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This script:
DROP TABLE IF EXISTS t1;
CREATE TABLE t1 (a VARCHAR(10)) ENGINE=MyISAM;
INSERT INTO t1 VALUES ('a'),('A');
SELECT * FROM t1 WHERE a='a' AND a <> _latin1'A' COLLATE latin1_bin;
correctly returns one row:
+------+ | a | +------+ | a | +------+
If I rewrite the query to use a derived table:
DROP TABLE IF EXISTS t1;
CREATE TABLE t1 (a VARCHAR(10)) ENGINE=MyISAM;
INSERT INTO t1 VALUES ('a'),('A');
SELECT * FROM (SELECT * FROM t1) AS table1 WHERE a='a' AND a <> _latin1'A' COLLATE latin1_bin;
it erroneously returns two rows:
+------+ | a | +------+ | a | | A | +------+
The problems happens because Item_direct_view_ref has cmp_context correctly set to STRING_RESULT, while its referenced field has cmp_context set to IMPOSSIBLE_RESULT. This makes equal field propagation replace the field to constant in <>.
The same bug causes ENUM column to return bad results:
DROP TABLE IF EXISTS t1;
CREATE TABLE t1 (a ENUM('5','6')) ENGINE=MyISAM;
INSERT INTO t1 VALUES ('5'),('6');
SELECT * FROM (SELECT * FROM t1) AS table1 WHERE a='5';
SELECT * FROM (SELECT * FROM t1) AS table1 WHERE a=1;
SELECT * FROM (SELECT * FROM t1) AS table1 WHERE a='5' AND a=1;
The first and the second query correctly return one row, equal to '5'. The third query erroneously returns empty set.
The same problem is also demonstrated in this script:
DROP TABLE IF EXISTS t1;
DROP VIEW IF EXISTS v1;
CREATE TABLE t1 (a varchar(10) character set cp1251 collate cp1251_ukrainian_ci, KEY (a)) ;
INSERT INTO t1 VALUES ('DD'), ('ZZ'), ('ZZ'), ('KK'), ('FF'), ('HH'),('MM'),('`1');
CREATE VIEW v1 AS SELECT * FROM t1;
SELECT * FROM t1 WHERE a <> 0 AND a = ' 1';
SELECT * FROM v1 WHERE a <> 0 AND a = ' 1';
The first SELECT returns empty set, the second SELECT returns one row.
The same problem is repeatable in this script:
DROP TABLE IF EXISTS t1;
CREATE TABLE t1 (a ENUM('5','6'));
INSERT INTO t1 VALUES ('5'),('6');
DROP VIEW IF EXISTS v1;
CREATE VIEW v1 AS SELECT * FROM t1;
SELECT * FROM t1 WHERE a='5' AND a<2;
SELECT * FROM v1 WHERE a='5' AND a<2;
The first SELECT correctly return one row, the second SELECT erroneously returns no rows.
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MySQL-5.7.8 returns a wrong result for this script:
DROP TABLE IF EXISTS t1; CREATE TABLE t1 (a VARCHAR(10)) ENGINE=MyISAM; INSERT INTO t1 VALUES ('a'),('A'); SELECT * FROM t1 WHERE a='a' AND a <> _latin1'A' COLLATE latin1_bin;The correct result is to return one row only.