Indexes, when added to the right columns, can provide massive performance gains for certain queries. Indexes aren't free though. It is worth noting that they take up disk space. The amount of disk space they take is generally irrelevant, but it is at least worth being aware of. Especially if you're deal with a massive table.
You can check the current size of an index on disk with PostgreSQL's
pg_relation_size
function.
First, you'll want to look up the name of the index you're curious about.
Running \d table_name (replacing table_name with the name of the table the
index is on) will show you everything about the table including its indexes and
their names.
Then run the following query:
select pg_size_pretty(pg_relation_size('index_users_on_email'));
pg_size_pretty
----------------
41 MB
(1 row)This one is pretty small. They can get pretty big though. I've seen some that take up over 1GB on disk.