EXPLAIN EXTENDED

From Stack Overflow:

My table looks like:

person_id	car_id	miles
1	1	100
1	2	200
2	3	1000
2	4	500

I need to total the miles for each person and then average those totals.

This is a good question to demonstrate how to improve MySQL queries using covering indexes.

The query should return a single value, which is SUM(miles) / COUNT(DISTINCT person_id). Since this query does not relate the fields to each other (only total sum and total count matter), it could be performed using only the indexes on miles and person_id. However, MySQL is not able to do this by default and it requires a little help from developer’s side.

Let’s create a sample table:

Table creation details

CREATE TABLE filler (
        id INT NOT NULL PRIMARY KEY AUTO_INCREMENT
) ENGINE=Memory;

CREATE TABLE persons (
        id INT NOT NULL PRIMARY KEY,
        person_id INT NOT NULL,
        miles INT NOT NULL,
        stuffing VARCHAR(200) NOT NULL,
        KEY ix_persons_person (person_id),
        KEY ix_persons_miles (miles)
) ENGINE=InnoDB;

DELIMITER $$

CREATE PROCEDURE prc_filler(cnt INT)
BEGIN
        DECLARE _cnt INT;
        SET _cnt = 1;
        WHILE _cnt <= cnt DO
                INSERT
                INTO    filler
                SELECT  _cnt;
                SET _cnt = _cnt + 1;
        END WHILE;
END
$$

DELIMITER ;

START TRANSACTION;
CALL prc_filler(1000000);
COMMIT;

INSERT
INTO    persons
SELECT  id, ((id - 1) % 1000) + 1, FLOOR(RAND(20091110) * 10000) + 1,
        RPAD('', 200, '*')
FROM    filler;

This table has 1,000,000 rows with 1,000 distinct person_id‘s. There are indexes on miles and person_id.

A plain query would look like this:

SELECT  SUM(miles) / COUNT(DISTINCT person_id) AS total
FROM    persons

select (sum(`20091110_indexes`.`persons`.`miles`) / count(distinct `20091110_indexes`.`persons`.`person_id`)) AS `total` from `20091110_indexes`.`persons`

The query takes more than 2 seconds.

If we look into the query plan we will see that a full table scan is used. The query needs both the values of person_id and miles and no index contains both values. MySQL browses all table records which are quite large.

But despite the fact that the query needs both values, it does not need to correspond the values. It may calculate the SUM first (using an index on miles), and COUNT(DISTINCT person_id) in another query. Since index records are much less in size that the table records, the separate aggregates on them will be calculated much faster.

And we can improve the query even more by replacing COUNT(DISTINCT person_id) with a COUNT over DISTINCT person_id in a subquery. The latter query will use INDEX FOR GROUP BY and therefore be almost instant.

Here’s how the new improved query would look like:

SELECT  SUM(miles) /
        (
        SELECT  COUNT(*)
        FROM    (
                SELECT  DISTINCT person_id
                FROM    persons
                ) q
        ) AS total
FROM    persons

select (sum(`20091110_indexes`.`persons`.`miles`) / (select count(0) AS `COUNT(*)` from (select distinct `20091110_indexes`.`persons`.`person_id` AS `person_id` from `20091110_indexes`.`persons`) `q`)) AS `total` from `20091110_indexes`.`persons`

This query uses separate indexes to calculate the aggregate values, and by using INDEX FOR GROUP-BY, COUNT(DISTINCT person_id) is calculated almost instantly.

Overall query performance is 750 ms, or thrice as fast as the original query.

From Stack Overflow:

When I run an SQL command like the one below, it takes more than 15 seconds:

SELECT  *
FROM    news
WHERE   cat_id = 4
ORDER BY
        id DESC
LIMIT   150000, 10

EXPLAIN shows that its using where and the index on (cat_id, id)

LIMIT 20, 10 on the same query only takes several milliseconds.

This task can be reformulated like this: take the last 150,010 rows in id order and return the first 10 of them

It means that though we only need 10 records we still need to count off the first 150,000.

The table has an index which keeps the records ordered. This allows us not to use a filesort. However, the query is still far from being efficient: 15 seconds for 150,000 records (which are already ordered) is way too much.

To better understand the reason behind the low performance let’s look into this picture:

Read the rest of this entry »

Answering questions asked on the site.

Princess asks:

Hello, I’ve got a problem with SQL.

