EXPLAIN EXTENDED

How to create fast database queries

Empty row if condition does not match

with 5 comments

Just found that in a Google referer to the blog:

I want SQL to return blank row even if the condition does not match

This may be useful for certain ORMs which always expect a single row as a result of a query.

Say, we have a query like that:

SELECT  *
FROM    mytable
WHERE   id = 42

and want it to return a single row (possibly consisting of NULL values) no matter what.

If we had a join and the condition in the ON clause:

SELECT  m.*
FROM    values v
JOIN    mytable m
ON      m.id = v.value

, we could just rewrite an INNER JOIN to a LEFT JOIN.

SELECT  m.*
FROM    values v
LEFT JOIN
mytable m
ON      m.id = v.value

This way, we would have at least one record returned for each entry in values.

In our original query we don't have a table to join with. But we can easily generate it:

SELECT  m.*
FROM    (
SELECT  42 AS value
) v
LEFT JOIN
mytable m
ON      m.id = v.value

If id is a PRIMARY KEY on mytable, this query would return exactly one record, regardless of whether it such an id exists in mytable or not.

Written by Quassnoi

January 3rd, 2013 at 11:00 pm

Posted in MySQL

Happy New Year!

with 2 comments

It's been a really long year.

The blog has not been updated during this year. I focused on improving my SQL skills and helping other folks to improve theirs.

During this year Explain Extended has become a team. We now do SQL consulting and database development. I'll write a post about it soon, but send me a message if you're impatient to wait.

Now, to the new year post.

New Year has always seemed like the most global thing to me. It does not happen to everyone at once. When I was a kid I wanted to go to space and see the New Year marching through the planet with my own eyes (and it's not that I don't want it anymore).

Blue Marble

Now we have satellite images and such, but with a little effort we can see how our planet looks from space using SQL.

To do this, we need some map data, basic math and pretty simple SQL. I'll use PostgreSQL for that.
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Written by Quassnoi

December 31st, 2012 at 11:00 pm

Posted in Uncategorized

Happy New Year!

with 18 comments

This winter is anomalously warm in Europe, there is no snow and no New Year mood. So today we will be drawing a snowflake in PostgreSQL.

#1. A little theory

Core of a snowflake is six large symmetrical ice crystals growing from the common center. Out of these larger crystals other, smaller, crystals grow.

The overall shape of the snowflake is defined by how large do crystals grow and where exactly are they attached to each other.

These things are defined by fluctuations in air and temperature conditions around the snowflake. Because the flake itself is very small, in any given moment the conditions are nearly identical around each crystal, that's why the offspring crystals pop up in almost same places and grow to almost same lengths. Different flakes, though, constantly move to and from each other and are subject to very different fluctuations, and that's why they grow so unique.

Except for the root crystals (of which there are six), the child icicles grow in symmetrical pairs. More than that, each branch grows their own children (also in pairs), so on each step there are twice as many crystals, but they all share almost same length and angle. This gives the snowflake its symmetrical look.

So we can easily see that, despite the fact there may be many child crystals, the shape of a snowflake is defined by a relatively small number of parameters: how many children each crystal produces, where are they attached to it, at which angle they grow and to which length.

Now, let's try to model it.
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Written by Quassnoi

December 31st, 2011 at 11:00 pm

Posted in Miscellaneous

What’s UNPIVOT good for?

with 5 comments

Answering questions asked on the site.

Karen asks:

… I've always thought PIVOT and UNPIVOT are signs of a poorly designed database. I mean, is there a legitimate use for them if your model is OK?

I've made an actual use for them in a project I've been working on for the last several months (which is partly why there were no updates for so long!)

Part of the project is a task management system where each task has several persons related to it. There can be the creator of the task, the person the task is assigned to, the actual author of the task (on behalf of whom the task is created), and the task can be possible completed by a person or deleted by a person. A total of 5 fields related to persons.

Now, we need to take all tasks within a certain time range and list all people involved in them.

Let's create a sample table and see how would we do that.

