The use and performance analysis of explain in MySQL

MySQL provides an EXPLAIN command, which can analyze the SELECT statement and output the detailed information of the SELECT execution for developers to optimize. The usage of the EXPLAIN command is very simple. The knowledge of MySQL performance analysis and explain usage is in this article What we mainly want to introduce, let us introduce this process through some practical examples!

1. Use the explain statement to view the analysis results

For example,

explain select * from test1 where id=1;

will appear:

id  selecttype  table  type possible_keys  key key_len  ref rows  extra各列。

Among them,

type=const means it is found through the index once;

key=primary, means the primary key is used;

type=all, means full table scan;

key=null means that the index is not used. type=ref, because it is considered to be multiple matching rows at this time. In joint query, it is usually REF.

2. Combined index in MYSQL

Assume that the table has id, key1, key2, key3, and the three form a combined index, then

For example:

where key1=....     
where key1=1 and key2=2     
where key1=3 and key3=3 and key2=2

According to the leftmost principle, these can all be indexed, such as from test where key1=1 order by key3. If you use explain analysis, only the normal_key index is used, but only for where clause works, and the subsequent order by needs to be sorted.

3. Use slow query analysis

In my.ini:

long_query_time=1
log-slow-queries=d:\mysql5\logs\mysqlslow.log

Record data exceeding 1 second in the slow query log

You can use mysqlsla to analyze it. You can also use mysqlreport, such as

DMS analyzes the percentages of select, update, insert, delete, replace, etc.

4. MYISAM and INNODB locking# In

##myisam, pay attention to the table lock. For example, after multiple

UPDATE operations, when you select again, you will find that the SELECT operation is locked and you must wait for all UPDATE operations to be completed. After that, if you can SELECT

innodb, it will be different. Row lock is used, and the above problem does not exist.

5. MYSQL transaction configuration item

innodb_flush_log_at_trx_commit=1

means that when the transaction is committed, the transaction

log will be written to the disk immediately, and the data and index will also be updated at the same time. .

innodb_flush_log_at_trx_commit=0

When the transaction is submitted, the transaction log is not written to the disk immediately. It is written every 1 second.

innodb_flush_log_at_trx_commit=2

When the transaction is submitted, the transaction log is written to the disk file immediately (here Just write to the kernel buffer, but not flush to the disk immediately, but flush to the disk every 1 second, updating data and index at the same time

explain usage

EXPLAIN tbl_name或：EXPLAIN [EXTENDED] SELECT select_options

The former can get the field structure of a table, etc., and the latter mainly gives some relevant index information, and the focus of today’s discussion is the latter

Example##. #

mysql> explain select * from event;  
+—-+————-+——-+——+—————+——+———+——+——+——-+  
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |  
+—-+————-+——-+——+—————+——+———+——+——+——-+  
| 1 | SIMPLE | event | ALL | NULL | NULL | NULL | NULL | 13 | |  
+—-+————-+——-+——+—————+——+———+——+——+——-+  
1 row in set (0.00 sec)

The meaning of each

attributeid

select the sequence number of the query

select_type

select the type of query , mainly to distinguish between ordinary queries and complex queries such as joint queries and subqueries.

table

The rows output

refer to the table used by the joint query. The type.

type shows the access type, which is a more important indicator. The result values ​​​​from best to worst are:

type

Generally speaking, it is necessary to ensure that the query reaches at least the range level. , it is best to reach ref.

possible_keys

Indicates which index MySQL can use to find the row in the table. If it is empty, there is no relevant index. Performance can be achieved by checking the WHERE clause to see if certain fields are referenced, or by checking that the field is not suitable for indexing.

key

Displays the key that MySQL actually decides to use if no index is selected. The key is NULL.

key_len

Displays the key length that MySQL decides to use. If the key is NULL, the length is NULL. The documentation suggests that special attention can be paid to the actual use of mysql in a multiple primary key. Which part.

ref

Shows which field or constant is used with key

rows

This number indicates how much data mysql needs to traverse. Found is not accurate on innodb

Extra

If it is Only index, this means that the information is retrieved only using the information in the index tree, which is faster than scanning the entire table. .

If it is where used, it means that the where restriction is used.

If it is impossible where, it means that there is no need to use where.

If this information is not found. It will be very difficult to display Using filesort or Using temporary. The indexes of WHERE and ORDER BY often cannot take into account both. If the index is determined according to WHERE, then Using filesort will inevitably be caused in ORDER BY. This depends on filtering first and then sorting. Is it cost-effective to sort first and then filter?

Summary:

That’s all the knowledge about MySQL performance analysis and explain usage is introduced here. I believe that my friends also have a certain understanding of this. understanding!

Related recommendations:

The role of EXPLAIN in Mysql

Introduction to mysql slow query and EXPLAIN

##MySQL query performance analysis tool-explain keyword analysis

mysql explain type connection type example

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