Question

The problem: we have a very complex search query. If its result yields too few rows we expand the result by UNIONing the query with a less strict version of the same query.

We are discussing wether a different approach would be faster and/or better in quality. Instead of UNIONing we would create a custom sql function which would return a matching score. Then we could simply order by that matching score.

Regarding performance: will it be slower than a UNION?

We use PostgreSQL.

Any suggestions would be greatly appreciated.

Thank you very much
Max

Answer 1

You want to order by the “return value” of your custom function? Then the database server can’t use an index for that. The score has to be calculated for each record in the table (that hasn’t been excluded with a WHERE clause) and stored in some temporary storage/table. Then the order by is performed on that temporary table. So this easily can get slower than your union queries (depending on your union statements of course).