The following SQL demonstrates how an index on an expression that can be
non-NULL even if all its arguments are NULL can lead to an incorrect answer
in a LEFT JOIN.

<blockquote><verbatim>
CREATE TABLE t1(x);
CREATE TABLE t2(y, z);
INSERT INTO t1 VALUES('key');
INSERT INTO t2 VALUES('key', -1);
SELECT count(*) FROM t1 LEFT JOIN t2 ON (x=y) WHERE ifnull(z, 0) >=0;
CREATE INDEX t2i ON t2(y, ifnull(z, 0));
SELECT count(*) FROM t1 LEFT JOIN t2 ON (x=y) WHERE ifnull(z, 0) >=0;


This problem has been in the code ever since support for indexes on
expression was added by check-in [2131a5ca53f0e9b0b98a9dd9a20e495]
on 2015-09-04.  The problem first appeared in the 3.9.0 release.

This problem was discovered during internal testing and analysis
and has not been observed in the wild.