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How to create fast database queries

Archive for March, 2009

Analytic functions: optimizing SUM and ROW_NUMBER

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In the previous articles I wrote about emulating numerous analytic function in MySQL.

Using methods described above, it's possible to emulate almost all analytic functions present in Oracle and SQL Server.

Here are these methods in a nutshell:

  • Select all table rows ordered by PARTITION BY columns, then by ORDER BY columns of the analytic function
  • Track the grouing sets by using session variables initialized in the first subquery
  • If the analytic function needs some precalculations to be evaluated (like, count of the rows in the grouping set, sum of the values etc), join the table with the precalculated aggregates
  • Use state session variables to calculate the analytic function and store intermediate values between rows
  • Initialize state session variables whenever the grouping set changes

This may sound confusing, but if you take a look on the examples from the previous articles, it will become clear as a bell.

This methods work and work well, if you need to select all rows from the tables.

But what if you need to implement some filtering? Do we really need to count millions of rows if we need first three? Do we really need to inspect all rows to find a maximum if we have an index?

Of course, no.

Analytic functions can be optimized as well as any other queries.
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Written by Quassnoi

March 11th, 2009 at 11:00 pm

Posted in MySQL

Analytic functions: FIRST_VALUE, LAST_VALUE, LEAD, LAG

with 5 comments

In the previous articles I wrote about emulating some of the analytic functions in MySQL.

Today, I'll write about four more userful functions: FIRST_VALUE, LAST_VALUE, LEAD and LAG.

These functions also do not have aggregate analogs.

FIRST VALUE(column) returns the value of column from the first row of the grouping set.

LAST_VALUE(column) returns the value of column from the last row of the grouping set.

This can be illustrated by the following query:
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Written by Quassnoi

March 10th, 2009 at 11:00 pm

Posted in MySQL

Analytic functions: NTILE

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In the previous article we dealt with analytic functions SUM, AVG and ROW_NUMBER().

Now we will try to emulate NTILE.

NTILE(N) is a special function that has no aggregate analog. It divides each grouping set of rows into N subranges, based on ORDER BY clause, and returns the subrange number for each row.
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Written by Quassnoi

March 9th, 2009 at 11:00 pm

Posted in MySQL

Analytic functions: SUM, AVG, ROW_NUMBER

with 5 comments

In one of the previous articles I wrote about emulating some of analytic functions in MySQL.

Now, I'd like to cover this question more extensively.

A quick reminder: an analytic function is a function that behaves like an aggregate function with one exception: aggregate function returns one last row for each aggregated set, while an analytic function returns intermediate results too.

An analytic function can be made out of almost all aggregate functions by adding keyword OVER to them with two additional clauses: PARTITION BY and ORDER BY.

PARTITION BY is analog of GROUP BY. ORDER BY defines order in which the intermediate rows will be evaluated.

The behaviour of analytic functions can probably be best illustrated with an example:
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Written by Quassnoi

March 8th, 2009 at 11:00 pm

Posted in MySQL

Selecting friends

with 2 comments

If you are building a Yet Another Great Social Network Service to beat MySpace, you'll certainly need to keep a list of friends there, so that Alice may communicate in private with Bob, and they both can show pictures to Chris, and Eve cannot eavesdrop on them and the rest of them can do all these kinds of things these people are supposed to do.

On most networks, friendship is an irreflexive symmetric binary relation:

  • Symmetric means that if Alice is a friend of Bob, then Bob is a friend of Alice too.
  • Irreflexive means that Alice is never a friend to herself.

As it's a many-to-many relation, we sure need a separate table for it.

But how will we keep it? Should we keep the relation in the table as is (i. e. two separate rows for Alice/Bob and Bob/Alice), or keep just one row and reconstruct the relation using the set operators?

Let's check.
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Written by Quassnoi

March 7th, 2009 at 11:00 pm

Posted in MySQL

Advanced row sampling

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Yesterday I wrote an article on how to emulate analytiс function ROW_NUMBER() that is present in SQL Server and Oracle, but absent in MySQL.

Today, we will try to optimize this query.
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Written by Quassnoi

March 6th, 2009 at 9:00 pm

Posted in MySQL

Row sampling

with 2 comments

Sometimes we need to get a sample row from a table satisfying a certain condition. Like, get a first row for each month.

MS SQL and Oracle supply analytical function ROW_NUMBER() for this purpose.

Let's create a simple table to illustrate our needs and see how do we query it.
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Written by Quassnoi

March 5th, 2009 at 9:00 pm

Posted in MySQL

Aggregate concatenation

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Aggregate concatenation functions help creating a concatenated list out of a recordset. Useful for reports, hierarchical trees, etc.

MySQL supplies GROUP_CONCAT for this purpose. SYS_CONNECT_BY PATH and FOR XML can be used in Oracle and MS SQL.

In PostgreSQL, we cannot use these tricks, but we can create our own aggregate function. And this function will also accept two more extremely useful parameters: DELIMITER and IS_DISTINCT.
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Written by Quassnoi

March 4th, 2009 at 9:00 pm

Posted in PostgreSQL

First common ancestor

with 2 comments

From Stack Overflow:

Let's say that we have we have a table with the classic manager_id recursive relationship:

Users (user_id int, manager_id int) (refers to user_id)

If you randomly select 2 rows in the table, or 2 nodes, how do you find the lowest level common ancestor? My platform is SQL Server 2005 (Transact-SQL), but any ANSI compliant SQL will also work...

Very nice question.

This may be useful to check against any kind of common ancestry in a tree structure: classes, folders, etc.
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Written by Quassnoi

March 3rd, 2009 at 11:00 pm

Posted in SQL Server

Zen update

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Yesterday I wrote an article about selecting random rows efficiently.

But today on Stack Overflow:

Hi

I wish to attach a column to my table which will be a random number from a sequential list = to the number of rows.

So, if my table had 999 rows, then the numbers 1 to 999 would be assigned randomly and uniquely.

Now, I figured that I could add a dummy TempRandomColumn=Rand(), sort by that and add the numbers sequentially using PHP. But that means 999 MySQL statements.

Is there a way to do this using a single MySQL statement?

Thanks for any pointers.

Well, it's just that simple:

Creating tables here:

and performing an update:

SET @r := 0;
UPDATE t_zen
SET    zen_order = (@r := @r + 1)
ORDER BY
       RAND(20030302)

ORDER BY RAND() certainly has some beauty in it.

Written by Quassnoi

March 2nd, 2009 at 11:00 pm

Posted in MySQL