连接中过滤器的 SQL 性能与结果中的

Question

I need to join 2 tables but one of them has a constant filter that has to be applied, my question is what option is better in terms of performance:

1. Having the constant filter on the "FROM" and setting an alias like this:

    SELECT <result fields> FROM (SELECT <table A fields> FROM filtered_table WHERE field = value) AS A LEFT OUTER JOIN tableB ON A.id = tableB.id

2. Setting the filter in the WHERE of the main query:

    SELECT <result fields> FROM filtered_table AS A LEFT OUTER JOIN tableB ON A.id = tableB.id WHERE a.field = value

Answer 1

Most databases will treat the two forms the same -- they are logically the same and the optimizer knows this.

Some databases tend to materialize subqueries. MySQL is one of these. In that database, the subquery would probably be more expensive, because it writes out the subquery.

I would recommend writing the code without a subquery. It is both simpler to write and read and it should never have worse performance than the subquery method.