Markus Schiltknecht wrote:
> Hi,
>
> when using LIMIT, how do I tell the planner to only call a function for
> rows it returns?
>
> An example: I want to fetch the top five categories. A function
> get_category_text_path(cat_id int) returns the textual representation of
> the category. For that I do something like:
>
> SELECT id, get_category_text_path(id)
> FROM category
> ORDER BY rank
> LIMIT 5
>
> Unfortunately this takes very long because it calls
> get_category_text_path() for all of the 450'000 categories in the table.
> But I only need the full text path of the top five rows.
SELECT id, get_category_text_path(id)
FROM (
SELECT id FROM category ORDER BY rank LIMIT 5
) AS foo
HTH
--
Richard Huxton
Archonet Ltd