SQLite

The query below gets a different result with and without the index. The result without the index is correct.

CREATE TABLE t1(x);
INSERT INTO t1 VALUES(1);
CREATE TABLE t2(y, z);
SELECT ifnull(z, '!!!') FROM t1 LEFT JOIN t2 ON (x=y);
CREATE INDEX t2i ON t2(y, ifnull(z, '!!!'));
SELECT ifnull(z, '!!!') FROM t1 LEFT JOIN t2 ON (x=y);

This problem appears to have been introduced by check-in [a47efb7c8520a01110ce3] on 2017-04-11, and first released with version 3.19.0. The problem was discovered during internal testing and has not been observed in the wild.