DISTINCT

Tuple filtering is applied for a DISTINCT when the following criteria are met:
Note: The set of column references must be an in-order prefix of the index.
Here is the most common case in which tuple filtering will be applied:
SELECT DISTINCT c1 FROM t1
Equality predicates allow tuple filtering on the following:
SELECT DISTINCT c2
FROM t1
WHERE c1 = 5

SELECT DISTINCT c2, c4
FROM t1
WHERE c1 = 5 and c3 = 7
-- the columns don't have to be in the
-- same order as the index
SELECT DISTINCT c2, c1
FROM t1
Related concepts
GROUP BY