Add an index on derivation_output_details.id

Where hash is not null, since this helps with the fixed output package
derivations query.

sqitch/deploy/package_derivations_not_null_hash_index.sql

@@ -0,0 +1,7 @@
+-- Deploy guix-data-service:package_derivations_not_null_hash_index to pg
+
+BEGIN;
+
+CREATE INDEX derivation_output_details_id_hash_not_null_idx ON derivation_output_details (id) WHERE hash IS NOT NULL;
+
+COMMIT;

sqitch/revert/package_derivations_not_null_hash_index.sql

@@ -0,0 +1,7 @@
+-- Revert guix-data-service:package_derivations_not_null_hash_index from pg
+
+BEGIN;
+
+-- XXX Add DDLs here.
+
+COMMIT;

sqitch/sqitch.plan

@@ -82,3 +82,4 @@ systems_table 2021-04-22T08:12:10Z Christopher Baines <mail@cbaines.net> # Add a
 some_indexes 2021-05-17T17:36:38Z Christopher Baines <mail@cbaines.net> # Add some indexes
 package_metadata_location_id_index 2021-05-27T19:51:13Z Canan Talayhan <canan.t.talayhan@gmail.com> # Add index for location id
 packages_replacement 2021-04-24T04:52:57Z Christopher Baines <mail@cbaines.net> # Add packages.replacement_package_id
+package_derivations_not_null_hash_index 2021-07-11T14:19:32Z Christopher Baines <mail@cbaines.net> # Add an index to package_derivations

sqitch/verify/package_derivations_not_null_hash_index.sql

@@ -0,0 +1,7 @@
+-- Verify guix-data-service:package_derivations_not_null_hash_index on pg
+
+BEGIN;
+
+-- XXX Add verifications here.
+
+ROLLBACK;