Re: index theory

Karel Zak kirjutas K, 16.10.2002 kell 15:19:
> 
>  Hi,
> 
>  I have SQL query:
> 
>  SELECT * FROM ii WHERE i1='a' AND i2='b';
> 
>  There're indexes on i1 and i2. I know best solution is use one
>  index on both (i1, i2).
> 
>  The EXPLAIN command show that optimalizer wants to use one index:
> 
> test=# explain SELECT * FROM ii WHERE i1='a' AND i1='b';
>                                    QUERY PLAN                                    
> ---------------------------------------------------------------------------------
>  Index Scan using i1 on ii  (cost=0.00..4.83 rows=1 width=24)
>    Index Cond: ((i1 = 'a'::character varying) AND (i1 = 'b'::character varying))
> 
> 
>  It's right and I undererstand why not use both indexes. But I talked
> about it with one Oracle user and he said me Oracle knows use both indexes 
> and results from both index scans are mergeted to final result -- this is maybe 
> used if full access to table (too big rows?) is more expensive than 2x index 
> scan and final merge. Is in PG possible something like this?

There has been some talk about using bitmaps generated from indexes as
an intermediate step.

----------
Hannu