Comments on: Optimizing MySQL: Importance of JOIN Order

By: Karthik Murugan

Karthik Murugan — Sun, 20 Dec 2009 16:17:42 +0000

Simple but great stuff. I hit with the same problem and your article helped me. Thanks Teddy.

By: Teddy

Teddy — Sun, 09 Aug 2009 09:20:42 +0000

Why? Is there something wrong with it?

By: Tobias J

Tobias J — Sun, 09 Aug 2009 07:26:28 +0000

Do you proof read your articles?
If so, you should have someone else do it for you.

Good topic though.

By: Teddy

Teddy — Thu, 30 Jul 2009 07:05:03 +0000

I absolutely understand what you are writing about, Fred. But in this case I’ve just pasted whole statement as it was originaly writen by the authors of that software. I realize that there are more deficiencies, but this one was used for demonstration purposes only. Nevertheless, I appreciate your comment, I’m sure it will be helpfull for all readers.

By: Fred

Fred — Wed, 29 Jul 2009 02:51:29 +0000

unix_timestamp(art.StartDate) = unix_timestamp(now())

will NEVER use the fields index,
because the date is calculated/converted.

The indexed field must be “pure”,
for instance, if you have an indexed datetime field and do date comparison with it, it won’t uses the index on that field.

You should rewrite it as:

art.StartDate >= current_date
art.ExpiryDate >= current_date

or if it’s a DateTime

art.StartDate >= now()
art.ExpiryDate >= now()

If you really need some calculation to be done, then you must rewrite the condition such that the indexed field is pure and the condition is not and can be computed before the index is used…

For instance:

DATE_SUB(CURRENT_DATE,INTERVAL 30 DAY) <= art.ExpiryDate;

== (constant <= field)

is faster than:

CURRENT_DATE <= DATE_ADD(art.ExpiryDate,INTERVAL 30 DAY);

== (constant <= computed(field))

Also, you are using two fields to be ordered by which are not "explicitly" being stated in your SELECT uncomputed…

Normally you should never uses *,
except for debugging, if you upgrade your schema, then your application logic may break.

Did you try EXPLAIN SELECT ?