Add Get The Size Of A Table as a postgres til

postgres/get-the-size-of-a-table.md

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+# Get The Size Of A Table
+
+In [Get The Size Of A Database](get-the-size-of-a-database.md), I showed a
+PostgreSQL administrative function, `pg_database_size()`, that gets the size
+of a given database. With the `pg_relation_size()` function, we can get the
+size of a given table. For instance, if we'd like to see the size of the
+`reservations` table, we can executing the following query:
+
+```sql
+> select pg_relation_size('reservations');
+ pg_relation_size
+------------------
+          1531904
+```
+
+This gives us the size of the `reservations` table in bytes. As you might
+expect, the referenced table needs to be part of the connected database and
+on the search path.
+
+See [the Postgres docs](http://www.postgresql.org/docs/current/static/functions-admin.html) for more details.