Optimizations for derived tables

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Derived tables are subqueries in the FROM clause. Prior to MariaDB 5.3/MySQL 5.6, they were too slow to be usable. In MariaDB 5.3/MySQL 5.6, there are two optimizations

  • Derived table merge
  • Automatic index creation for derived tables

which provide adequate performance.

Derived table merge

The idea

Users of "big" database systems are used to using FROM subqueries as a way to structure their queries. For example, if one's first thought was that they need to select cities with population greater than 10,000 people, and then that from these cities one needs to select those that are located in Germany, one could write this SQL:

select * 
from 
  (select * from City where Population > 10*1000) as big_city
where 
  big_city.Country='DEU'

For MySQL, such syntax was a taboo. If you run EXPLAIN for it, you can see why:

mysql> explain select * from (select * from City where Population > 1*1000) as big_city where big_city.Country='DEU' ;
+----+-------------+------------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table      | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+------------+------+---------------+------+---------+------+------+-------------+
|  1 | PRIMARY     | <derived2> | ALL  | NULL          | NULL | NULL    | NULL | 4068 | Using where |
|  2 | DERIVED     | City       | ALL  | Population    | NULL | NULL    | NULL | 4079 | Using where |
+----+-------------+------------+------+---------------+------+---------+------+------+-------------+
2 rows in set (0.60 sec)

this

Automatic

See also

Optimizing Subqueries in the FROM Clause in MySQL 5.6 manual

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