Hướng dẫn học Microsoft SQL Server 2008 part 60 pptx

By default a clustered index is created automatically when the primary key constraint is created.To remove an index use theDROP INDEXcommand with both the table and index name: DROP INDE

By default a clustered index is created automatically when the primary key constraint is created.

To remove an index use theDROP INDEXcommand with both the table and index name:

DROP INDEX OrderDetail.IxOrderID;

Indexes, once created, do not automatically maintain their efficiency Updates can frag-ment the index and affect the index page’s fill factor While this chapter frag-mentions index maintenance, Chapter 42, ‘‘Maintaining the Database,’’ details the administrative requirements for index

performance.

Composite indexes

A composite index is a clustered or clustered index that includes multiple key columns Most

non-clustered indexes are composite indexes If you use SQL Server Management Studio’s Index Properties

form, composite indexes are created by adding multiple columns to the index in the General page

When creating a composite index with code, it must be declared in aCREATE INDEXDDL statement

after the table is created The following code sample creates a composite clustered index on theGuide

table in theCHA2database:

CREATE CLUSTERED INDEX IxGuideName

ON dbo.Guide (LastName, FirstName);

The order of the columns in a composite index is important In order for a search to take advantage

of a composite index it must include the index columns from left to right If the composite index is

lastname,firstname, a search forfirstnamewill not use the index, but a search forlastname,

orlastnameandfirstname, will

SQL Server can index words within columns using Integrated Full-Text Search, covered in Chapter 19, ‘‘Using Integrated Full-Text Search.’’

Primary keys

A primary key can be initially defined as a clustered or non-clustered index However, in order for the

index type to be changed, the primary key constraint must be dropped and recreated — a painful task if

numerous foreign keys are present or the table is replicated For more information on designing primary

keys, please see the section ‘‘Primary keys,’’ earlier in this chapter and Chapter 3, ‘‘Relational Database

Design.’’

Filegroup location

If the database uses multiple named filegroups, the index may be created on a certain filegroup with the

ON filegroupnameoption:

CREATE NONCLUSTERED INDEX IndexName

ON Table (Columns)

ON filegroupname;

This option is useful for spreading the disk I/O throughput for very heavily used databases For

example, if a web page is hit by a zillion users per minute, and the main page uses a query that involves

two tables and three indexes, and several disk subsystems are available, then placing each table and

index on its own disk subsystem will improve performance Remember that a clustered index must be in

the same location as the table because the clustered index pages and the data pages are merged

The physical location of tables and indexes may be further configured using filegroups and

partitioning For more details on table and index partitioning, refer to Chapter 68,

‘‘Parti-tioning.’’

Index options

SQL Server 2008 indexes may have several options, including uniqueness, space allocation, and

perfor-mance options

Unique indexes

AUNIQUE INDEXoption is more than just an index with a unique constraint; index optimizations are

available to unique indexes A primary key or a unique constraint automatically creates a unique index

In Management Studio, a unique index is created by checking the Unique option in the General page of

the Index Properties form

In code, an index is set as unique by adding theUNIQUEkeyword to the index definition, as follows:

CREATE UNIQUE INDEX OrderNumber

ON [Order] (OrderNumber);

Include columns

SQL Server 2005 added the capability to include non-key columns in the leaf level of non-clustered

indexes These non-sorted columns are great for defining covering indexes Multiple columns may be

included, and included columns can be added in the Index Properties dialog or with code:

CREATE INDEX ixGuideCovering

ON dbo.Guide (LastName, FirstName)

INCLUDE (Title)

Filtered indexes

New for SQL Server 2008, filtered indexes enable DBAs to create indexes that include less than the

entire table’s worth of data Essentially, aWHEREclause is added to the index definition These are

perfect for highly tuned covering indexes Filtered indexes can be configured in the Filter page of the

Management Studio’s Index Properties dialog, or using T-SQL The following DDL statement creates a

filtered non-clustered index that includes only the work orders that are active Older work orders are

ignored by the index:

CREATE INDEX IxActiveProduction

ON Production.WorkOrders (WorkOrderID, ProductID)

WHERE Status = ‘Active’

Index fill factor and pad index

An index needs a little free space in the tree so that new entries don’t require restructuring of the index

When SQL Server needs to insert a new entry into a full page, it splits the page into two pages and

writes two half-full pages back to the disk This causes three performance problems: the page split itself,

the new pages are no longer sequential, and less information is on each page so more pages must be

read to read the same amount of data

Because the index is a balanced tree (b-tree), each page must hold at least two rows The fill factor and

the pad index affect both the intermediate pages and the leaf node, as described in Table 20-6

