Re: Index on immutable function call

In response to Philippe Lang :
> Hi,
>
> I'm trying to figure out how to use an index on an immutable function
> call in order to speed up queries.
> My goal is to make query...
>
> select * from indexed_table WHERE data1 >
> this_is_a_long_transformation(data2);
>
> ... as fast as
>
> select * from indexed_table WHERE data1 > data2;
>
> ... with the help of the index "long_transformation_index".
>
>
> Unfortunately, Postgreql does not use the index at all.
>
> What am I doing wrong? I use the default query tuning options of
> Postgresql 8.3.7.

The planner doesn't know the actual parameter for the function, so he
picked out the wrong plan. You can force the planner to re-planning with
a dynamical statement within the function and EXECUTE that.

http://www.postgresql.org/docs/current/static/plpgsql-statements.html#PLPGSQL-STATEMENTS-EXECUTING-DYN

In short: build a string that contains your query and EXECUTE that
string, within your function.

Regards, Andreas
--
Andreas Kretschmer
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