Re: Problem with index when using function

Thanks if you started to look into this, but nevermind.  I figured it
out.  Turns out I only needed to mark the function as STABLE:

http://www.postgresql.org/docs/8.0/static/xfunc-volatility.html

"A STABLE function cannot modify the database and is guaranteed to
return the same results given the same arguments for all calls within a
single surrounding query. This category allows the optimizer to optimize
away multiple calls of the function within a single query. In
particular, it is safe to use an expression containing such a function
in an index scan condition. (Since an index scan will evaluate the
comparison value only once, not once at each row, it is not valid to use
a VOLATILE function in an index scan condition.)"

-- Dante



D. Dante Lorenso wrote:
> All,
>
> I have a weird situation where my index IS used when I use a query that
> hard-codes a value but it does NOT use the index when the value is
> returned from a PGSQL function:
>
> ======================================================================
> DOES NOT WORK
> ======================================================================
>
> svcprod=# EXPLAIN SELECT
>     COALESCE(SUM(start_count), 0) AS start_count,
>     COALESCE(SUM(end_count), 0) AS end_count,
>     COALESCE(SUM(total_playtime), 0) AS total_playtime
>   FROM audio_file_stats
>   WHERE afile_id = num2id(1173, 1075, 'audio_file', 'audio_id',
> 'afile_id')::bigint;
>
>     QUERY PLAN
> ----------------------------------------------------------------------
>  Aggregate  (cost=118677.35..118677.36 rows=1 width=19)
>    ->  Seq Scan on audio_file_stats  (cost=0.00..118675.33 rows=268
> width=19)
>          Filter: (afile_id = num2id(1173::bigint, 1075::bigint,
> 'audio_file'::character varying, 'audio_id'::character varying,
> 'afile_id'::character varying))
> (3 rows)
>
> ======================================================================
> WORKS
> ======================================================================
>
> byoaudio=# EXPLAIN SELECT
>     COALESCE(SUM(start_count), 0) AS start_count,
>     COALESCE(SUM(end_count), 0) AS end_count,
>     COALESCE(SUM(total_playtime), 0) AS total_playtime
>   FROM audio_file_stats
>   WHERE afile_id = 48702;
>
> QUERY PLAN
> ----------------------------------------------------------------------
>  Aggregate  (cost=672.69..672.70 rows=1 width=19)
>    ->  Index Scan using audio_file_stats_idx_afile_id on
> audio_file_stats  (cost=0.00..670.73 rows=261 width=19)
>          Index Cond: (afile_id = 48702)
> (3 rows)
>
> ======================================================================
>
> The function I use is defined as using returning a BIGINT which is the
> same datatype as is used by the index:
>
> FUNCTION "public"."num2id" (in_acct_id bigint, in_value bigint,
>   in_table_name varchar, in_input_column varchar,
>   in_output_column varchar) RETURNS bigint
>
> Can someone help explain what is being done wrong here?  I'm using 8.2.4.
>
> -- Dante
>
>
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