The value n may be 1 or 0:
When 1 (true) SQL queries involving DISTINCT and GROUP BY run more efficiently by making better use of indices (if available).Caution:
The values returned by such queries are collated in the same way they are stored within the index. This means the results of such queries may be uppercase. This may have an effect on case-sensitive applications.
When 0 (false) optimization of DISTINCT queries is disabled.
0 or 1. The default is 1.
On the page System Administration > Configuration > SQL and Object Settings > General SQL Settings, for the DISTINCT optimization turned ON setting, choose Yes (true) or No (false).