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Re: Index Keys of the table
Posted by: Rick James
Date: October 30, 2012 11:59AM

Should I create the KEY as following?
KEY(Name, Gender, Phone, Email, DOB) -- Only good when "name=..." in included

OR following will be better?
KEY1(Name)
KEY2(Gender) -- rarely worthwhile indexing on low-cardinality fields
KEY3(Phone)
KEY4(Email)
KEY5(DOB)

Read about "compound" indexes here: http://mysql.rjweb.org/doc.php/index1

SELECT * FROM contacts WHERE Name = ?; -- INDEX(Name ...)
SELECT * FROM contacts WHERE Phone = ?; -- INDEX(Phone ...)
SELECT * FROM contacts WHERE Name = ? AND Gender = ?; -- Since Name is virtually unique, the test for Gender is unlikely to be useful
SELECT * FROM contacts WHERE Phone = ? AND Email = ?; -- INDEX(Phone, Email ...) or INDEX(Email, Phone, ...); but the issue of uniqueness makes a compound key not that useful.
SELECT * FROM contacts WHERE Name = ? AND Gender = ? AND Phone = ?; -- ditto

Assuming that there are very few duplicate names, duplicate phones, and duplicate emails, I recommend:
INDEX(Name)
INDEX(Phone)
INDEX(Email) -- (although you did not have a use case for this)
INDEX(DOB) -- (although you did not have a use case for this)

If a user can search just on Gender (WHERE Gender = 'M'), a table scan is almost as good as having an index.

If a user can search for things other than exact equality (eg, WHERE Name LIKE 'Lee%'), then my recommendations may or may not be good enough. Please provide any such, so we can discuss further. Also, how many rows in the table?

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