I think that the PostgreSQL query planner just thinks that it's not worth applying the index as the LIMIT is too small.

Also, there could be reasons PostgreSQL doesn't use the index, for example:

• PostgreSQL doesn't use indexes when datatypes don't match properly, you may need to include appropriate casts, or redefine you index.

• Your planner settings might be causing problems.

For optimizing your query you might want to refer to some form of documentation for query performance optimization.