CODEWITHSUNDEEP
mysql
Alan
Alan

Reputation:

MySQL - force not to use cache for testing speed of query

I'm testing the speed of some queries in MySQL. The database is caching these queries making it difficult for me to get reliable results when testing how fast these queries are.

Is there a way to disable caching for a query?

System: MySQL 4 on Linux webhosting, I have access to PHPMyAdmin.

Thanks

Upvotes: 406

Views: 265611

Answers (11)

Robert Máslo
Robert Máslo

Reputation: 458

You must change SQL string. Because SQL string is a cache key. For example, add a timestamp to a SQL comment.

Function for PHP:

function db_RunSQL($SQL, $NoCacheMode=false)
{
$SQL = (($NoCacheMode) ? '/*'.time().'*/ ' : '') . $SQL;
return mysqli_query(db_SavedConnect(), $SQL);
}

Upvotes: 0

Ian
Ian

Reputation: 13842

Whilst some of the answers are good, there is a major caveat.

The mysql queries may be prevented from being cached, but it won't prevent your underlying O.S caching disk accesses into memory. This can be a major slowdown for some queries especially if they need to pull data from spinning disks.

So whilst it's good to use the methods above, I would also try and test with a different set of data/range each time, that's likely not been pulled from disk into disk/memory cache.

Upvotes: 4

Jarod Elliott
Jarod Elliott

Reputation: 15670

Try using the SQL_NO_CACHE (MySQL 5.7) option in your query. (MySQL 5.6 users click HERE )

eg.

SELECT SQL_NO_CACHE * FROM TABLE

This will stop MySQL caching the results, however be aware that other OS and disk caches may also impact performance. These are harder to get around.

Upvotes: 605

Sergio Costa
Sergio Costa

Reputation: 759

I'd Use the following:

SHOW VARIABLES LIKE 'query_cache_type';
SET SESSION query_cache_type = OFF;
SHOW VARIABLES LIKE 'query_cache_type';

Upvotes: 15

newtover
newtover

Reputation: 32084

Using a user-defined variable within a query makes the query resuts uncacheable. I found it a much better indicator than using SQL_NO_CACHE. But you should put the variable in a place where the variable setting would not seriously affect the performance:

SELECT t.*
FROM thetable t, (SELECT @a:=NULL) as init;

Upvotes: 7

John Carter
John Carter

Reputation: 55271

Another alternative that only affects the current connection:

SET SESSION query_cache_type=0;

Upvotes: 141

mediobit
mediobit

Reputation: 749

Any reference to current date/time will disable the query cache for that selection:

SELECT *,NOW() FROM TABLE

See "Prerequisites and Notes for MySQL Query Cache Use" @ http://dev.mysql.com/tech-resources/articles/mysql-query-cache.html

Upvotes: 74

barbushin
barbushin

Reputation: 5305

There is also configuration option: query_cache_size=0

To disable the query cache at server startup, set the query_cache_size system variable to 0. By disabling the query cache code, there is no noticeable overhead. If you build MySQL from source, query cache capabilities can be excluded from the server entirely by invoking configure with the --without-query-cache option.

See http://dev.mysql.com/doc/refman/5.1/en/query-cache.html

Upvotes: 32

wbharding
wbharding

Reputation: 4573

One problem with the 

SELECT SQL_NO_CACHE * FROM TABLE

method is that it seems to only prevent the result of your query from being cached. However, if you're querying a database that is actively being used with the query you want to test, then other clients may cache your query, affecting your results. I am continuing to research ways around this, will edit this post if I figure one out.

Upvotes: 21

Jinesh
Jinesh

Reputation: 841

If you want to disable the Query cache set the 'query_cache_size' to 0 in your mysql configuration file . If its set 0 mysql wont use the query cache.

Upvotes: 2

djt
djt

Reputation: 469

You can also run the follow command to reset the query cache.

RESET QUERY CACHE

Upvotes: 25

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