Create an index for the hash component of derivation filenames

This is to aid rendering of narinfo files. They're requested with the path
/HASH.narinfo, so to quickly find the relevant derivation, this index can be
used.

sqitch/deploy/derivations_hash_index.sql

@@ -0,0 +1,7 @@
+-- Deploy guix-data-service:derivations_hash_index to pg
+
+BEGIN;
+
+CREATE INDEX derivations_hash ON derivations (substring(file_name from 12 for 32));
+
+COMMIT;

sqitch/revert/derivations_hash_index.sql

@@ -0,0 +1,7 @@
+-- Revert guix-data-service:derivations_hash_index from pg
+
+BEGIN;
+
+DROP INDEX derivations_hash;
+
+COMMIT;

sqitch/sqitch.plan

@@ -38,3 +38,4 @@ sort_out_duplicate_builds 2019-12-05T12:43:53Z   <chris@phact> # Sort out duplic
 add_some_database_indexes 2019-12-05T15:53:04Z Christopher Baines <mail@cbaines.net> # Add some indexes
 derivation_output_sets 2019-12-05T23:19:05Z Christopher Baines <mail@cbaines.net> # Describe the sets of derivation outputs
 builds_add_derivation_output_details_set_id 2019-12-07T18:25:38Z Christopher Baines <mail@cbaines.net> # Add a derivation_output_details_set_id column to builds
+derivations_hash_index 2019-12-24T22:54:19Z Christopher Baines <mail@cbaines.net> # Add index on derivations for the hash component of the filename

sqitch/verify/derivations_hash_index.sql

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