migration doc

docs/source/migrating_to_1.18.rst

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+Migrating to Koji 1.18
+======================
+
+..
+  reStructured Text formatted
+
+You should consider the following changes when migrating to 1.18:
+
+DB Updates
+----------
+
+This release has a few schema changes:
+
+    * Several new indexes to speed operations
+    * A ``cg_id`` field has been added to the ``build`` table
+    * A new ``build_reservations`` table
+    * A new ``build_notifications_block`` table
+    * Updates to the data in the ``archivetypes`` table
+
+As in previous releases, we provide a migration script that updates the
+database.
+
+::
+
+    # psql koji koji  </usr/share/doc/koji/docs/schema-upgrade-1.17-1.18.sql
+
+
+Other changes
+-------------
+
+There are numerous other changes in 1.18 that should not have a direct impact
+on migration. For details see:
+:doc:`release_notes_1.18`