On 3/19/2014 7:27 PM, Mogens Melander wrote:
> A function in a where equals what ?
> On Wed, March 19, 2014 15:05, Morgan Tocker wrote:
>> Hi Christophe,
>>> Considering the following simple query :
>>> SELECT * FROM Status WHERE DWProcessed = 0 AND PreviousStatus NOT IN
>>> ('PENDING', 'ACCEPTED') AND SubscribeDate < DATE_SUB(NOW(), INTERVAL 24
>>> Which of these filters are processed first ?
>>> I'd like the first filter (DWProcessed / Lowest cardinality and indexed)
>>> being processed first, but I can't really find any useful information
>>> about this .
>>> Is there any performance impact on query processing, about the order of
>>> WHERE clauses ?
>> When a MySQL server receives a query, it goes through a process called
>> query optimization and tries to determine the best way to execute it
>> (based on availability of indexes etc). You can think of this as similar
>> to how GPS software picks the fastest route - it is very similar.
>> The order of the WHERE clause does not matter, and in fact more
>> complicated transformations happen in query optimization automatically.
>> For Example:
>> SELECT * FROM Status WHERE 1=1 AND DWProcessed = 0;
>> 1=1 is detected as a tautology and removed.
The entire WHERE expression becomes a row-by-row TRUE/FALSE test for
which rows should become part of the results. The expressions are
evaluated in whichever order the Optimizer thinks will get us to the
answer with the least effort.
In this particular query case, there are three tests to perform
1) DWProcessed = 0
2) PreviousStatus NOT IN ('PENDING', 'ACCEPTED')
3) SubscribeDate < DATE_SUB(NOW(), INTERVAL 24 HOUR)
The NOT IN test of #2 is actually shorthand for a negated OR expression.
It is evaluated like this
(PreviousStatus <> 'PENDING') AND (PreviousStatus <> 'ACCEPTED')
According to the original WHERE clause, all three terms are AND-ed
together which means that if any one of those tests fail for a
particular row, that row does not become part of the final result.
The inner workings of the optimizer are complex. There is quite a bit
already written about how it works. And, you can review that section of
the code if you are truly adventurous . MySQL is open source and it's
all there for anyone to review.
I also suggest that a good introduction to the entire process is in
sections 22.214.171.124 through 126.96.36.199 of the manual. These provide a nice
overview of the various access and simplification methods that the
Optimizer can choose from while trying to work out the optimal method of
resolving a SQL command.
MySQL Senior Principal Technical Support Engineer
Oracle USA, Inc. - Hardware and Software, Engineered to Work Together.
Office: Blountville, TN