Use index of geometry inside user-defined function

Question

create function are_close_to_each_other(g1 geometry, g2 geometry)
returns boolean
language sql immutable as $$
   select ST_DWithin(g1, g2, 100);
$$;

select x.*, y.*
from sometable x, sometable y
where are_close_to_eachother(x.geom, y.geom);

I have an index on sometable (geom). Will this index be used inside function are_close_to_each_other? If not, how to make it use it?

Answer 1

It will be used the same way it would be used if the query looked like this:

select x.*, y.*
from sometable x, sometable y
where ST_DWithin(x.geom, y.geom, 100);

Documentation states that ST_DWithin uses available indexes.

You can make sure by running:

explain analyze
select x.*, y.*
from sometable x, sometable y
where are_close_to_each_other(x.geom, y.geom);

If there is "index scan" in the output it means that the indexes were used.