September 20 2012 · mysql · optimization

Optimize MySQL query by indexing

Database optimization is very crucial for any production grade application to achieve scalability and high performance. MySQL optimization involves different levels, from optimizing SQL queries, configuring system variables, buffer and cache, refining database structure. This article is going to focus on the first part: optimizing SQL queries to reduce response time.

Optimizing SQL query can be restructuring your query, adding indexes. The (*)sec returned when a query executed is the benchmark on how different optimizations affect the response time. This article will cover optimization of SELECT statments. The optimization of DML statements will be left for later posts.

The EXPLAIN syntax

EXPLAIN command is a very important tool to understand how query is executed in MySQL. When you type EXPLAIN SELECT..., MySQL doesn't actually perform the query. It just analyzes and tell you how the query will be executed. Some of the output information:

select_type | table | type | possible_keys | key | key_len | rows | extra

Based on these information, we can start optimize our query. When a SELECT is executed, there are two ways that MySQL will retrieve the records. It can either scan the table row by row or do an index look up to find the records explicitly. Most of the times, the later case is more desirable and can improve performance significantly. Or that is to say, there should be an used index, i.e, key value in result of explain command

Simple SELECT example:

Let's consider a USER table:

|id | first_name | last_name  |
|---|------------|----------- |

Do a SELECT statement based on primary key id:



| id | select_type | table | type  | possible_keys | key     | key_len | ref   | rows | Extra |
|  1 | SIMPLE      | user  | const | PRIMARY       | PRIMARY | 4       | const |    1 |       |

Let's do another SELECT:

EXPLAIN SELECT * FROM user WHERE first_name = 'khue';

We will see the result:

| id | select_type | table | type  | possible_keys | key     | key_len | ref   | rows | Extra       |
|  1 | SIMPLE      | user  | ALL   | NULL          | NULL    | NULL    | NULL  |   18 | Using where |

No key is used to look up the result. MySQL optimizer performs a full table scan (row by row) to retrieve the record. To improve the performance, simply add the index to first_name column:

ALTER TABLE user ADD INDEX first_name_index (first_name);

When you have an index on first_name, optimizer will still be able to use this index when you only select based on part of the key only:

SELECT * FROM user WHERE first_name = 'kh%';

| id | select_type | table | type  | possible_keys   | key             | key_len | ref   | rows | Extra       |
|  1 | SIMPLE      | user  | ALL   | first_name_index| first_name_index|       30| const |   1  | Using where |

SELECT based on multiple columns:

How about searching for record based on two column:

SELECT * FROM user WHERE first_name = 'khue' AND last_name = 'vu';

You can add a multiple column index:

ALTER TABLE user ADD INDEX first_last_index (first_name, last_name);

With the above index, the optimizer will use first_last_index for SELECT with WHERE clause like:

first_name = 'abc' AND last_name = 'xyz'
first_name = 'abc' AND (last_name = 'x' OR last_name = 'y')

But not for:

last_name = 'xyz'
first_name = 'abc' OR last_name = 'xyz'

What if you have two separate indexes on these two columns? MySQL optimizer will try to merge the indexes or select one which it think will filter out more rows.

SELECT * FROM user WHERE first_name = 'abc' OR last_name = 'xyz'

With the above statement, optimizer won't use any index, either it is a combined index on two columns first_name and last_name or one index on each column. There is a trick to that:

SELECT * FROM user WHERE first_name = 'abc' UNION ALL SELECT * FROM user WHERE last_name = 'xyz'

Use EXPLAIN we will have:

| id | select_type  | table      | type | possible_keys  | key            |
|  1 | PRIMARY      | user       | ref  | first_name     | first_name     |
|  2 | UNION        | user       | ref  | last_name      | last_name      |
| NULL | UNION RESULT | <union1,2> | ALL  | NULL           | NULL         |

The optimizer will execute two SELECT statement, one PRIMARY and one UNION, and then UNION the result set. On each SELECT statement, it uses the respective index to retrieve rows. As a result, we achieve better performance than the previous statement, in which MySQL do a full table scan.

Is index always necessary?

At the begining of this article, I have mentioned that most of the time, key look up is better than full table scan. There is, however, case when full table scan is better. When you have a small table(less than 10 entries), performing a full table scan is faster than key look up. Or when the result set constitutes a large part of the table (more than 30%), full table scan to fetch result rows is faster than key look up and fetch rows later.

Fortunately, MySQL optimizer internally takes on the job of calculating the expenses by different approaches and choose the right one for you. It decides whether accessing by index will be more expensive than table scan or vice versa; which index to use among the available.

When you think MySQL optimizer is not doing the right thing, you should run ANALYZE on the table your query applies to. ANALYZE gives optimizer up-to-date statistical analysis of tables (size, indexes, etc...) thus, it has a better idea on how much resources is needed for each approach. If the optimizer still fails you after running ANALYZE, try making an index hint to it. You can suggest, force it to use or ignore an index.

What next?

So far we have shortly discussed on how to optimize MySQL query - SELECT statement - through the use of EXPLAIN. Other topics I want to extend around this theme are:

Hope I can spend time on them soon. In the mean time, there are plenty of resources to explore on MySQL optimization. You can check out the Optimization Chapter on MySQL Reference Manual as a start.

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