Comments on: How expensive is a WHERE clause in MySQL?

By: gizg

gizg — Tue, 02 Dec 2008 20:55:21 +0000

All well and good being a student with plenty of time on your hands to do such trivial (read: useless) tests!
HOWEVER, try and write a query in a real world web app that can do without a WHERE clause!?!?!?!?!

By: Baron Schwartz

Baron Schwartz — Tue, 04 Nov 2008 13:29:26 +0000

Pep, yes, the index is half the physical size. The row estimate is only an estimate. The stats are inexact.

By: Pep Pla

Pep Pla — Tue, 04 Nov 2008 10:18:14 +0000

Show table status gives half the size of the table to the index.

Also explain gives half (???) the rows if using the index.

I can understand that the index takes less space than the table as all the keys are the same but why the optimizer believes that using the index requieres half the rows? because is a b-tree?

Pep

By: Pep Pla

Pep Pla — Tue, 04 Nov 2008 09:49:20 +0000

Sheri,

MySQL Optimizer does something called: Constant condition removal.
This means that a condition like this TRUE or 5=5 is going to be
removed by the optimizer prior to query execution.

Mark,
Is it possible that using a index is faster because counting index records is
faster than counting table records? It is important to realize that all the
index keys are the same so probably the table is larger than the index and
executing the query takes less io -> faster. (I’ll check table size vs. index size)

Pep

By: Sheeri K. Cabral

Sheeri K. Cabral — Sat, 01 Nov 2008 20:45:00 +0000

How much of this is actually overhead because of the comparison versus the where?

ie, can you compare with

WHERE TRUE
and
WHERE 1=1
and
WHERE ‘a’='a’? That would be much more interesting, because the 60% or 87% are likely because of the access and comparison costs. But how much is the actual WHERE costing?

There’s also explain plan overhead.

By: MC.Spring

MC.Spring — Sat, 01 Nov 2008 01:26:26 +0000

I think you should use none function in the where case for the last statement, while that maybe took a little time down.

By: Martin

Martin — Fri, 31 Oct 2008 14:49:57 +0000

Well that is three things I have learned in one post – awesome!
Thx
Marty

By: Baron Schwartz

Baron Schwartz — Fri, 31 Oct 2008 14:42:33 +0000

InnoDB has a hidden 6-byte primary key.

By: Martin

Martin — Fri, 31 Oct 2008 14:21:24 +0000

I’m confused. Where did the Primary Key come from in:
CREATE TABLE `t` (`a` date NOT NULL) ENGINE=InnoDB DEFAULT CHARSET=latin1;
?

By: Baron Schwartz

Baron Schwartz — Fri, 31 Oct 2008 14:17:14 +0000

Of course — because it’s using the primary key.

My point in asking those questions was to get you thinking about the query plan. You can’t compare the primary key to the secondary key and say that the difference is due to where clauses or not having where clauses. I think you’re misunderstanding how the storage engine api and InnoDB indexes work.