I am building a website in ASP.NET which is connected to a MySQL database.
I have a complex query and I have noticed that first time after a I re-start MySQL this query is slow (5-8 sec), following happens almost immediately (< ~1 sec).
I thought this had to do with data being cached by MySQL, so I tried to do a
FLUSH QUERY CACHE and even a RESET QUERY CACHE - but this seems not to affect the response-time of the query.
Any suggestion on how I can completely clear the cache without actually restarting the DB server.
You can use the SQL_NO_CACHE option.
Example:
SELECT SQL_NO_CACHE id, name FROM customer;
For more information, check the MySQL documentation for SQL Query Cache SELECT Options
Related
I know that SQL query will use query cache to receive data instead of reprocess all of the data. Here the question I would like to ask,
I working with a server of database and I'm one of the developer that working on it and I need to do performance testing on queries that i handling
If I clear the query cache
example using FLUSH QUERY CACHE; or RESET QUERY CACHE;,
will it affect others developer or it only clears away my local query cache?
If it will affect others, is there any way to clear locally or allow my query won't use the query cache for testing
Two clarifications to begin with:
MySQL query cache is a server-side feature, there's no such thing as "local cache". You're probably confused by the LOCAL keyword in FLUSH command. As docs explain it's just an alias for NO_WRITE_TO_BINLOG (thus it's related to replication and "local" means "this server").
MySQL will only return cached data if you've enabled the feature and either made it default or opted-in with the SQL_CACHE hint. In my experience, most servers do not have it by default.
Let's now answer your question. At The MySQL Query Cache we can read:
The query cache is shared among sessions, so a result set generated by
one client can be sent in response to the same query issued by another
client.
Which makes sense: a cache that cannot reuse stored data is not as useful.
I don't know what you want to test exactly. Your data should always be fresh:
The query cache does not return stale data. When tables are modified,
any relevant entries in the query cache are flushed.
However you might want to get an idea of how long the query takes to run. You can always opt out with the SQL_NO_CACHE keyword:
The server does not use the query cache. It neither checks the query
cache to see whether the result is already cached, nor does it cache
the query result.
Just take into account that a query that runs for the second time might run faster even without cache because part of the data segments might be already loaded into RAM.
Try using the SQL_NO_CACHE option in your query.This will stop MySQL caching the results
SELECT SQL_NO_CACHE * FROM TABLE
With SQL Server for cached data, you can use DBCC DROPCLEANBUFFERS and force a manul CHECKPOINT.
However it works at the Server (instance) level:
Use DBCC DROPCLEANBUFFERS to test queries with a cold buffer cache without shutting down and restarting the server.
To drop clean buffers from the buffer pool, first use CHECKPOINT to produce a cold buffer cache. This forces all dirty pages for the current database to be written to disk and cleans the buffers. After you do this, you can issue DBCC DROPCLEANBUFFERS command to remove all buffers from the buffer pool.
edited*
SQL Query buffer cache is global but not local. If the buffer or query cache is drop, it drops it globally, and will affect all user using the database server.
I'm running a database on a MySQL server that has dozens of databases, so
RESET QUERY CACHE
is not possible because I would be resetting it for all other databases.
I was curious whether it's possible to reset the query cache for just one database, or one query? Or is it possible to write a SELECT statement that doesn't use the query cache?
Update: I tried SELECT SQL_NO_CACHE and set session query_cache_type=0 but these didn't make any difference... which makes me think I'm running into disk or OS caches.
All I really want to do is benchmark... you'd think that MySQL would provide a way...
Unfortunately for a single socket all databases will share same my.cnf parameters. You can bind MYSQL on a different socket and have different instances of mysql running, then you can have query cache enabled or disabled.
But there is another solution, you can use memcached and store queries there so you can access from there if the need be, so that you can disable query cache. I understand query cache can cause contention.
Can't we use SQL_NO_CACHE
Example: SELECT SQL_NO_CACHE id, name FROM customer;
I am trying to profile two different queries that do the same thing to find which one is faster. For testing, I have put SQL_NO_CACHE into both queries to prevent the query cache from messing up the timing.
Query A is consistently 50ms.
Query B is 100ms the first time it is run and 10ms if I run it a second time shortly after.
Why is Query B faster the second time? The query cache should not be speeding up the queries. Could it be that the first run of query B loads the data from disk into memory so that the second query is running in memory and faster? Is there a way to test this? I tried to test this myself by doing select * from the table before I ran Query B, but it still exhibited the same behavior. Is SQL_NO_CACHE perhaps not working to disable the query cache?
Query B looks something like this:
SELECT SQL_NO_CACHE foo,bar FROM table1 LEFT JOIN table2 ON table1.foo=table2.foo WHERE bar=1
Depending on the storage engine you're using, yes it is most probably being loaded from a data cache and not a query cache.
MyISAM provides no storage engine level caching for data, and only caches indexes. However, the operating system often serves up data from its own caches which may well be speeding up your query execution.
You can try benchmarking the query in a real scenario, just log that specific query to the database every time its executed (along with its execution time).
Depending on the size of your indexes and your table type, it may be that indexes are not in memory the first time the query is run. So MySQL will pull indexes into memory the first time the query is run, causing a significant slowdown. The next time, most of what MySQL needs may in memory, resulting in the performance gain.
Is your app making a connection and doing the authentication handshake on the first query? If so the 2nd query will already have an open/authenticated connection to execute from. Try running it a 3rd time and see if the 2nd and 3rd tries are close to the same time.
I have a slow MySQL query in my application that I need to re-write. The problem is, it's only slow on my production server and only when it's not cached. The first time I run it, it will take 12 seconds, then any time after that it'll be 500 milliseconds.
Is there an easy way to test this query without it hitting the query cache so I can see the results of my refactoring?
MySQL supports to prevent caching single queries. Try
SELECT SQL_NO_CACHE field_a, field_b FROM table;
alternatively you can diasble the query cache for the current session:
SET SESSION query_cache_type = OFF;
See http://dev.mysql.com/doc/refman/5.1/en/query-cache.html
To add to johannes's good answer, what I do is
RESET QUERY CACHE;
This has the slight added advantage of not requiring any changes to either the statements I'm executing or the connection.
A trivial thing to do is to alter the statement you're executing somehow, such as put a random number in a comment, because a queries are located in the cache only if they are byte-identical to some previous 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
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.
Another alternative that only affects the current connection:
SET SESSION query_cache_type=0;
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
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
You can also run the follow command to reset the query cache.
RESET QUERY CACHE
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.
I'd Use the following:
SHOW VARIABLES LIKE 'query_cache_type';
SET SESSION query_cache_type = OFF;
SHOW VARIABLES LIKE 'query_cache_type';
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;
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.
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.
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);
}