From PostgreSQL wiki

Selecting Distinct Values

The term "loose indexscan" is used in some other databases for the operation of using a btree index to retrieve the distinct values of a column efficiently; rather than scanning all equal values of a key, as soon as a new value is found, restart the search by looking for a larger value. This is much faster when the index has many equal keys.

Postgres does not support loose indexscans natively, but they can be emulated using a recursive CTE as follows:

WITH RECURSIVE
     t AS (SELECT min(col) AS col FROM tablename
           UNION ALL
           SELECT (SELECT min(col) FROM tablename WHERE col > t.col) FROM t WHERE t.col IS NOT NULL)
SELECT col FROM t WHERE col IS NOT NULL
UNION ALL
SELECT NULL WHERE EXISTS(SELECT * FROM tablename WHERE col IS NULL);

The recursive part of the query processes one new row in each pass, finding the smallest column value greater than that on the previous pass; the main query adds in a single null result if needed. The above query is therefore equivalent to

SELECT DISTINCT col FROM tablename;

but runs orders of magnitude faster when "col" has only a proportionally small number of distinct values.

Making use of a non-leading column of an index

The same technique can be used to allow a query to benefit from an index which has a leading column with few distinct values (and which would not naturally be specified in the query), and a 2nd column which is highly specific and is used by the query. The leading column can be introduced into the query in a way which does not alter the meaning.

To set up the example:

CREATE TABLE tablename (col integer NOT NULL, col2 integer NOT NULL);
INSERT INTO tablename SELECT floor(random()*5) AS col, (random()*1000000)::integer AS col2 FROM generate_series(1,10000000);
CREATE INDEX ON tablename (col,col2 );

The natural specification of the query, which cannot use the index and takes 1758 ms:

SELECT count(*) FROM tablename WHERE col2=9854 ;

The modified version which can benefit from the index, and runs in 2 ms:

SELECT count(*) FROM tablename WHERE col2=9854 AND col IN 
(
  WITH RECURSIVE
     t AS (SELECT min(col) AS col FROM tablename
           UNION ALL
           SELECT (SELECT min(col) FROM tablename WHERE col > t.col) FROM t WHERE t.col IS NOT NULL)
  SELECT col FROM t
);

Writing a correct version of this query would be much more complex if col could be NULL