Using an index to materialize a function - Mailing list pgsql-general

If I have a slow_function(), and I create an index of
slow_function(field), will Postgres use that index to avoid having to
recompute the function?

Example:

SELECT slow_function(field1) FROM table1 WHERE id = 5

It won't use the index on field1 to _find_ the record.  Can it use it
to compute the field?