This article will focus on describing the four types of masql database indexes. How to create a database index? Columns appearing in WHERE and JOIN need to be indexed, but not entirely so, because MySQL only does <, <=, =, >, >=, BETWEEN, IN, and sometimes LIKE. Use indexes. I hope this article can help everyone. First, let’s understand what an index is. To sum it up in one sentence: Index is the key to fast search.
The establishment of MySQL index is very important for the efficient operation of MySQL. The following introduces several common MySQL index types
In database tables, indexing fields can greatly improve query speed. Suppose we create a mytable table:
The code is as follows:
CREATE TABLE mytable( ID INT NOT NULL, username VARCHAR(16) NOT NULL );
We randomly There are 10,000 records inserted, including one: 5555, admin.
When searching for the record of username="admin" SELECT * FROM mytable WHERE username='admin';, if an index has been established on username, MySQL can accurately find the record without any scanning. On the contrary, MySQL will scan all records, that is, 10,000 records will be queried.
Indexes are divided into single column indexes and combined indexes. A single-column index means that an index only contains a single column. A table can have multiple single-column indexes, but this is not a combined index. Combined index, that is, one index contains multiple columns.
MySQL index types include:
1. Ordinary index
This is the most basic index, it has no restrictions. It has the following creation methods:
1. Create an index
The code is as follows:
CREATE INDEX indexName ON mytable(username(length));
If it is CHAR, VARCHAR type, length can be less than the actual length of the field; if it is BLOB and TEXT type, length must be specified, the same below.
2. Modify the table structure
The code is as follows:
ALTER mytable ADD INDEX [indexName] ON (username(length)) -- Specify it directly when creating the table
CREATE TABLE mytable( ID INT NOT NULL, username VARCHAR(16) NOT NULL, INDEX [indexName] (username(length)) );
-- Syntax to delete index:
DROP INDEX [indexName] ON mytable;
2. Unique index
It is similar to the previous ordinary index, except that the value of the index column must be unique, but it is allowed There is a null value. In the case of a composite index, the combination of column values must be unique. It has the following creation methods:
The code is as follows:
CREATE UNIQUE INDEX indexName ON mytable(username(length))
-- Modify the table structure
ALTER mytable ADD UNIQUE [indexName] ON (username(length))
-- Directly specify
CREATE TABLE mytable when creating the table( ID INT NOT NULL, username VARCHAR(16) NOT NULL, UNIQUE [indexName] (username(length) ) );
3. Primary key index
It is a special unique index that does not allow null values. Generally, the primary key index is created at the same time when creating the table:
The code is as follows:
CREATE TABLE mytable( ID INT NOT NULL, username VARCHAR(16) NOT NULL, PRIMARY KEY(ID) ) ;
Of course you can also use the ALTER command. Remember: a table can only have one primary key.
4. Combined index
In order to vividly compare single column index and combined index, add multiple fields to the table:
The code is as follows:
CREATE TABLE mytable( ID INT NOT NULL, username VARCHAR(16) NOT NULL, city VARCHAR(50) NOT NULL, age INT NOT NULL );
In order to further extract the efficiency of MySQL, it is necessary to Consider building a composite index. Just build name, city, age into an index:
The code is as follows:
ALTER TABLE mytable ADD INDEX name_city_age (name(10),city,age);[code]
When creating the table, the usernname length is 16, and 10 is used here. This is because generally the name length will not exceed 10, which will speed up the index query, reduce the size of the index file, and improve the update speed of INSERT.
If you create single-column indexes on usernname, city, and age respectively, so that the table has three single-column indexes, the query efficiency will be very different from the above combined index, which is far lower than our combined index. . Although there are three indexes at this time, MySQL can only use the single-column index that it thinks seems to be the most efficient.
