Optimize searching for users on Postgres (#9782)

The searches on Postgres using LIKE with prefix matching will only
use an index if it has the text_pattern_ops flag (forcing C collation).
This drops the old indexes and adds properly optimized ones.

store/sqlstore/upgrade.go

@@ -521,6 +521,14 @@ func UpgradeDatabaseToVersion56(sqlStore SqlStore) {
 
 	// migrating user's accepted terms of service data into the new table
 	sqlStore.GetMaster().Exec("INSERT INTO UserTermsOfService SELECT Id, AcceptedTermsOfServiceId as TermsOfServiceId, :CreateAt FROM Users WHERE AcceptedTermsOfServiceId != \"\" AND AcceptedTermsOfServiceId IS NOT NULL", map[string]interface{}{"CreateAt": model.GetMillis()})
+
+	if sqlStore.DriverName() == model.DATABASE_DRIVER_POSTGRES {
+		sqlStore.RemoveIndexIfExists("idx_users_email_lower", "lower(Email)")
+		sqlStore.RemoveIndexIfExists("idx_users_username_lower", "lower(Username)")
+		sqlStore.RemoveIndexIfExists("idx_users_nickname_lower", "lower(Nickname)")
+		sqlStore.RemoveIndexIfExists("idx_users_firstname_lower", "lower(FirstName)")
+		sqlStore.RemoveIndexIfExists("idx_users_lastname_lower", "lower(LastName)")
+	}
 	//saveSchemaVersion(sqlStore, VERSION_5_6_0)
 	//}
 }