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Optimisation of a query using Group by and a Join
Posted by: Alex Brown
Date: October 19, 2015 12:20PM

Currently, there's no indexes

Orders Table:

ID
CustomerID

Users Table
ID
Email

The query is

UPDATE orders,
(SELECT id, c.email, t1.needed FROM customers c
JOIN (SELECT min(id) as needed, email FROM customers GROUP by email HAVING COUNT(*) > 1) t1 ON c.email = t1.email
WHERE c.email IN(SELECT email FROM customers GROUP by email HAVING COUNT(*) > 1)) t2
SET customerID = t2.needed
WHERE customerID = t2.id


There's about 300k rows in Customer table, and it's still running 1 hour later!

Is there anything I can do to improve the performance of the query?

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