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

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

5. MYSQL transaction configuration item

innodb_flush_log_at_trx_commit=1

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

innodb_flush_log_at_trx_commit=2

explain usage

EXPLAIN tbl_name或：EXPLAIN [EXTENDED] SELECT select_options

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)

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

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

type

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.

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!

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