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Test case:
CREATE TABLE `t1` (
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`id` int(10) unsigned NOT NULL auto_increment,
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`t` date,
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PRIMARY KEY (`id`),
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KEY `t` (`t`)
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);
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INSERT INTO t1 (t) values ('2013-11-14'), ('2013-11-15'),('2013-11-16'),('2013-11-17'),('2013-11-18'),('2013-11-19'),('2013-11-20');
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EXPLAIN UPDATE t1 SET t = CURDATE() WHERE id = (SELECT * FROM (SELECT id FROM t1 ORDER BY t ASC LIMIT 1) tmp) LIMIT 1;
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FLUSH STATUS;
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UPDATE t1 SET t = CURDATE() WHERE id = (SELECT * FROM (SELECT id FROM t1 ORDER BY t ASC LIMIT 1) tmp) LIMIT 1;
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SHOW STATUS LIKE 'sort%';
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returns:
id select_type table type possible_keys key key_len ref rows Extra
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1 PRIMARY t1 ALL NULL NULL NULL NULL 7 Using where
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2 SUBQUERY <derived3> ALL NULL NULL NULL NULL 2
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3 DERIVED t1 ALL NULL NULL NULL NULL 7 Using filesort
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SHOW STATUS LIKE 'sort%';
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Variable_name Value
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Sort_merge_passes 0
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Sort_range 0
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Sort_rows 0
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Sort_scan 0
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