Add Get The Size Of An Index as a postgres til

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

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+# Get The Size Of An Index
+
+Want to get an idea of how much disk space that additional index is taking
+up? You can query for it with the same methods discussed in [Get The Size Of
+A Table](get-the-size-of-a-table.md) and [Pretty Print Data
+Sizes](pretty-print-data-sizes.md).
+
+For instance, if I have a table with a `users_pkey` index and a
+`users_unique_lower_email_idx` index, I can check the sizes like so:
+
+```sql
+> select pg_size_pretty(pg_relation_size('users_pkey'));
+ pg_size_pretty
+----------------
+ 240 kB
+
+> select pg_size_pretty(pg_relation_size('users_unique_lower_email_idx'));
+ pg_size_pretty
+----------------
+ 704 kB
+```
+
+[source](https://www.niwi.nz/2013/02/17/postgresql-database-table-indexes-size/)