Add Get The Size On Disk Of An Index as a Postgres til

postgres/get-the-size-on-disk-of-an-index.md

@@ -0,0 +1,29 @@
+# Get The Size On Disk Of An Index
+
+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](https://www.postgresql.org/docs/current/functions-admin.html).
+
+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:
+
+```sql
+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.