I run this script:
DROP TABLE IF EXISTS t1;
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CREATE TABLE t1 (a INT);
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INSERT INTO t1 VALUES (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);
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INSERT INTO t1 SELECT * FROM t1;
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INSERT INTO t1 SELECT * FROM t1;
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INSERT INTO t1 SELECT * FROM t1;
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INSERT INTO t1 SELECT * FROM t1;
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INSERT INTO t1 SELECT * FROM t1;
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INSERT INTO t1 SELECT * FROM t1;
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INSERT INTO t1 SELECT * FROM t1;
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INSERT INTO t1 SELECT * FROM t1;
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INSERT INTO t1 SELECT * FROM t1;
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INSERT INTO t1 SELECT * FROM t1;
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INSERT INTO t1 SELECT * FROM t1;
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INSERT INTO t1 SELECT * FROM t1;
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INSERT INTO t1 SELECT * FROM t1;
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INSERT INTO t1 SELECT * FROM t1;
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INSERT INTO t1 SELECT * FROM t1;
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EXPLAIN SELECT * FROM t1 WHERE a IN (1,2,(SELECT MAX(a) FROM t1));
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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 | t1 | ALL | NULL | NULL | NULL | NULL | 328184 | Using where |
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| 2 | SUBQUERY | t1 | ALL | NULL | NULL | NULL | NULL | 328144 | |
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+------+-------------+-------+------+---------------+------+---------+------+--------+-------------+
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2 rows in set (6.69 sec)
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Notice, the EXPLAIN query was slow.