Why MySql doesn't use my index and how to avoid the “Using temporary; Using filesort”?

Question

I have a MySql query that looks like the following:

SELECT trace_access.Employe_Code, trace_access.Employe_Prenom, 
trace_access.Employe_Nom, , trace_access.Evenement_Date, trace_access.Evenement_Heure
FROM trace_access 
INNER JOIN emp
ON trace_access.Employe_Code = emp.Employe_CodeEmploye
LEFT JOIN user
ON emp.Employe_ID = user.Employe_ID
LEFT JOIN role 
ON role.User_ID = user.User_ID
WHERE trace_access.Employe_Nom Not Like "TEST%NU" 
ORDER BY trace_access.Evenement_Date DESC , trace_access.Evenement_Heure DESC

The table "trace_access"contains almost 20 million entries.

When I explain the query:

explain query

My question is why MySql didn't use the key for the emp table and how to avoid "Using temporary; Using filesort" ??

I have tried to froce it to use my index but that didn't work. The query lasts one hour and more, writes a file on /tmp folder that exceeds 8Go !!!

Any help ?

Thanks a lot.

Answer 1

With your current execution plan (to start with emp ), a full table scan is required and there is no usefull index on emp . This can make sense when you only get a relatively small amount of rows after your join to trace_access , which probably isn't the case here.

To prevent the filesort , you need an index that supports your order by . So add, if it doesn't exist yet, the index trace_access(Evenement_Date, Evenement_Heure) .

This might already be enough to make MySQL start with trace_access . If not, replace INNER JOIN emp with STRAIGHT_JOIN emp . This will force MySQL to do so.

Also add an index for emp(Employe_CodeEmploye) .

Depending on your data, you can try to add Employe_Code and/or Employe_Nom as 3rd and/or 4th column to the index on trace_access , though it will probably not have much effect.