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Editor to share with you how the explain implementation plan key_len calculation rules are, I believe that most people do not know much about it, so share this article for your reference, I hope you will learn a lot after reading this article, let's go to know it!

Its rules are as follows:

1. All index fields, if not null is not set, need to be added by one byte.

two。 Fixed-length fields, four bytes for int, three bytes for date, and n characters for char (n).

3. For the field varchar (n), there are n characters + two bytes.

4. The number of bytes occupied by a character varies with different character sets. Latin1-encoded, one character occupies one byte, gbk-encoded, one character takes two bytes, utf8-encoded, one character takes three bytes.

Examples are as follows:

Mysql > desc person

+-+ +

| | Field | Type | Null | Key | Default | Extra | |

+-+ +

| | id | varchar | NO | PRI | |

| | name | varchar | YES | MUL | NULL |

| | age | int (10) | YES | | NULL |

+-+ +

3 rows in set (0.00 sec)

Check the index:

Mysql > show index from person

+ -+

| | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |

+ -+

| | person | 0 | PRIMARY | 1 | id | A | 2 | NULL | NULL | | BTREE |

| | person | 1 | idx_name | 1 | name | A | 2 | NULL | NULL | YES | BTREE |

+ -+

2 rows in set (0.00 sec)

Mysql > explain select name from person

+-- +

| | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |

+-- +

| | 1 | SIMPLE | person | index | NULL | idx_name | 103 | NULL | 2 | Using index |

+-- +

1 row in set (0.00 sec)

According to the above calculation rules:

All index fields, if not null is not set, need to be added by one byte

For the field varchar (n), there are n characters + two bytes

Therefore: key_len=100+2+1=103

These are all the contents of this article entitled "what are the key_len calculation rules in the explain implementation Plan?" Thank you for reading! I believe we all have a certain understanding, hope to share the content to help you, if you want to learn more knowledge, welcome to follow the industry information channel!

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