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(at)nasby(dot)net
EnterpriseDB http://enterprisedb.com 512.569.9461 (cell)