https://mariadb.com/kb/en/mariadb/semi-join-subquery-optimizations/

http://dev.mysql.com/doc/refman/5.7/en/subquery-optimization.html

https://mariadb.com/kb/en/mariadb/loosescan-strategy/

https://mariadb.com/kb/en/mariadb/semi-join-materialization-strategy/

https://www.quora.com/SQL-What-is-the-difference-between-a-correlated-and-an-uncorrelated-subquery

https://mariadb.com/kb/en/mariadb/optimization-strategies/

https://mariadb.com/kb/en/mariadb/optimization-and-tuning/

MYSQL 优化