Understanding why this MySQL outer select slows down the query

Question

I have a Companies table and a Zipcodes table and I want to get the samllest subset of companyIDs before I apply additional processing. So often I put that in an inner select, grabbing the companyIDs on some WHERE condition and then go some aggregation or calculation (like maybe a distance calculation) on the outside.

For some reason I'm not understanding why this outer SELECT is slow while the inner query is blazing fast.

I know it has to do with the JOIN on the inside. Because removing it fixes the problem. Somehow MySQL is deriving the tables... SELECT _i1.companyID FROM ( SELECT companyID FROM Companies JOIN Zipcodes ON Zipcodes.zipcode = Companies.companyZipcode WHERE Companies.companyType = 'plumbing' ) _i1 Time: .8s
Without the outer query: .00x seconds

There are indexes on Zipcodes.zipcode and Companies.companyZipcode and Companies.companyType and Companies.companyID is the Primary Key.

Doing an EXPLAIN shows that the outer query causes MySQL to have a select_type of Derived instead of Simple.

Answer 1

当您有子查询时，需要创建一个临时表并将数据插入其中。