FirstMatch strategy

The idea

FirstMatch is an execution strategy for Semi-join subqueries.

It is very similar to how IN/EXISTS subqueries were executed in MySQL 5.x.

Let's take the usual example of search for countries with big cities:

select * from Country 
where Country.code IN (select City.Country 
                       from City 
                       where City.Population > 1*1000*1000)
      and Country.continent='Europe'

Suppose, our execution plan is to find countries in Europe, and then for each found country check if it has any big cities. Regular inner join execution will look as follows:

Since Germany has got two big cities (in this diagram), it will be put into query output two times. FirstMatch strategy solves this by short-cutting execution as soon as the first match was found:

FirstMatch in action

EXPLAIN will look as follows:

MariaDB [world]> explain select * from Country  where Country.code IN (select City.Country from City where City.Population > 1*1000*1000) and Country.continent='Europe';
+----+-------------+---------+------+--------------------+-----------+---------+--------------------+------+----------------------------------+
| id | select_type | table   | type | possible_keys      | key       | key_len | ref                | rows | Extra                            |
+----+-------------+---------+------+--------------------+-----------+---------+--------------------+------+----------------------------------+
|  1 | PRIMARY     | Country | ref  | PRIMARY,continent  | continent | 17      | const              |   60 | Using index condition            |
|  1 | PRIMARY     | City    | ref  | Population,Country | Country   | 3       | world.Country.Code |   18 | Using where; FirstMatch(Country) |
+----+-------------+---------+------+--------------------+-----------+---------+--------------------+------+----------------------------------+
2 rows in set (0.00 sec)

This execution plan is similar to one you would get in MySQL:

MySQL [world]> explain select * from Country  where Country.code IN (select City.Country from City where City.Population > 1*1000*1000) and Country.continent='Europe';
+----+--------------------+---------+----------------+--------------------+-----------+---------+-------+------+------------------------------------+
| id | select_type        | table   | type           | possible_keys      | key       | key_len | ref   | rows | Extra                              |
+----+--------------------+---------+----------------+--------------------+-----------+---------+-------+------+------------------------------------+
|  1 | PRIMARY            | Country | ref            | continent          | continent | 17      | const |   60 | Using index condition; Using where |
|  2 | DEPENDENT SUBQUERY | City    | index_subquery | Population,Country | Country   | 3       | func  |   18 | Using where                        |
+----+--------------------+---------+----------------+--------------------+-----------+---------+-------+------+------------------------------------+
2 rows in set (0.01 sec)