Question

I have a performance problem on a query.

First table is a Customer table which has millions records in it. Customer table has a column of email address and some other information about customer.

Second table is a CommunicationInfo table which contains just Email addresses.

And What I want in here is; how many times the email address in CommunicationInfo table repeats in Customers table. What could be the the most performer query.

The basic query that I can explain this situation is;

Select ci.Email, count(*) from Customer c left join 
CommunicationInfo ci on c.Email1 = ci.Email or c.Email2 = ci.Email 
Group by ci.Email

But sure, it takes about 5, 6 minutes in execution.

Thanks in Advance.

Answer 1

this query is about as good as it gets if you have an index on Customer.Email and another on CommunicationInfo.Email

Select
    c.Email, count(*)
    from Customer c
        left join CommunicationInfo  ci on c.Email1 = ci.Email
        left join CommunicationInfo ci2 on c.Email2 = ci2.Email
    Group by c.Email