Change a package_descriptions index to use MD5 of the description

To avoid the index data being too large.

This was first seen in guix commit 1bb4fd64b7bbe5a17eda6f0ca8105283c038f7c8:

  psql-query-error (fatal-error PGRES_FATAL_ERROR ERROR: index row size 2808
  exceeds maximum 2712 for index "package_descriptions_locale_description_key"
  HINT: Values larger than 1/3 of a buffer page cannot be indexed.

sqitch/deploy/change_package_descriptions_index.sql

@@ -0,0 +1,11 @@
+-- Deploy guix-data-service:change_package_descriptions_index to pg
+
+BEGIN;
+
+ALTER TABLE package_descriptions DROP CONSTRAINT
+  package_descriptions_locale_description_key;
+
+CREATE UNIQUE INDEX package_descriptions_locale_description_key
+  ON package_descriptions USING btree (locale, MD5(description));
+
+COMMIT;

sqitch/revert/change_package_descriptions_index.sql

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

sqitch/sqitch.plan

@@ -67,3 +67,4 @@ build_status_add_unique_index 2020-06-30T17:19:30Z Christopher Baines <mail@cbai
 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
 add-tsvectors-per-locale 2020-07-16T18:19:44Z daniela <daniela@linux-ijv5> # Add tsvectors per locale
+change_package_descriptions_index 2020-08-23T11:56:00Z Christopher Baines <mail@cbaines.net> # Change the package_descriptions index

sqitch/verify/change_package_descriptions_index.sql

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