I have a table which contains the production details for the factories. They are not factories and items of course but I cannot disclose the project and need to obfuscate so let’s pretend they are :)

I need to select the items for which the first 5 factories have low production rate.

I tried to do the query like this:

SELECT  ProductionItem
FROM    FactoryProductions
WHERE   5 >= FactoryID
        AND 100 >= ProductionAmount

which returns correct results but is slow.

I have an index on (FactoryID, ProductionAmount).

There are 13 Factories and 2,300,000 Items

This is in MySQL

This is a nice illustration of how index range evaluation works in MySQL and how to optimize it.

We will assume that the ProductionAmount is usually much higher than 100

Now, let’s create a sample table:
Read the rest of this entry »

From Stack Overflow:

Are the following queries efficient in MySQL:

SELECT  *
FROM    table
WHERE   field & number = number
-- to find values with superset of number's bits

SELECT  *
FROM    table
WHERE   field | number = number
-- to find values with subset of number's bits

, if an index for the field has been created?

If not, is there a way to make it run faster?

An index can be used for the following things:

1. To limit the number of records scanned
2. To lower the number of row lookups

When doing a full table scan, every record should be fetched and examined. If the table contains say, 1,000,000 records, and each record is 100 bytes long, then 100 Mb worth of data should be processed. These data are usually scanned sequentially.

An index row, on the other hand, contains the key value, the row pointer and some additional information (pointers to the parents and children). On a MyISAM table, each row of the index on an INT column occupies 10 bytes (4 (sizeof(INT)) + 6 (default MyISAM pointer size)) plus some overhead (block headers, leaf pointers etc).

Even if we cannot build a range on the index and need to look over all index values, 10 Mb is far less than 100 Mb.

However, scanning the index has two drawbacks:

1. Traversing the B-Tree is more slow than a full table scan, since the former is not sequential
2. When the condition we search for matches the index key, we should follow the pointer to fetch the unindexed values from the table row. This also takes some time.

Both these things can be optimized:

• Oracle has INDEX FAST FULL SCAN access method. It scans the index rows in their physical order, not logical. The key order is not maintained in such a scan, but we not always need it anyway.

  This makes the index scanning sequential (and taking the constant time)

• PostgreSQL has bitmap access method. PostgreSQL cannot disregard the index order as Oracle does so it has to traverse the tree anyway. But instead of following the pointer to fetch the values it fills a special bitmap with a bit set for each matching row.

  The order of bits corresponds to the physical order of rows.

  When it’s over with setting bits it just reads the bitmap and fetches the rows with the bits set. Since the bits are ordered, the rows fetched are ordered too, so this scan is also sequential. Scanning the bitmap takes constant time (which is usually negligible since bitmaps are very small), pure time for fetching the rows depends on the number of bits set but is no longer than a full scan (since the scan is sequential).

MySQL, however, does neither of these optimizations.

Read the rest of this entry »

Continuing the series:

What is better to store hierarchical data: nested sets model or adjacency list (parent-child) model?

For detailed explanations of the terms, see the first article in the series:

• Adjacency list vs. nested sets: PostgreSQL

This is the last article of the series which covers MySQL.

MySQL differs from the other systems, since it is the only system of the big four that does not support recursion natively. It has neither recursive CTE‘s nor CONNECT BY clause, not even rowset returning functions that help to emulate recursion in PostgreSQL 8.3.

MySQL supports a thing that all other systems either lack or implement inefficiently: session variables. They can be set in a SELECT clause and can be used to keep some kind of a state between the rows as they are processed and returned in a rowset.

This of course is against the whole spirit of SQL, since SQL implies operations on whole sets and session variables operate on rows and are totally dependent on the order they are returned or processed. But if used properly, this behavior can be exploited to emulate some things that MySQL lacks: analytic functions, efficient random row sampling etc.

Hierarchical functions are among the things that need to be emulated in MySQL using session variables to keep the function state.

Here’s the old article in my blog that shows how to do this:

• Hierarchical queries in MySQL

On the other hand, MySQL implements one more thing that is useful for nested sets model: SPATIAL indexes.
Read the rest of this entry »

Which method is best to select values present in one table but missing in another one?

This:

SELECT  l.*
FROM    t_left l
LEFT JOIN
        t_right r
ON      r.value = l.value
WHERE   r.value IS NULL

, this:

SELECT  l.*
FROM    t_left l
WHERE   l.value NOT IN
        (
        SELECT  value
        FROM    t_right r
        )

or this:

SELECT  l.*
FROM    t_left l
WHERE   NOT EXISTS
        (
        SELECT  NULL
        FROM    t_right r
        WHERE   r.value = l.value
        )

Finally, it’s MySQL time.

