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LooseScan strategy

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LooseScan is an execution strategy for Semi-join subqueries.

The idea

We will demonstrate LooseScan strategy by example. Suppose, you're looking for countries that have satellites. If we ignore the fact that some satellites are co-owned by multiple countries, this can be achieved by using this subquery:

select * from Country  
where 
  Country.code in (select country_code from Satellite)

MariaDB [world]> explain select * from Country where Country.code in (select country_code from Satellite);
+----+-------------+-----------+--------+---------------+--------------+---------+------------------------------+------+-------------------------------------+
| id | select_type | table     | type   | possible_keys | key          | key_len | ref                          | rows | Extra                               |
+----+-------------+-----------+--------+---------------+--------------+---------+------------------------------+------+-------------------------------------+
|  1 | PRIMARY     | Satellite | index  | country_code  | country_code | 9       | NULL                         |  932 | Using where; Using index; LooseScan |
|  1 | PRIMARY     | Country   | eq_ref | PRIMARY       | PRIMARY      | 3       | world.Satellite.country_code |    1 | Using index condition               |
+----+-------------+-----------+--------+---------------+--------------+---------+------------------------------+------+-------------------------------------+

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