PSQL: Get The Size Of An Index

Want to get an idea of how much disk space that additional index is taking up?

For instance, if I have a table with a users_key index and a users_unique_lower_email_idx index, I can check the sizes like so:

> select pg_size_pretty(pg_relation_size('users_key'));
 pg_size_pretty
----------------
 240 kB

> select pg_size_pretty(pg_relation_size('users_unique_lower_email_idx'));
 pg_size_pretty
----------------
 704 kB

Via jbranchaud/til.