MariaDB [test]> select val from (SELECT val from (SELECT val FROM ( SELECT SUM(1) AS val limit 1) tab1 ) as tab2 ) as tab3 WHERE val=1;
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+------+
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| val |
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+------+
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| 1 |
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+------+
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1 row in set (0.00 sec)
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MariaDB [test]> select val from (SELECT val from (SELECT val FROM ( SELECT SUM(1) AS val) tab1 ) as tab2 ) as tab3 WHERE val=1;
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Empty set (0.00 sec)
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MariaDB [test]> explain select val from (SELECT val from (SELECT val FROM ( SELECT SUM(1) AS val) tab1 ) as tab2 ) as tab3 WHERE val=1;
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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 | PRIMARY | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Impossible WHERE noticed after reading const tables |
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| 4 | DERIVED | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Impossible HAVING |
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+------+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+
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2 rows in set (0.00 sec)
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MariaDB [test]> set optimizer_switch='derived_merge=off';
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Query OK, 0 rows affected (0.00 sec)
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MariaDB [test]> select val from (SELECT val from (SELECT val FROM ( SELECT SUM(1) AS val) tab1 ) as tab2 ) as tab3 WHERE val=1;
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+------+
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| val |
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+------+
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| 1 |
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+------+
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1 row in set (0.01 sec)
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