TABLE 20-6

Fill Factor and Pad Index

Fill Factor Intermediate Page(s) Leaf Node

1–99 One free entry or≤ fill factor if pad index ≤ Fill factor

The fill factor only applies to the detail, or leaf, node of the index, unless thePAD INDEXoption is

applied to the fill factor ThePAD INDEXoption directs SQL Server to apply the looseness of the fill

factor to the intermediate levels of the b-tree as well

Best Practice

The best fill factor depends on the purpose of the database and the type of clustered index If the database

is primarily for data retrieval, or the primary key is sequential, a high fill factor will pack as much as

possible in an index page If the clustered index is nonsequential (such as a natural primary key), then the

table is susceptible to page splits, so use a lower page fill factor and defragment the pages often

The index’s fill factor will slowly become useless as the pages fill and split The main-tenance plan must include periodic re-indexing to reset the fill factor Chapter 42,

‘‘Maintaining the Database,’’ includes information on how to maintain indexes.

Using Management Studio, the fill factor is set in the Index Properties Options page In T-SQL code,

include the fill factor and index pad options after theCREATE INDEXcommand The following code

example creates theOrderNumberindex with 15 percent free space in both the leaf nodes and the

intermediate pages:

CREATE NONCLUSTERED INDEX IxOrderNumber

ON dbo.[Order] (OrderNumber)

WITH FILLFACTOR = 85, PAD_INDEX = ON;

Limiting index locks and parallelism

The locking behavior of queries using the index may be controlled using theALLOW_ROW_LOCKSand

ALLOW_PAGE_LOCKSoptions Normally these locks are allowed

Index sort order

SQL Server can create the index as a descending index

Any query using anORDER BYclause will still be sorted ascending unless the query’sORDER BY

specifically statesDESC

TheASCorDESCoption follows the column name in theCREATE INDEXDDL command

The Ignore Dup Key index option

TheIGNORE_DUPLICATE_KEYoption doesn’t affect the index, but rather how the index affects data

modification operations later

Normally, transactions are atomic, meaning that the entire transaction either succeeds or fails as a

logi-cal unit However, theIGNORE_DUPLICATE_KEYoption directsINSERTtransactions to succeed for all

rows accepted by the unique index, and to ignore any rows that violate the unique index

This option does not break the unique index Duplicates are still kept out of the table, so the

consis-tency of the database is intact, but the atomicity of the transaction is violated Although this option

might make importing a zillion questionable rows easier, I personally don’t like any option that weakens

the ACID (atomic, consistent, isolated, durable) properties of the database

The following command is the same as the previousCREATE UNIQUE INDEXcommand, but with the

IGNORE_DUPLICATE_KEYoption:

CREATE UNIQUE INDEX OrderNumber

ON [Order] (OrderNumber)

WITH IGNORE_DUP_KEY = ON

The Drop Existing index option

TheDROP EXISTINGoption directs SQL Server to drop the current index and rebuild the new index

from scratch This may cause a slight performance improvement over rebuilding every index if the index

being rebuilt is a clustered index and the table also has nonclustered indexes, because rebuilding a

clus-tered index forces a rebuild of any non-clusclus-tered indexes

The Statistics Norecompute index option

The SQL Server Query Optimizer depends on data-distribution statistics to determine which index is

most significant for the search criteria for a given table Normally, SQL Server updates these statistics

automatically However, some tables may receive large amounts of data just prior to being queried, and

the statistics may be out of date For situations that require manually initiating the statistics update, the

STATISTICS NORECOMPUTE = ONindex option disables automatic statistics, but for nearly all indexes

this option should be ignored

Sort in tempdb

TheSORT_IN_TEMPDB = ONoption modifies the index-creation method by forcing it to usetempdbas

opposed to memory If the index is routinely dropped and recreated, this option may shorten the

index-creation time For most indexes, this option is neither required nor important

Disabling an index

An index may be temporarily disabled, or taken offline, using the Use Index check box in the Index

Properties➪ Options page Using T-SQL, to disable an index use theALTER INDEXDDL command

with theDISABLEoption:

ALTER INDEX [IxContact] ON [dbo].[Contact] DISABLE

During some intensive data import operations, it’s faster to drop the index and recreate it than to

update the index with every newly inserted row The benefit of disabling an index is that the metadata

for the index is maintained within the database, rather than depending on the code to recreate the

correct index

Disabling a clustered index effectively disables the table.

