use better index for sessions

Fixes: https://pagure.io/koji/issue/1372

docs/schema-upgrade-1.17-1.18.sql

@@ -8,5 +8,7 @@ BEGIN;
 UPDATE archivetypes SET extensions = 'tar tar.gz tar.bz2 tar.xz tgz' WHERE name = 'tar';
 INSERT INTO archivetypes (name, description, extensions) VALUES ('vhdx', 'Hyper-V Virtual Hard Disk v2 image', 'vhdx');
 
+-- add better index for sessions
+CREATE INDEX sessions_expired ON sessions(expired);
 
 COMMIT;

docs/schema.sql

@@ -118,6 +118,7 @@ CREATE TABLE sessions (
 ) WITHOUT OIDS;
 CREATE INDEX sessions_master ON sessions(master);
 CREATE INDEX sessions_active_and_recent ON sessions(expired, master, update_time) WHERE (expired IS NOT TRUE AND master IS NULL);
+CREATE INDEX sessions_expired ON sessions(expired);
 
 -- Channels are used to limit which tasks are run on which machines.
 -- Each task is assigned to a channel and each host 'listens' on one