Question

Ok I need to build a query based on some user input to filter the results.

The query basically goes something like this:

SELECT * FROM my_table ORDER BY ordering_fld;

There are four text boxes in which users can choose to filter the data, meaning I’d have to dynamically build a “WHERE” clause into it for the first filter used and then “AND” clauses for each subsequent filter entered.

Because I’m too lazy to do this, I’ve just made every filter an “AND” clause and put a “WHERE 1” clause in the query by default.

So now I have:

SELECT * FROM my_table WHERE 1 {AND filters} ORDER BY ordering_fld;

So my question is, have I done something that will adversely affect the performance of my query or buggered anything else up in any way I should be remotely worried about?

Answer 1

MySQL will optimize your 1 away.

I just ran this query on my test database:

EXPLAIN EXTENDED
SELECT  *
FROM    t_source
WHERE   1 AND id < 100

and it gave me the following description:

select `test`.`t_source`.`id` AS `id`,`test`.`t_source`.`value` AS `value`,`test`.`t_source`.`val` AS `val`,`test`.`t_source`.`nid` AS `nid` from `test`.`t_source` where (`test`.`t_source`.`id` < 100)

As you can see, no 1 at all.

The documentation on WHERE clause optimization in MySQL mentions this:

• Constant folding:

  (a<b AND b=c) AND a=5
  -> b>5 AND b=c AND a=5
  

• Constant condition removal (needed because of constant folding):

  (B>=5 AND B=5) OR (B=6 AND 5=5) OR (B=7 AND 5=6)
  -> B=5 OR B=6
  

Note 5 = 5 and 5 = 6 parts in the example above.