As always, we will create the sample tables:

Read the rest of this entry »

Answering questions asked on the site.

Magerio asks:

Hello, thanks for your great post about advanced row sampling, it helped me a lot.

I have a question.

I am using your SQL query to show a portfolio of products and these products have a certain integer quality level (the lower, the better).

What if I want to show the best 3 products of each category?

Just a quick reminder: the article mentioned describes a query to emulate ROW_NUMBER() in MySQL. Since MySQL does not support analytic (window) functions natively, we had to do some kind of a trick to mimic their behaviour.

The original article described a query to obtain 15 first id‘s for each grouper. The idea behind this query was quite simple:

1. Take a list of groupers using DISTINCT
2. For each grouper, find the 15th id using LIMIT in a correlated subquery. We have now a set of (grouper, id15) pairs: for any given grouper we have the id of the 15th row (ordered by id).
3. Join this resultset with the main table so that for each grouper, the id‘s less or equal to the 15th id are selected. We then will have 15 lowest id‘s for each grouper that is exactly what we were after.

This emulated ROW_NUMBER() OVER (PARTITION BY grouper ORDER BY id): a function with a single PARTITION BY expression and a single ORDER BY expression.

But in your case there are multiple ORDER BY expressions. It may be not so obvious (since the question mentions only one ordering criterion, namely, quality). However, the solution above relied on the fact that the field in ORDER BY was unique, so we now that there are exactly 15 rows with id less or equal to the id returned with LIMIT 15. Since this is most probably not the case with quality, we should include id into the ORDER BY clause so that it makes the ordering conidition unique.

A subquery in MySQL, however, can only return a single value.

To work around this, we should use two subqueries that will return us a single column in an ORDER BY condition.

Let’s create a sample table:
Read the rest of this entry »

From Stack Overflow:

I have a database that stores products available on the market and products still in development in two separate tables (market_product and dev_product).

A third table (substance) contains all substances a product can made of.

Other two tables (marked_product_comp and dev_product_comp) mantains product compositions.

I want to select products still in development that are made of the same ingredients of marketed products.

This means that we need to return every dev_product that has at least one market_product with complete substance match.

In the following example:

we should return development product 1, since it’s made of apples and bananas, and there is at least one product already on market (namely, market product 2) that is made of exactly same things and nothing else.

We should not return development product 3, because these is no complete match with market product 3, since is daikons are used to make the latter but not the former.

In one of the previous articles I wrote about a similar task in SQL Server:

• Matching whole sets

However, this used FULL OUTER JOIN which MySQL lacks.

Let’s see how to implement this task in MySQL:

Read the rest of this entry »

From Stack Overflow:

I have a table that holds data about items that existed at a certain time (regular snapshots taken).

Simple example:

Timestamp	ID
1	A
1	B
2	A
2	B
2	C
3	A
3	D
4	D
4	E

In this case, item C gets created sometime between snapshot 1 and 2, sometime between snapshot 2 and 3 B and C disappear and D gets created, etc.

The table is reasonably large (millions of records) and for each timestamp there are about 50 records.

What’s the most efficient way of selecting the item ids for items that disappear between two consecutive timestamps?

So for the above example I would like to get the following:

Previous snapshot	Current snapshot	Removed
1	2	NULL
2	3	B, C
3	4	A

If it doesn’t make the query inefficient, can it be extended to automatically use the latest (i. e. MAX) timestamp and the previous one?

We basically need to do the following things here:

1. Split the table into sets grouped by timestamp
2. Compare each set with the one of previous timestamp
3. Find the values missing in the current set and concatenate them

This is possible to do using only the standard ANSI SQL operators, however, this will be inefficient in MySQL.

Let’s create a sample table and see how to work around this:

Read the rest of this entry »

When you need to pass a list of parameters into a MySQL, there are at least two method to do this:

• Issue a dynamical query which would fill an IN list with constant values
• Fill a temporary table with these values then use it in a JOIN

The second method is considered better for many reasons (you can easily reuse the values, it’s more injection-proof etc.)

Leaving all these issues aside, let’s compare these methods performance-wise.

We will create a table of 50,000 records, pass a list of 500 parameters (using both methods) and see which is faster:

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