It is easy for MySQL replication to become bottleneck when Master server is not seriously loaded and the more cores and hard drives the get the larger the difference becomes, as long as replication remains single thread process. At the same time it is a lot easier to optimize your system when your replication runs [...]
Extending Index for Innodb tables can hurt performance in a surprising way
May 22, 2010 By 10 Comments
One schema optimization we often do is extending index when there are queries which can use more key part. Typically this is safe operation, unless index length increases dramatically queries which can use index can also use prefix of the new index are they ? It turns there are special cases when this is not [...]
When the subselect runs faster
March 18, 2010 By 9 Comments
A few weeks ago, we had a query optimization request from one of our customer. The query was very simple like:
1 | SELECT * FROM `table` WHERE (col1='A'||col1='B') ORDER BY id DESC LIMIT 20 OFFSET 0 |
This column in the table is looks like this:
1 | `col1` enum('A','B','C','CD','DE','F','G','HI') default NULL |
The table have 549252 rows and of course, there is an index on the col1. MySQL estimated the cardinality of that index as [...]
What does Using filesort mean in MySQL?
March 5, 2009 By 27 Comments
If you were interviewing to work at Percona, and I asked you “what does Using filesort mean in EXPLAIN,” what would you say? I have asked this question in a bunch of interviews so far, with smart people, and not one person has gotten it right. So I consider it to be a bad interview [...]
Profiling MySQL stored routines
January 19, 2009 By 4 Comments
These days I’m working with a customer who has an application based entirely on stored routines on MySQL side. Even though I haven’t worked much with stored procedures, I though it’s going to be a piece of cake. In the end – it was, but there’s a catch.
Four ways to optimize paginated displays
September 24, 2008 By 27 Comments
A paginated display is one of the top optimization scenarios we see in the real world. Search results pages, leaderboards, and most-popular lists are good examples. You know the design pattern: display 20 results in some most-relevant order. Show a “next” and “previous” link. And usually, show how many items are in the whole list [...]
Finding what Created_tmp_disk_tables with log_slow_filter
September 22, 2008 By 3 Comments
Whilst working with a client recently I noticed a large number of temporary tables being created on disk.
1 2 3 4 5 | show global status like 'Created_tmp%' | Created_tmp_disk_tables | 91970 | | Created_tmp_files | 19624 | | Created_tmp_tables | 1617031 | |
The MySQL optimizer, the OS cache, and sequential versus random I/O
April 28, 2008 By 17 Comments
In my post on estimating query completion time, I wrote about how I measured the performance on a join between a few tables in a typical star schema data warehousing scenario. In short, a query that could take several days to run with one join order takes an hour with another, and the optimizer chose [...]
Updated msl (microslow) patch, installation walk-through!
April 20, 2008 By 17 Comments
For a couple of months there have been no updates to our msl patch, however recently I managed some time to change this. The functionality was extended a little bit and what’s even more important the patch is available for all the recent MySQL releases. To remind anyone who has not yet come across this [...]
Speeding up GROUP BY if you want aproximate results
March 7, 2008 By 24 Comments
Doing performance analyzes today I wanted to count how many hits come to the pages which get more than couple of visits per day. We had SQL logs in the database so It was pretty simple query:
1 | select sum(cnt) from (select count(*) cnt from performance_log_080306 group by page having cnt>2) pv; |
Unfortunately this query ran for over half an hour badly overloaded server and I had to kill [...]
