Add Restart A Sequence as a postgres til.

postgres/restart-a-sequence.md

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+# Restart A Sequence
+
+In postgres, if you are truncating a table or doing some other sort of
+destructive action on a table in a development or testing environment, you
+may notice that the id sequence for the primary key just keeps plugging
+along from where it last left off. The sequence can be reset to any value
+like so:
+
+```sql
+> alter sequence my_table_id_seq restart with 1;
+ALTER SEQUENCE
+```
+
+[source](http://www.postgresql.org/docs/current/static/sql-altersequence.html)