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Explain plan--join type
Posted by: Amy Ng
Date: October 05, 2011 11:10PM

Dear ladies and gentlemen :)

How are you doing? This is my 1st post :)

I am learning various join types from the output of the explain plan, and found an article here: http://dev.mysql.com/doc/refman/5.0/en/explain-output.html. However, I am still struggling to understand the concept. Could someone please tell me the differences between these 3 join types?

1. const:If the table has at most one matching row, then why the article stated "const is used when you compare ALL parts of a PRIMARY KEY or UNIQUE index to constant values". What does it mean "ALL parts of the PK"?

2. eq_ref: what does it really mean "One row is read from the table for each combination of rows from the previous tables"? Can someone elaborate a bit?

3. ref:


Thank you for reading my post.

--Amy

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