Add some indexes

This is from reading the slow query log on data.guix.gnu.org.

sqitch/deploy/some_indexes.sql

@@ -0,0 +1,13 @@
+-- Deploy guix-data-service:some_indexes to pg
+
+BEGIN;
+
+CREATE INDEX nar_urls_nar_id ON nar_urls (nar_id);
+
+CREATE INDEX load_new_guix_revision_jobs_unprocessed
+  ON load_new_guix_revision_jobs (id) WHERE succeeded_at IS NULL;
+
+CREATE INDEX guix_revision_lint_warnings_guix_revision_id
+  ON guix_revision_lint_warnings (guix_revision_id);
+
+COMMIT;

sqitch/revert/some_indexes.sql

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

sqitch/sqitch.plan

@@ -79,3 +79,4 @@ guix_revision_package_derivations_add_package_derivation_index 2020-10-27T16:58:
 increase_derivation_inputs_statistics_targets 2020-12-27T10:34:58Z Christopher Baines <mail@cbaines.net> # Increase stats targets on derivation_inputs fields
 remove_guix_revisions_store_path 2021-02-02T20:06:18Z Christopher Baines <mail@cbaines.net> # Drop guix_revisions.store_path
 systems_table 2021-04-22T08:12:10Z Christopher Baines <mail@cbaines.net> # Add a systems table
+some_indexes 2021-05-17T17:36:38Z Christopher Baines <mail@cbaines.net> # Add some indexes

sqitch/verify/some_indexes.sql

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