Mysql creates index to improve system running speed

Suppose we create a mytable table:
CREATE TABLE mytable( ID INT NOT NULL, username VARCHAR(16) NOT NULL ); We randomly insert 10,000 records into it, 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:
(7) Precautions for using indexes
When using indexes, there are some tips and precautions as follows:
◆The index will not contain columns with NULL values ​​
As long as the column contains NULL values, it will not Will 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 this composite index. Therefore, when designing the database, we should not let the default value of the field be NULL.
◆Use short indexes
to index the string, specifying a prefix length if possible. 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.
◆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.
◆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 “%aaa%” will not use the index but like “aaa%” will use the index.
◆Don’t operate on columns
select * from users where YEAR(adddate)<2007; will operate on each row, which will cause index failure and a full table scan, so we can change to
select * from users where adddate<'2007-01-01';
◆Do not use NOT IN and <> operations
The above has introduced the MySQL index types.
(6) Disadvantages of indexes
The benefits of using indexes are mentioned above, but excessive use of indexes will cause abuse. Therefore, the index will also have its shortcomings:
◆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.
◆Creating index files 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 to improve efficiency. If your MySQL has a large data table, you need to spend time researching and building the best indexes or optimizing query statements.
(5) Timing to create an index
Now we have learned how to create an index, so under what circumstances do we 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:
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 time, you need to index city and age, because the mytable table The userame also appears in the JOIN clause, and it is necessary to index it.
I just mentioned that only certain LIKEs need to be indexed. Because MySQL will not use the index when making queries starting with wildcard characters % and _. For example, the following sentence will use the index:
SELECT * FROM mytable WHERE username like'admin%', but the next sentence will not use the index:
SELECT * FROM mytable WHEREt Name like'%admin' Therefore, you should pay attention to the above differences when using LIKE.
(4) Composite index
To visually compare single-column indexes and composite indexes, add multiple fields to the table:
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 establishing a combined index. Just build name, city, age into an index:
ALTER TABLE mytable ADD INDEX name_city_age (name(10),city,age); When creating the table, the length of usernname 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 username, city, and age respectively, so that the table has three single-column indexes, the query efficiency will be very different from the above-mentioned 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 are there no combined indexes 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:
SELECT * FROM mytable WHREE username="admin" AND city="Zhengzhou" SELECT * FROM mytable WHREE username="admin" The following ones will not be used:
SELECT * FROM mytable WHREE age=20 AND city="Zhengzhou" SELECT * FROM mytable WHREE city="Zhengzhou"
(3) Primary key index
It is A special unique index that does not allow null values. Generally, the primary key index is created when creating the table:
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.
(2) Unique index
It is similar to the previous ordinary index, except that the value of the index column must be unique, but null values ​​are allowed. In the case of a composite index, the combination of column values ​​must be unique. It has the following creation methods:
◆Create index
CREATE UNIQUE INDEX indexName ON mytable(username(length)) ◆Modify the table structure
ALTER mytable ADD UNIQUE [indexName] ON (username(length)) ◆Create the table directly Specify
CREATE TABLE mytable( ID INT NOT NULL, username VARCHAR(16) NOT NULL, UNIQUE [indexName] (username(length)) );
(1) Ordinary index
This is the most basic index, it has no restrictions. It has the following creation methods:
◆Create index
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.
◆Modify the table structure
ALTER mytable ADD INDEX [indexName] ON (username(length)) ◆Specify 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;
Index is the key to fast search. The establishment of MySQL index is very important for the efficient operation of MySQL. Here are some common MySQL index types.
In database tables, indexing fields can greatly improve query speed.

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