Combined Primary Key and Auto Increment Primary Key

[Chris White]

Hi all,

I had somewhat of a performance question.  I have an association table with 2 
unique values which will always be selected by one of the values (never by 
id).  That said, I'm wondering which would be a better gain, having this:

CREATE TABLE association_sample (
   `id` INTEGER NOT NULL AUTO_INCREMENT PRIMARY KEY,
    association_id1 INTEGER NOT NULL,
    association_id2 INTEGER NOT NULL
)ENGINE=InnoDb;

or

CREATE TABLE association_sample (
    association_id1 INTEGER NOT NULL,
    association_id2 INTEGER NOT NULL,
    PRIMARY KEY(association_id1,association_id2)
)ENGINE=InnoDb;

note that this table will mostly consist of table writes (updates, inserts, 
deletes).
-- 
Chris White
PHP Programmer
Interfuel

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