indexing - Indexes in Postgresql -

we have table users (userid, username, category, pincode) in postgresql

if create indexes like:

1. create index category_idx(category) on users;
2. create index category_pincode_idx(category, pincode) on users;

and query table users as:

select * users category = somevalue;  

which index applied?

executing following statement reveal index execution planner pick depending on current data set:

explain select * users category = somevalue; 

please advised planner might chose not use index if amount of data not warrant use of index because full table scan on few rows might quicker.