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Run this script:
DROP TABLE IF EXISTS t1,t2;
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CREATE TABLE t1 (a DOUBLE PRIMARY KEY);
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INSERT INTO t1 VALUES (1.8446744073709552e19);
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DROP TABLE IF EXISTS t2;
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CREATE TABLE t2 (a BIGINT UNSIGNED);
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INSERT INTO t2 VALUES (18446744073709551614),(18446744073709551615);
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SELECT t1.* FROM t1 JOIN t2 USING(a);
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SELECT t1.* FROM t1 LEFT JOIN t2 USING(a);
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Both SELECT queries return two rows:
+-----------------------+
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| a |
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+-----------------------+
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| 1.8446744073709552e19 |
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| 1.8446744073709552e19 |
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+-----------------------+
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2 rows in set (0.00 sec)
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This is correct, because comparison is done as DOUBLE, and the two BIGINT values are mapped into the same DOUBLE value:
mysql> SELECT CAST(a AS DOUBLE) FROM t2;
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+-----------------------+
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| CAST(a AS DOUBLE) |
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+-----------------------+
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| 1.8446744073709552e19 |
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| 1.8446744073709552e19 |
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+-----------------------+
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2 rows in set (0.01 sec)
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Now add a primary key on t2 and rerun the same SELECT queries:
ALTER TABLE t2 ADD PRIMARY KEY(a);
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SELECT t1.* FROM t1 JOIN t2 USING(a);
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SELECT t1.* FROM t1 LEFT JOIN t2 USING(a);
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Now the first query with a natural join returns 0 rows,
and the second query with a left join return 1 row.
This looks wrong. Both queries should return 2 rows.
EXPLAIN for the natural join reports eq_ref join method:
mysql> EXPLAIN SELECT t1.* FROM t1 JOIN t2 USING(a);
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+------+-------------+-------+--------+---------------+---------+---------+-----------+------+--------------------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
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+------+-------------+-------+--------+---------------+---------+---------+-----------+------+--------------------------+
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| 1 | SIMPLE | t1 | index | PRIMARY | PRIMARY | 8 | NULL | 1 | Using index |
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| 1 | SIMPLE | t2 | eq_ref | PRIMARY | PRIMARY | 8 | test.t1.a | 1 | Using where; Using index |
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+------+-------------+-------+--------+---------------+---------+---------+-----------+------+--------------------------+
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2 rows in set (0.00 sec)
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which is wrong.
EXPLAIN for the LEFT join says that t2 is eliminated:
mysql> EXPLAIN SELECT t1.* FROM t1 LEFT JOIN t2 USING(a);
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+------+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
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| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
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+------+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
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| 1 | SIMPLE | t1 | index | NULL | PRIMARY | 8 | NULL | 1 | Using index |
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+------+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
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1 row in set (0.00 sec)
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which is also wrong.
The same wrong behaviour is repeatable if I change the data type for t2.a from BIGINT to DECIMAL(30).
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