Blob or text fields use hash values and prefix index optimizations to improve query speed

1. Create the table, the storage engine is MyISAM, and use the MD5 function to create a hash value for the large text field blob

CREATE TABLE t2 (ID varchar, content blob, Hash_value varchar) engine=myisam default Charset=utf8;

2. Inserting data

INSERT into T2 values (1, repeat (' world1 '), MD5 (content));

INSERT into T2 values (2, repeat (' world2 ',), MD5 (content));

INSERT into T2 values (3, repeat (' world3 ',), MD5 (content));

INSERT into T2 values (4, repeat (' world4 ',), MD5 (content));

INSERT into T2 values (5, repeat (' World5 ', MB), MD5 (content));

3. Use hash value query, only for exact match

SELECT * FROM T2 where HASH_VALUE=MD5 (repeat (' world4 ', 70));

Using a hash value query is faster than using a BLOB text field query, because using a hash value matches a much shorter length.

4. To make a fuzzy query, you can prefix the top n columns of a BLOB field

Create index Idx_blob on T2 (content (150));

Prefix index of the first 150 bytes of the field Content field in the T2 table

5. Use explain or desc to view execution plans, use indexes

Mysql> explain select * from T2 where the content like ' world4% ';

+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

| ID | Select_type | Table | Type | Possible_keys | Key | Key_len | Ref | Rows | Extra |

+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

| 1 | Simple | T2 | Range | Idx_blob | Idx_blob | 153 |    NULL | 1 | Using where |

+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

1 row in Set (0.01 sec)

Mysql> desc SELECT * FROM T2 where the content like ' world4% ';

+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

| ID | Select_type | Table | Type | Possible_keys | Key | Key_len | Ref | Rows | Extra |

+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

| 1 | Simple | T2 | Range | Idx_blob | Idx_blob | 153 |    NULL | 1 | Using where |

+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

1 row in Set (0.00 sec)

As you can see, the prefix index Idx_blob is used.