Re: Ignoring index on (A is null), (A is not null)

try this:
EXPLAIN [ANALYZE] SELECT a FROM table1 WHERE a IS NULL OR a>2;
SET enable_seqscan TO off;
EXPLAIN [ANALYZE] SELECT a FROM table1 WHERE a IS NULL OR a>2;

and compare the costs and times of both executions. This will tell you why postgresql is not using an index.

For example, if you have 1000 rows in your table, they will fit in only one page of the table, so postgresql will (correctly) think that fetching and procesing this only page will be faster than fetching the index page, procesing it, and fetching and procesing the table page.
Or perhaps there are so many rows that match your condition, that postgresql realizes that using and index or not it will still have to visit almost every page in the table.

Many things can cause postgresql to think that a seqscan is better than an indexscan, If after comparing the EXPLAINs you see that postgresql is wrong, you should tweak your postgresql.conf (for example the cpu_index_tuple_cost value).

hope it helps.

On Thu, 2003-10-30 at 08:34, Cestmir Hybl wrote:

Are you seeing this question as totally off-topic in this list, or there is
really no one who knows something about indexing "is null" bits in postgres?

Regards
CH


> Hi,
>
> suppose, for simplicity, there is a table with index like this:
>
> create table TABLE1 (
>   A integer
> );
> create index TABLE1_A on TABLE1 (A);
>
> My question is: why psql (7.3.3) does not use index when filtering by A IS
> NULL, A IS NOT
> NULL expressions?
>
> In fact, I need to filter by expression ((A is null) or (A > const)).
>
> Is there a way to filter by this expression using index?
>
> Functional index cannot be used (except strange solution with CASE-ing and
> converting NULL values into some integer constant)
>
>
>
> --------------------------------------------------------------------------
--
> --
>  Index Scan using table1_a on table1  (cost=0.00..437.14 rows=29164
width=4)
>    Index Cond: (a > 1000)
> --------------------------------------------------------------------------
--
> --
>  Seq Scan on table1  (cost=0.00..448.22 rows=1 width=4)
>    Filter: (a IS NULL)
> --------------------------------------------------------
>  Seq Scan on table1  (cost=0.00..448.22 rows=30222 width=4)
>    Filter: (a IS NOT NULL)
> ------------------------------------------------------------
>  Seq Scan on table1  (cost=0.00..523.77 rows=29164 width=4)
>    Filter: ((a IS NULL) OR (a > 1000))
> ------------------------------------------------------------
>
>
> CH


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