哪个是带过滤器的更快子查询，然后加入或加入查询，然后在 MYSQL 中过滤

Question

I have two table one is user table other is specialty table. Fields in user table: userid, username, userLocation Fields in specialty table: userid, userSpecialty

Now I want to join there two tables. Please let me know which approach will be better:

select * from ( select * from user where userLocation = 'value') u inner join specialty s on u.userid = s.userid; or

select * from user u inner join specialty s on u.userid = s.userid where userLocation = 'value';

Is it good practice to minimize the number of records where ever we can or SQL optimizer will do that automatically?

Answer 1

For best shot at optimal performance, give preference to the pattern in the second query, the query without the inline view.

For earlier version of MySQL (version 5.5 and earlier), the first query will require MySQL to run the inline view query, and materialize a derived table ( u ). Once that is done, the outer query will run against the derived table. And that table won't be indexed. For large sets, that can be a significant performance hit. For small sets, the performance impact for a single query isn't noticeable.

With the second query, the optimizer isn't forced to create and populate a derived table, so there's potential for better performance.

The existence of suitable indexes (or the non-existence of indexes) i0ndexes is going to have a much bigger impact on performance. And retrieving all columns including columns that aren't needed by the query (SELECT *) also has an impact on performance. Specifying a subset of the columns, the expressions that are actually needed, will give better performance, especially if a covering index is available to avoid lookups to the underlying data pages of the table.