BUG#16798: Inapplicable ref_or_null query plan and bad query result on random occasions
The bug was as follows: When merge_key_fields() encounters "t.key=X OR t.key=Y" it will try to join them into ref_or_null access via "t.key=X OR NULL". In order to make this inference it checks if Y<=>NULL, ignoring the fact that value of Y may be not yet known. The fix is that the check if Y<=>NULL is made only if value of Y is known (i.e. it is a constant). TODO: When merging to 5.0, replace used_tables() with const_item() everywhere in merge_key_fields(). mysql-test/r/innodb_mysql.result: Testcase for BUG16798 mysql-test/t/innodb_mysql.test: Testcase for BUG16798 sql/sql_select.cc: BUG#16798: Inapplicable ref_or_null query plan and bad query result on random occasions In merge_key_fields() don't call val->is_null() if the value of val is not known.
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