Add a index to derivation_output_details_sets

This helps with queries like:

  SELECT *
  FROM derivation_output_details_sets
  WHERE ARRAY[5563227] <@ derivation_output_details_ids;

sqitch/deploy/derivation_output_details_sets_derivation_output_details_ids_index.sql

@@ -0,0 +1,9 @@
+-- Deploy guix-data-service:derivation_output_details_sets_derivation_output_details_ids_index to pg
+
+BEGIN;
+
+CREATE INDEX derivation_output_details_sets_gin_idx
+  ON derivation_output_details_sets
+  USING GIN (derivation_output_details_ids);
+
+COMMIT;

sqitch/revert/derivation_output_details_sets_derivation_output_details_ids_index.sql

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

sqitch/sqitch.plan

@@ -65,3 +65,4 @@ build_status_nullable_timestamp 2020-06-27T12:34:29Z Christopher Baines <mail@cb
 build_add_build_server_build_id 2020-06-27T12:48:57Z Christopher Baines <mail@cbaines.net> # Add builds.build_server_build_id
 build_status_add_unique_index 2020-06-30T17:19:30Z Christopher Baines <mail@cbaines.net> # Add a unique index to the build_status table
 builds_remove_build_server_id_derivation_unique_constraint 2020-06-30T20:17:48Z Christopher Baines <mail@cbaines.net> # Allow multiple builds of the same derivation per build server
+derivation_output_details_sets_derivation_output_details_ids_index 2020-07-04T07:56:49Z Christopher Baines <mail@cbaines.net> # Add GIN index on derivation_output_details_ids

sqitch/verify/derivation_output_details_sets_derivation_output_details_ids_index.sql

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