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Written by Quassnoi

June 30th, 2011 at 11:00 pm

Posted in SQL Server

Shared Plan and Algorithm Network Cache (SPANC)

with 5 comments

Due to the nature of my work I have to deal with various database systems.

While SQL is more or less standardized, the optimizers are implemented differently in the different systems. Some systems cannot join tables with anything other than nested loops, the other can only GROUP BY using a sort, etc.

So when you write a join in, say, MySQL, you cannot expect it to be a sort merge join (and you should consider this fact when designing the query). Or, when you write a DISTINCT in SQL Server, you can't expect a loose index scan. These are limitations put by their optimizers.

However, in the last three months I noticed a great improvement in the queries where I could not expect any.

It started when I tried to debug this in SQL Server:

SELECT  DISTINCT order
FROM    orderItem

while yielded this plan:

Similar results were obtained on Oracle:

Plan hash value: 1345318323

---------------------------------------------------------------------------------------------------------------------------------
| Id  | Operation                                                         | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                                                  |             |       |   200 |     2  (50)| 00:00:01 |
|   1 |  REMOTE SPANC QUERY (SQLSERVER, MYSQL, POSTGRESQL, STACKOVERFLOW) |             |       |   200 |     2  (50)| 00:00:01 |
---------------------------------------------------------------------------------------------------------------------------------

, MySQL:

+----+-------------+-----------+-------+---------------+---------+---------+------+---------+-----------------------------------------------------+
| id | select_type | table     | type  | possible_keys | key     | key_len | ref  | rows    | Extra                                               |
+----+-------------+-----------+-------+---------------+---------+---------+------+---------+-----------------------------------------------------+
|  1 | SIMPLE      | orderItem | spanc | NULL          | ALL     | NULL    | NULL |         | Using Oracle, PostgreSQL, SQL Server, StackOverflow |
+----+-------------+-----------+-------+---------------+---------+---------+------+---------+-----------------------------------------------------+

and PostgreSQL:

Seq Scan on OrderItem  (cost=0.00..6.44 width=4)
 -> Remote Scan on SPANC (Oracle, MySQL, SQL Server, StackOverflow)   (cost=0.00..100.00 width=4)

Network analysis has shown weird encrypted activity between the servers in my internal network which host SQL Server, Oracle, PostgreSQL and MySQL servers.

Ultimately, there was unencrypted activity outside of the internal network which turned out to be an HTTP POST request followed by several GET polls to http://stackoverflow.com/questions/5518080/distinct-optimization.

It seems that the developers of major database systems agreed to share the knowledge about the most efficient query plans in some kind of a distributed storage (which probably is called SPANC as we can see in the query plans) and provide an interface to access each other's systems.

It also seems that these systems treat Stack Overflow as an external optimization engine where the most experienced developers can build their plans for them in a most efficient way.

I would be glad to have further clarification from the companies staff.

This also begs a question: how many of regular Stack Overflow participants are in fact query engines disguised as curious fellow developers?

It would be definitely nice to know.

Written by Quassnoi

April 1st, 2011 at 11:00 pm

Posted in Miscellaneous

MySQL: GROUP BY in UNION

with 2 comments

From Stack Overflow:

I have a query where I have a custom developed UDF that is used to calculate whether or not certain points are within a polygon (first query in UNION) or circular (second query in UNION) shape.

SELECT  a.geo_boundary_id, …
FROM     geo_boundary_vertex a, …
…
GROUP BY
a.geo_boundary_id
UNION
SELECT  b.geo_boundary_id, …
FROM     geo_boundary b, …
…
GROUP BY
b.geo_boundary_id

When I run an explain for the query I get filesort for both queries within the UNION.

Now, I can split the queries up and use the ORDER BY NULL trick to get rid of the filesort however when I attempt to add that to the end of a UNION it doesn't work.

How do I get rid of the filesort?

In MySQL, GROUP BY also implies ORDER BY on the same set of expressions in the same order. That's why it adds an additional filesort operation to sort the resultset if it does not come out naturally sorted (say, from an index).

This is not always a desired behavior, and MySQL manual suggests adding ORDER BY NULL to the queries where sorting is not required. This can improve performance of the queries.

