Optimize submission deletion

The nested select was producing a sequential lookup into the books
table, that took hours to resolv.

lib/database/database.go

@@ -246,6 +246,7 @@ CREATE INDEX IF NOT EXISTS visits_book_id_idx on visits(book_id);
 CREATE INDEX IF NOT EXISTS submissions_id_idx on submissions(submission_id);
 CREATE INDEX IF NOT EXISTS submissions_book_idx on submissions(book_id);
 CREATE INDEX IF NOT EXISTS submissions_user_idx on submissions(user_id);
+CREATE INDEX IF NOT EXISTS submissions_last_modified_idx on submissions(last_modified);
 
 -- Submissions constraint
 ALTER TABLE ONLY submissions

lib/database/submissions.go

@@ -100,11 +100,21 @@ func (db *pgDB) submissionsCleaner() {
 	for true {
 		time.Sleep(periodicity)
 		_, err := db.sql.Model(&Submission{}).
-			Where("book_id is null OR book_id in (SELECT id FROM books WHERE active is true)").
+			Where("book_id is null").
 			Where("last_modified < CURRENT_TIMESTAMP - interval ?", fmt.Sprintf("%d days", daysExpireSubmission)).
 			Delete()
 		if err != nil {
-			log.Error("Error deleting submissions: ", err)
+			log.Error("Error deleting null submissions: ", err)
+		}
+
+		_, err = db.sql.Model(&Submission{}).
+			TableExpr(`books`).
+			Where("book_id = books.id").
+			Where("books.active is true").
+			Where("last_modified < CURRENT_TIMESTAMP - interval ?", fmt.Sprintf("%d days", daysExpireSubmission)).
+			Delete()
+		if err != nil {
+			log.Error("Error deleting approved submissions: ", err)
 		}
 	}
 }