on t  (cost=0.00..17904.90 rows=499956 width=0) (actual 
> time=0.040..235.909 rows=1 loops=1)
>         Filter: (NOT (col IS DISTINCT FROM 123))
> Total runtime: 236.065 ms
> (4 rows)
> Time: 250.000 ms
> 
> explain analyze select count(*) from t where col is not null and 123 is not 
> null and col = 123 or col is null and 123 is null;
>                                                QUERY PLAN
> -----------------------------------------------------------------------------------------------------------
> Aggregate  (cost=8.13..8.14 rows=1 width=0) (actual time=0.267..0.268 
> rows=1 loops=1)
>   ->  Index Scan using i on t  (cost=0.00..8.13 rows=1 width=0) (actual 
> time=0.237..0.241 rows=1 loops=1)
>         Index Cond: (col = 123)
> Total runtime: 0.366 ms
> (4 rows)
> Time: 0.000 ms
> 
> I am on Windows XP Service pack 2 with PostgreSQL 8.2 beta2
> 
> Thanks,
> Jean-Pierre Pelletier
> e-djuster
> 
> 
> 
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-- 
Jim Nasby                                            jim@nasby.net
EnterpriseDB      http://enterprisedb.com      512.569.9461 (cell)