subject, predicate, object and labels are not indexed, thus, using a where clause on these fields for the DELETE statement does a full-table scan. Using *_hash columns instead will use the indexes.
It appears that preparing an INSERT statement on PostgreSQL actually makes it expecting to receive INSERTs and thus, it create some kind of locks for it. If instead, you only send him DELETE statements, it will indefinitely wait for an INSERT and will hung.
When `tx.Exec` fails and an error is returned, result is nil. Thus, `result.RowsAffected()` panics.
* Add a missing return statement in the error test.
* Add a second missing return statement just below to return the proper error message if we can't execute `RowsAffected` instead of saying that the triple doesn't exist.
