Can you apply LIMIT on MySQL before LEFT JOIN another?

问题

For example:

SELECT * FROM table_1 LIMIT 5 
LEFT JOIN table_2 AS table_1.id = table_2.id 
WHERE 1

Otherwise the engine takes all of table_1 before applying the join, then limiting which can slow the query down massively (with massive tables).

回答1:

You can do it by joining on a subquery instead of an actual table. Something like this should work:

SELECT * FROM
    (SELECT * FROM table_1 LIMIT 5) as subq
    LEFT JOIN table_2 ON subq.id = table_2.id WHERE 1

来源：https://stackoverflow.com/questions/7405432/can-you-apply-limit-on-mysql-before-left-join-another