What does eq_ref and ref types mean in MySQL explain

I'll try an explanation...

eq_ref – imagine that you have two tables. Table A with columns (id, text) where id is a primary key. Table B with the same columns (id, text) where id is a primary key. Table A has the following data:

1, Hello 2, How are

Table B has the following data:

1, world!2, you?

Imagine eq_ref as JOIN between A and B:

select A.text, B.text where A.ID = B.ID

This JOIN is very fast because for each row scanned in table A there can be only ONE row in table B which satisfies the JOIN condition. One and no more than one. That is because B.id is UNIQUE.
Here you are: pseudo code which illustrates the processing at server side:

foreach (rowA in A){    if (existsInBRowWithID(rowA.id)    {        addToResult(rowA.text, getRowInBWithID(rowA.id).text);    }}

ref - Now imagine another table C with columns (id, text) in which id an index but a non UNIQUE one. Table C has the following data:

1, John!1, Jack!

Imagine ref as JOIN between A and C:

select A.text, C.text where A.ID = C.ID

Here you are: pseudo code illustrating the server side processing:

foreach (rowA in A){    foreach (rowC in C)    {        if (rowA.id == rowC.id)        {            addToResult(rowA.text, rowC.text);        }    }}

This JOIN is NOT as fast as the former one because for each row scanned in table A there are SEVERAL possible rows in table C which may satisfy the JOIN condition (nested loops). That is because C.ID is NOT UNIQUE.

I hope that helps...

Cheerz!

The "type" refers to the join type made in your request. From best to worst, here is the list :

• system
• const
• eq_ref
• ref
• range
• index
• all

You will find a more detailed explanation at the MySQL documentation : http://dev.mysql.com/doc/refman/5.0/en/explain-output.html