Quick DISTINCT scans
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.
This technique appears in RunTimeStatistics as a