Review of MySQL (IX): index

MySql Review ( Nine )： Indexes

What is index ？

Indexes are used to quickly find rows with a specific value on a column . No index ,MySQL Have to start with the first record , Then read the entire table until it finds the relevant rows . The bigger the watch , The more time it takes .

--  Although index can improve retrieval efficiency , But you can't add indexes at will , Because the index is also an object in the database , It also requires constant database maintenance . There are maintenance costs .
--  such as ： The data in the table is often modified , This is not suitable for adding indexes , Because once the data is modified , The index needs to be reordered , For maintenance .

--  Adding an index is to a field , Or add an index to some fields .

select ename,sal from emp where ename = 'SMITH';
--  When ename When the field is not indexed , above sql Statement will perform a full table scan , scanning ename All values in the field .
--  When ename When adding an index to a field , above sql Statement will scan... According to the index , Rapid positioning .

Create index

If the index is not used , Inquire about Wages are greater than 1500 Will perform a full table scan

An index is equivalent to the contents of a Book

The primary key will be automatically added to the index , Therefore, it is more efficient to query according to the primary key .

If often based on sal The query , And encountered a performance bottleneck , First, check whether the program has algorithm problems , Think again about sal Index , Index as follows ：

1. Create index object

   create unique index Index name on Table name ( Name );

create unique index u_ename on emp(ename);

2. Modify index name

   alter table Table name add unique index Index name ( Name );

Look at the index

show index from emp;

Use index

Be careful ： It must not be used select * … You can see type!=all 了 , Indicates that the index is used

explain select sal from emp where sal > 1500;

Delete index

DROP INDEX index_name ON talbe_name
ALTER TABLE table_name DROP INDEX index_name
ALTER TABLE table_name DROP PRIMARY KEY
--  among , The first two statements are equivalent , Delete the table_name Index in index_name.
--  The first 3 Only delete PRIMARY KEY Use... When indexing , Because a watch can only have one PRIMARY KEY Indexes , 
mysql> ALTER TABLE EMP DROP INDEX test_index;

--  The index is no longer used after deletion , The query will perform a full table scan .

Other knowledge of the index

• The underlying data structure of the index is ：B + Tree

• The realization principle of index ？
  adopt B Tree Narrow the scanning range , The underlying index is sorted , Partition , The index will carry the data in the table " Physical address ", After the data is finally retrieved through the index , Get the associated physical address ,
  After the data is retrieved through the physical index , Get the associated physical address , Locate the data in the table by physical address , Efficiency is the highest .
  select ename from emp where ename = ‘SMITH’;
  Convert to by index ：
  select ename from emp where Physical address = 0x123;

• Classification of indexes ？
  Single index ： Add an index to a single field
  Composite index ： Add an index to the union of multiple fields
  primary key ： An index is automatically added to the primary key
  unique index ： Yes unique The constrained fields are automatically indexed
  …

• When does the index expire ？
  select ename from emp where ename like ’ %A% ';
  Suo Yin
  …

• When does the index expire ？
  select ename from emp where ename like ’ %A% ';
  When fuzzy query , The first wildcard uses %, At this time, the index is invalid .