Establishing such a combined index is actually equivalent to establishing the following three sets of combined indexes:
usernname,city,age usernname,city usernname Why is there no combined index like city and age? This is a result of the "leftmost prefix" of the MySQL composite index. The simple understanding is to only start the combination from the leftmost one. Not only queries containing these three columns will use this combined index, the following SQL will use this combined index:
[code]
SELECT * FROM mytable WHREE username="admin" AND city ="Zhengzhou" SELECT * FROM mytable WHREE username="admin"
The following ones will not be used:
The code is as follows:
SELECT * FROM mytable WHREE age=20 AND city="Zhengzhou" SELECT * FROM mytable WHREE city="Zhengzhou"
5. How to create an index
So far we have learned how to create an index, then we Under what circumstances do you need to create an index? Generally speaking, columns appearing in WHERE and JOIN need to be indexed, but this is not entirely true because MySQL only indexes <, <=, =, >, >=, BETWEEN, IN, and sometimes LIKE will use the index. For example:
The code is as follows:
SELECT t.Name FROM mytable t LEFT JOIN mytable m ON t.Name=m.username WHERE m.age=20 AND m.city='Zhengzhou'
At this point, it is necessary to index city and age. Since the userame of the mytable table also appears in the JOIN clause, it is also necessary to index it.
I just mentioned that only LIKE needs to be indexed at certain times. Because MySQL will not use the index when making queries starting with wildcard characters % and _. For example, the following sentence will use the index:
The code is as follows:
SELECT * FROM mytable WHERE username like'admin%'
The next sentence will not use the index:
The code is as follows:
SELECT * FROM mytable WHEREt Name like'�min'
Therefore, you should pay attention to the above differences when using LIKE.
6. Index Disadvantages
The above all talk about the benefits of using indexes, but excessive use of indexes will cause abuse. Therefore, the index will also have its shortcomings:
1. Although the index greatly improves the query speed, it will also reduce the speed of updating the table, such as INSERT, UPDATE and DELETE on the table. Because when updating the table, MySQL not only needs to save the data, but also save the index file.
2. Creating an index file will occupy disk space. Generally, this problem is not serious, but if you create multiple combined indexes on a large table, the index file will expand quickly.
Indexes are only one factor in improving efficiency. If your MySQL has a large amount of data tables, you need to spend time researching and building the best indexes or optimizing query statements.
7. Things to note when using indexes:
When using indexes, there are some tips and precautions:
1. The index will not contain Columns with NULL values
As long as the column contains NULL values, they will not be included in the index. As long as one column in the composite index contains a NULL value, then this column will be invalid for the composite index. Therefore, when designing the database, we should not let the default value of the field be NULL.
2. Use short index
to index the string. If possible, you should specify a prefix length. For example, if you have a CHAR(255) column, if most values are unique within the first 10 or 20 characters, then do not index the entire column. Short indexes not only improve query speed but also save disk space and I/O operations.
3. Index column sorting
MySQL query only uses one index, so if the index has been used in the where clause, the columns in order by will not use the index. Therefore, do not use sorting operations when the default sorting of the database can meet the requirements; try not to include sorting of multiple columns. If necessary, it is best to create composite indexes for these columns.
4.Like statement operation
Generally, the use of like operation is not encouraged. If it must be used, how to use it is also a problem. Like “�a%” will not use the index but like “aaa%” will.
5. Do not perform operations on columns
The code is as follows:
select * from users where YEAR(adddate)<2007;
will be in each row This will cause the index to fail and perform a full table scan, so we can change it to:
The code is as follows:
select * from users where adddate<'2007-01-01' ;
6. Do not use NOT IN and <> operations
Above, the MySQL index types are introduced. I hope to be helpful.
Two methods of indexing:
B-tree index
Note: It is called btree index, the big aspect Look, they all use balanced trees, but in terms of specific implementation, each storage engine is slightly different. For example, strictly speaking, the NDB engine uses T-tree
Myisam, in innodb, uses B-tree index by default , the theoretical query time complexity of B-tree is O(log2 (N-1)), N is the number of rows in the data table
hash index
In tables using memory storage engine, the default is hash The theoretical query time complexity of index and hash is O(1)
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Advantages and Disadvantages of Indexing Page 1/2
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