Details
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Type:
Bug
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Status: Closed
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Priority:
Minor
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Resolution: Fixed
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Affects Version/s: None
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Fix Version/s: None
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Component/s: None
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Labels:
Description
If the table expression for a derived table contains a LIMIT clause then the estimate of the number of rows read from the derived table may be badly off in mariadb-5.3:
MariaDB [test]> create table t1 (a int);
Query OK, 0 rows affected (0.02 sec)
MariaDB [test]> insert into t1 values
-> (8), (3), (4), (7), (9), (5), (1), (2);
Query OK, 8 rows affected (0.00 sec)
Records: 8 Duplicates: 0 Warnings: 0
MariaDB [test]> select * from (select * from t1 limit 3) t;
------
| a |
------
| 8 |
| 3 |
| 4 |
------
3 rows in set (0.00 sec)
MariaDB [test]> explain select * from (select * from t1 limit 3) t;
------------------------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
------------------------------------------------------------------+
| 1 | PRIMARY | <derived2> | ALL | NULL | NULL | NULL | NULL | 8 | |
| 2 | DERIVED | t1 | ALL | NULL | NULL | NULL | NULL | 8 |
------------------------------------------------------------------+
2 rows in set (0.00 sec)
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Launchpad bug id: 917990
Re: Bad estimate of the number of rows in derived table with LIMIT clause
Lp bug #916551 is a duplicate of this bug.