Quick DISTINCT scans

Derby can use a hash table instead of a sorter to eliminate duplicates when performing a DISTINCT in the following cases:

• There is a single table in the query block.
• An ORDER BY clause is not merged into the DISTINCT.
• All entries in the SELECT list are simple column references.
• There are no predicates in the query block.

This technique allows for minimal locking when performing the scan at the READ COMMITTED isolation level.