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Displaying posts with tag: tricks (reset)
Tricking the Optimizer, or How Checking Bug Reports Help to Solve Real Problems

I've got several useful habits over the years of work in MySQL Support. One of them is to start working on every problem with search for known MySQL bugs related to the problem at hand. I'd like to share one recent case where this habit helped me to get a solution for customer almost instantly.

It was one of rare cases when customer opened a support request with a very clear question and even a test case. The problem was described very precisely, more or less as follows (with table and column names, and data changed for this blog post, surely).

Let's assume we have two tables created like these:

mysql> create table t1(id int auto_increment primary key, c1 varchar(2), c2 varchar(100));Query OK, 0 rows affected (0.27 sec)

mysql> create table t2(id int auto_increment primary key, t1_id int, ctime datetime, cvalue decimal(10,2), key(t1_id, ctime));
Query OK, 0 …

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Quick and dirty concurrent operations from the shell

Let’s say that you want to measure something in your database, and for that you need several operations to happen in parallel. If you have a capable programming language at your disposal (Perl, Python, Ruby, PHP, or Java would fit the bill) you can code a test that sends several transactions in parallel.

But if all you have is the shell and the mysql client, things can be trickier. Today I needed such a parallel result, and I only had mysql and bash to accomplish the task.

In the shell, it’s easy to run a loop:

for N in $(seq 1 10)
mysql -h host1 -e "insert into sometable values($N)"

But this does run queries sequentially, and each session will open and close before the next one starts. Therefore there is no concurrency at all.
Then I thought that the method for parallel execution in the shell is to run things in the background, and then collect the …

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Never say "there is no way"

Reading a recent MySQL book, I saw an example of SHOW CREATE TABLE that comes with backticks (`) around the table and column names, and a comment:
Unfortunately, there is no way to remove this from generated syntax with this command.(Emphasis mine).
Here's how it goes:

mysql> show create table mytest\G
*************************** 1. row ***************************
Table: mytest
Create Table: CREATE TABLE `mytest` (
`id` int(11) NOT NULL,
`description` varchar(50) DEFAULT NULL,
1 row in set (0.00 sec)

Of course, there is a way!

mysql> pager tr -d '`'
PAGER set to 'tr -d '`''
mysql> show create table mytest\G
*************************** 1. row ***************************
Table: mytest
Create Table: CREATE TABLE …
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Bzr and launchpad tricks: firefox plugin

If you work with bazaar, you have seen its URIs. You can find the complete list is in the bzr help urlspec. Although I commonly use only a subset of that, like bzr+ssh:// and

In addition I often use Launchpad aliases, such as lp:~maria-captains/maria/5.3-serg/, lp:maria/5.3, and lp:869001.

And finally, there are common abbreviations that we have used in MySQL, and others that we use in MariaDB, for example bug#12345 and wl#90.

What’s annoying, I need to remember that wl#90 corresponds to and type the latter in the location bar of the …

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Less known facts about MySQL user grants

Reading MySQL security: inconsistencies I remembered a few related experiments that I did several years ago when I was studying for the MySQL certification. The first fact that came to mind is about the clause "WITH GRANT OPTION", which can only be given on the full set of options, not on a single grant. For example

GRANT INSERT,DELETE,UPDATE on world.* to myuser identified by 'mypass';
GRANT SELECT on world.* to myuser identified by 'mypass' WITH GRANT OPTION;
show grants for myuser\G
*************************** 1. row ***************************
Grants for myuser@%: GRANT USAGE ON *.* TO 'myuser'@'%' IDENTIFIED BY PASSWORD '*6C8989366EAF75BB670AD8EA7A7FC1176A95CEF4'
*************************** 2. row ***************************
Grants for myuser@%: GRANT SELECT, INSERT, UPDATE, DELETE ON `world`.* TO 'myuser'@'%' WITH GRANT …
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Quick benchmarking trick

I have been doing quite a lot of benchmarking recently.
I needed to find a safe way of measuring the time spend by the database doing a long task, like catching up on a huge backlog of accumulated replication updates. The problem with measuring this event is that I can record when it starts, but I can't easily detect when it finishes. My initial approach was to monitor the database and count the tables rows to see when the task was done, but I ended up affecting the task performance with my additional queries. So I thought of another method.
Since I had control on what was sent from the master to the slave, I used the following:
The initial time is calculated as the minimum creation time of the databases that I know are created during the exercise. Let's say that I had 5 databases named from db1 to db5:

set @START = (select min(create_time) from information_schema.tables where table_schema like "db%")

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implementing table quotas in MySQL

I have just seen Limiting table disk quota in MySQL by Shlomi Noach, and I could not resist.
You can actually implement a disk quota using an updatable view with the CHECK OPTION.
Instead of giving the user access to the table, you give access to the view (at least for inserting, see the caveat at the end), and you will get a genuine MySQL error when the limit is reached.

drop table if exists logs;
create table logs (t mediumtext) engine=innodb;

drop function if exists exceeded_logs_quota ;
create function exceeded_logs_quota()
returns boolean
return (
select CASE
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A hidden options file trick

I was listening today to the OurSQL Episode 36: It's Not Our (De)fault! Part 1. As usual, Sheeri and Sarah are very informational and entertaining while explaining the innards of MySQL and their best practices.
Being a DBA oriented show, there was an omission in this podcast. There was no mention of custom groups that you can have for your my.cnf. This is mostly useful for developers. If your application requires some specific settings, instead of using a separated configuration file, you can use a different group, and then instruct your client applications to use that group.
By default, all client applications read the "[client]" group.
But you can tell your client to read a group that you can call whatever you like.
For example, with this configuration file,

[client] …
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Beware Starting Slaves in the Position in the file

I’ve seen many a good DBA make the master of starting slaves from the position in the file, most recently this week, that I want to bring it to everyone’s attention. Of course I mean the underlying issue and not the names of the DBA because that would be cruel.

In the typical scenario where this is an issue, the sequence of events is roughly the same with some small variation. A cold backup or a snapshot is restored onto a new server to build out a new slave. The binary log position from the file, which is part of the backup, is used to start replication. Eventually after a short while, someone notices data discrepancies on the new slave compared to the master or replication stops due to an error.

The problem can be best looked by looking the slave status output in MySQL like below:

mysql> show slave status\G
*************************** 1. row *************************** …
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LOAD DATA: a tricky replication issue

When you are importing large amounts of data from other sources LOAD DATA is a common method of inserting data into a table.
It is one of the old commands implemented in MySQL. As such it is very fast, and it has been optimized for both MyISAM and InnoDB.
All is well when you are loading data into a standalone server. All is almost well when you are using replication. LOAD DATA used to be a problem in old versions of MYSQL, prior to 4.1. With recent versions of MySQL, it is replicated correctly, and sometimes efficiently.
The trouble starts when the data file is big. The definition of big and the amount of trouble that you can get depends on many factors. That's why users may not realize that this problem exists, even with fairly large files, and then being hit by this disaster when the file is only a little larger than the previous …

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