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SQL Indexes(索引)

目录

Indexes

Using Clustered Indexes

Using Nonclustered Indexes

Declaring Indexes

Using Indexes

Finding Rows Without Indexes

Finding Rows in a Heap with a Nonclustered Index

Finding Rows in a Clustered Index

Finding Rows in a Clustered Index with a Nonclustered Index

总结:

Database Tuning


Indexes

  • Index = data structure used to speed access to tuples of a relation, given values of one or more attributes.(用于给定一个或者多个属性后加速对于关系的访问)
  • Could be a hash table, but in a DBMS it is always a balanced search tree with giant nodes (a full disk page) called a B+ tree.

Using Clustered Indexes

  • Each Table Can Have Only One Clustered Index(每张表只能有一个聚焦索引)
  • The Physical Row Order of the Table and the Order of Rows in the Index Are the Same(物理储存顺序与聚焦索引的顺序一样)
  • Key Value Uniqueness Is Maintained Explicitly or Implicitly(键值的维护是显式或者隐式的)

Using Nonclustered Indexes

  • Nonclustered Indexes Are the SQL Server Default(非聚焦索引是SQL_Server的默认索引类型)
  • Existing Nonclustered Indexes Are Automatically Rebuilt When:
  1. An existing clustered index is dropped
  2. A clustered index is created
  3. The DROP_EXISTING option is used to change which columns define the clustered index

以上情况会导致非聚焦索引会重建:

  1. 已存在的聚焦索引被删除(会导致非聚焦索引储存的数据指针失效,所以需要重建)
  2. 新的聚焦索引被创建
  3. 修改了聚焦索引的列(可能会导致物理储存顺序重新排列)

总结来说的话就是,影响了物理储存顺序就可能会导致非聚焦索引的顺序失效

Declaring Indexes

No standard!

Typical syntax:

CREATE INDEX BeerInd ON
Beers(manf);
CREATE INDEX SellInd ON
Sells(bar, beer);

Using Indexes

  • Given a value v, the index takes us to only those tuples that have v in the attribute(s) of the index.(通过给定的值v,能够快速找到包含v属性的元组)

  • Example: use BeerInd and SellInd to find the prices of beers manufactured by Pete’s and sold by Joe.

SELECT price FROM Beers, Sells
WHERE manf = ’Pete’’s’ AND
Beers.name = Sells.beer AND
bar = ’Joe’’s Bar’;
  • Use BeerInd to get all the beers made by Pete’s.

  • Then use SellInd to get prices of those beers, with bar = ’Joe’’s Bar’

Finding Rows Without Indexes

Finding Rows in a Heap with a Nonclustered Index

Finding Rows in a Clustered Index

Finding Rows in a Clustered Index with a Nonclustered Index

总结:

  • 非聚集索引的叶子结点存放的是聚集索引的关键字,聚集索引叶子结点存放的是数据本身
  • 所以使用非聚焦索引查询数据时,应该是先找到非聚焦索引的叶子结点上的聚焦索引的关键字,然后通过这个关键字,从聚焦索引找到存放了数据的叶子结点

Database Tuning

  • A major problem in making a database run fast is deciding which indexes to create.(一个主要的问题就是,要决定那些索引是需要被创建的)
  • Pro: An index speeds up queries that can use it.(优点:能够提升查询的速度)
  • Con: An index slows down all modifications on its relation because the index must be modified too.(缺点:是的关系上的修改效率降低,因为修改的同时索引也需要修改)

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