Tag: SQL

Slides from my talk: “Programmatic Queries: things you can code with SQL”

MySQL

Here are the slides from my talk Programmatic Queries: things you can code with SQL held on October 25th on the Percona Live event, London.

Programmatic Queries: PDF

I wish to thank those who attended my talk.

The topic of the talk was irregular and, to some extent, controversial. Should one really rely on internal implementation for optimizing her queries? Sergei Golubchik was quick to suggest that even in current and coming versions of MariaDB, some things I spoke about may not hold true. I accept.

I was in urge to complete my talk within the time frame. I was happy to hear later on that my talk was well received.

There were homework to my talk. I thought I would make some contest picking the best answers, stay tuned.

Speaking on Percona Live, London: “Programmatic Queries: things you can code with SQL”

MySQL

I’ll be speaking at the Percona Live event, held in London, October 24, 25, 2011.

My session is called Programmatic Queries: things you can code with SQL. It’s a short 30 minute talk, in which I present underlying knowledge of the programmatic nature of SQL queries within MySQL, and how to take advantage of such knowledge so as to build faster, shorter, and sometimes unexpected queries.

This is not about stored routine programming, a classic programmatic aspect of MySQL, but rather about expected order of execution: of row evaluation, of control flow statements, of table inference, of time issues.

I have far too many examples, some real-world problem solvers, and some less common in daily use, to be able to deliver them all on this session. I will pick up those which seem most interesting to me, or those best presenting the programmatic nature of the query. As time allows I may add more examples, or look into interesting future possibilities.

I hope to see you there.

Three wishes for a new year

MySQL

It’s another new year by Jewish calendar. And what do I wish for in the following year?

  1. World peace
  2. Good health to all
  3. Have some way to turn SHOW commands into SELECT statements, server side. I’m fervently trying to hack around this. Stored routines, export/import from file, text manipulation, I don’t care! I want to SELECT seconds_behind_master somehow. Without plugins.

PS, none of my last year’s wishes came true. I’ll settle for two out of three.

MySQL eval()

MySQL

I’ve just implemented an eval() call for MySQL. It is implemented with SQL, using a stored procedure. So this is not some plugin: you can use it from within your normal database server.

Just what is an eval() call?

In some programming languages it would mean: get some text, and execute it as though it were complied code. So, dynamic coding.

In SQL: get the text of query which generates SQL statements in itself (either DML or DDL), and invoke those implied SQL statements.

A simple example

Best if I present Mass killing of MySQL Connections by Peter Zaitsev. The thing is to execute a query, typically on INFORMATION_SCHEMA, which uses metadata so as to generate SQL queries/commands. Peter’s example is:

select concat('KILL ',id,';') from information_schema.processlist where user='root'

The above query generates KILL commands for all users called ‘root’. I do many such queries in common_schema: like creating the GRANT statements for accounts, the DROP KEY statements for redundant keys, the ADD and DROP statements for foreign keys etc.

So the problem is you have to export those statements to file, then execute them from file: either using SOURCE, as in Peter’s example, or from shell prompt, piping file contents into mysql client.

You can now eval()

I’ve been on family holiday for a couple of weeks, which meant no need to think of work. Which means more time to think of SQL (darn!). And I’ve found the way to do it completely within the server (no external files required). Continue reading » “MySQL eval()”

MySQL Global status difference using single query

MySQL

Have just read MySQL Global status difference using MySQL procedures / functions, by Andres Karlsson. Have commented, but realized I did not provide with a direct answer. In the comment, I suggested checking out a solution based on views, found in common_schema. But the solution in common_schema is split into two views, due to the fact views cannot handle derived tables subqueries.

Well, here’s a single query to do that: it checks GLOBAL_STATUS twice, 10 seconds apart in the following sample. It uses SLEEP() to actually wait between the two reads. Yes, you can do that with a query.

The following query shows all GLOBAL_STATUS values that have changed during the sample period.

[UPDATE] query updated to work with MySQL 5.6 optimizer Continue reading » “MySQL Global status difference using single query”

Finding CURRENT_USER for any user

MySQL

A MySQL account is a user/host combination. A MySQL connection is done by a user connecting from some host.

However, the user/host from which the connection is made are not the same as the user/host as specified in the account. For example, the account may be created thus:

CREATE USER 'temp'@'10.0.0.%' IDENTIFIED BY '123456';

The host as specified in the above account is a wildcard host. A connection by the ‘temp’ user from ‘10.0.0.3’ can map into that account. It thus happens that the connected user is ‘temp’@’10.0.0.3’, yet the assigned account is ‘temp’@’10.0.0.%’.

MySQL provides with the USER() and CURRENT_USER() which map to the connected user and the assigned account, respectively, and which lets the current session identify the relation between the two. Read more on this on the MySQL docs.

The problem

And the trouble is: MySQL only provides this functionality for the current session. Surprisingly, given a user/host combination, I cannot get MySQL to tell me which account matches those details.

The inconsistency

And I care because there is an inconsistency. Namely, when I do SHOW PROCESSLIST MySQL tells me the user & host from which the connection is made. It does not tell me the account for which the process is assigned. Continue reading » “Finding CURRENT_USER for any user”

Announcing common_schema: common views & routines for MySQL

MySQL

Today I have released common_schema, a utility schema for MySQL which includes many views and functions, and is aimed to be installed on any MySQL server.

What does it do?

There are views answering for all sorts of useful information: stuff related to schema analysis, data dimensions, monitoring, processes & transactions, security, internals… There are basic functions answering for common needs.

Some of the views/routines simply formalize those queries we tend to write over and over again. Others take the place of external tools, answering complex questions via SQL and metadata. Still others help out with SQL generation.

Here are a few highlights:

There’s more. Take a look at the common_schema documentation for full listing. And it’s evolving: I’ve got quite a few ideas already for future components.

Some of these views rely on heavyweight INFORMATION_SCHEMA tables. You should be aware of the impact and risks.

What do I need to install?

There’s no script or executable file. It’s just a schema. The distribution in an SQL file which generates common_schema. Much like a dump file.

Continue reading » “Announcing common_schema: common views & routines for MySQL”

Pop quiz answered: “what would be the results of the following queries?”

MySQL

The quiz presented poses with an uncommon, though valid SQL syntax: one is allowed to use quoted name aliases. Thus, it is valid to write:

SELECT Name AS 'n', Continent AS 'c' FROM countries

But what does the above mean? Let’s see the results of our three questions: Continue reading » “Pop quiz answered: “what would be the results of the following queries?””