Let's create a sample table and see:

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Written by Quassnoi

March 30th, 2011 at 11:00 pm

Posted in MySQL

MySQL: splitting aggregate queries

with 2 comments

Answering questions asked on the site.

Victor asks:

I have a table which I will call sale to protect the innocent:

Sale
id product price amount date

I need to retrieve ultimate values of price, amount and date for each product:

SELECT  product,
MIN(price), MAX(price),
MIN(amount), MAX(amount),
MIN(date), MAX(date)
FROM    sale
GROUP BY
product

The query only returns about 100 records.

I have all these fields indexed (together with product), but this still produces a full table scan over 3,000,000 records.

How do I speed up this query?

To retrieve the ultimate values of the fields, MySQL would just need to make a loose index scan over each index and take the max and min value of the field for each product.

However, the optimizer won't do when multiple indexes are involved. Instead, it will revert to a full scan.

There is a workaround for it. Let's create a sample table and see them:

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Written by Quassnoi

March 28th, 2011 at 11:00 pm

Posted in MySQL

Things SQL needs: SERIES()

with 9 comments

Recently I had to deal with several scenarios which required processing and aggregating continuous series of data.

I believe this could be best illustrated with an example:

id source value
1 1 10
2 1 20
3 2 15
4 2 25
5 1 45
6 3 50
7 3 35
8 1 40
9 1 10

The records are ordered by id, and within this order there are continuous series of records which share the same value of source. In the table above, the series are separated by thick lines.

We want to calculate some aggregates across each of the series: MIN, MAX, SUM, AVG, whatever:

source min max sum avg
1 10 20 30 15.00
2 15 25 40 20.00
1 45 45 45 45.00
3 35 50 85 42.50
1 10 40 50 25.00

This can be used for different things. I used that for:

  • Reading sensors from a moving elevator (thus tracking its position)
  • Recording user's activity on a site
  • Tracking the primary node in a server cluster

, but almost any seasoned database developer can recall a need for such a query.

As you can see, the values of source are repeating so a mere GROUP BY won't work here.

In the systems supporting window functions there is a workaround for that:

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Written by Quassnoi

February 18th, 2011 at 11:00 pm

Posted in Miscellaneous

Late row lookups: InnoDB

with 8 comments

Answering questions asked on the site.

Aryé asks:

Thanks for your article about late row lookups in MySQL.

I have two questions for you please:

  • Is this workaround specific to MyISAM engine?
  • How does PostgreSQL handle this?

The questions concerns a certain workaround for MySQL LIMIT … OFFSET queries like this:

SELECT  *
FROM    mytable
ORDER BY
id
LIMIT   10
OFFSET  10000

which can be improved using a little rewrite:

SELECT  m.*
FROM    (
SELECT  id
FROM    mytable
ORDER BY
id
LIMIT   10
OFFSET  10000
) q
JOIN    mytable m
ON      m.id = q.id
ORDER BY
m.id

For the rationale behind this improvement, please read the original article.

Now, to the questions.

The second questions is easy: PostgreSQL won't pull the fields from the table until it really needs them. If a query involving an ORDER BY along with LIMIT and OFFSET is optimized to use the index for the ORDER BY part, the table lookups won't happen for the records skipped.

Though PostgreSQL does not reflect the table lookups in the EXPLAIN output, a simple test would show us that they are done only LIMIT times, not OFFSET + LIMIT (like MySQL does).

Now, let's try to answer the first question: will this trick improve the queries against an InnoDB table?

To do this, we will create a sample table:

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Written by Quassnoi

February 11th, 2011 at 11:00 pm

Posted in MySQL

Happy New Year!

with 9 comments

Some say SQL is not good at graphics.

Well, they have some point. Database engines lack scanner drivers, there is no easy way to do sepia, and magic wand, let's be honest, is just poor.

However, you can make some New Year paintings with SQL.

Let's make a New Year clock showing 12 o'clock in Oracle.

Read the rest of this entry »

Written by Quassnoi

December 31st, 2010 at 11:00 pm

Posted in Miscellaneous