To re-enable an index, use theALTER INDEX REBUILD WITHcommand:

ALTER INDEX [PKContact0BC6C43E]

ON [dbo].[Contact]

REBUILD WITH

( PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, ALLOW_ROW_LOCKS = ON,

ALLOW_PAGE_LOCKS = ON, SORT_IN_TEMPDB = OFF, ONLINE = OFF )

Summary

The logical database schema often requires tweaking in order to serve as a physical schema It’s in the

nitty-gritty details of the physical-database schema that the logical design takes shape and becomes a

working database within the restrictions of the data types, keys, and constraints of the database

prod-uct Knowing the table-definition capabilities of SQL Server means that you can implement some project

features at the server-constraint level, rather than in T-SQL code in a trigger or stored procedure

A few key points about creating the physical schema:

■ DDL code is like any other source code Write it in script form and keep it under source code control

■ Be careful with autogrowth

■ Set up a separate filegroup for user tables and set it as the default filegroup, and keep user

tables off the primary filegroup

■ Natural keys are good for lookup tables Use surrogate keys for the other tables

■ Constraints are faster and more reliable than application code Always enforce the database

rules with constraints

■ Indexes are easy to create but complex to design, as covered in Chapter 64, ‘‘Indexing

Strategies.’’

Within this part of the book, ‘‘Developing with SQL Server,’’ this chapter has provided the code to build

the physical schema From here, the rest of this part continues the development discussion, with several

more chapters of T-SQL and a chapter of NET, and key development ideas

Programming with

T-SQL

IN THIS CHAPTER

The basics of T-SQL and batches

Working with local variables Controlling the flow of the batch

Exploring SQL Server objects with code

Working with temporary tables and table variables

Using multiple assignment variable select statements

Standard SQL Data Manipulation Language (DML) commands —SELECT,

INSERT,UPDATE, andDELETE— only modify or return data SQL

DML lacks both the programming structure to develop procedures and

algorithms, and the database-specific commands to control and tune the server

To compensate, each full-featured database product must complement the SQL

standard with some proprietary SQL language extension

Transact-SQL, better known as T-SQL, is Microsoft’s implementation of SQL

plus its collection of extensions to SQL The purpose of T-SQL is to provide a

set of procedural and administrative tools for the development of a transactional

database

T-SQL is often thought of as synonymous with stored procedures In reality it’s

much more than that, which is why this chapter is about T-SQL and the next

chapter covers stored procedures It may be employed in several different ways

within a SQL Server client/server application:

■ T-SQL is used within expressions as part of DML commands (INSERT,

UPDATE, andDELETE) submitted by the client process

■ It is used within blocks of code submitted to SQL Server from a client

as a batch or script

■ T-SQL functions are used as expressions within check constraints

■ T-SQL code is used within batches of code that have been packaged

within SQL Server as stored procedures, functions, or triggers

Truth be told, this book has been covering T-SQL programming since Chapter 8,

‘‘Introducing Basic Query Flow.’’ The DML commands are the heart of T-SQL

This chapter merely adds the programmatic elements required to develop

server-side procedural code The language features explained in this chapter

are the foundation for developing stored procedures, user-defined functions,

and triggers

What’s New with T-SQL Programming?

Microsoft has steadily enhanced Transact-SQL SQL Server 7 saw unique identifiers, SQL Server 2000

added user-defined functions, and SQL Server 2005 added common table expressions SQL Server

2008 adds a few juicy new features as well: Topping the list are table-valued parameters and the new

user-defined table type, which radically improves how client applications can send complex transactions to

SQL Server In addition, variables may be initialized when they’re declared, and there’s a new shortcut for

incrementing variables and columns

Transact-SQL Fundamentals

T-SQL is designed to add structure to the handling of sets of data Because of this, it does not provide

several language features that application development needs If you do a lot of application

program-ming development, you’ll find that T-SQL is in many ways the exact opposite of how you think when

programming in VB, C#, Java, or any other structured development language

Best Practice

SQL Server 2005 added the capability to create stored procedures, functions, and triggers using NET and

the common language runtime Nevertheless, T-SQL is the native language of SQL Server, and it remains

the best choice for any data-oriented task

T-SQL batches

A query is a single SQL DML statement, and a batch is a collection of one or more T-SQL statements.

The entire collection is sent to SQL Server from the front-end application as a single unit of code

SQL Server parses the entire batch as a unit Any syntax error will cause the entire batch to fail,

mean-ing that none of the batch will be executed However, the parsmean-ing does not check any object names or

schemas because a schema may change by the time the statement is executed

Terminating a batch

A SQL script file or a Query Analyzer window may contain multiple batches If this is the case, a

batch-separator keyword terminates each batch By default, the batch-batch-separator keyword isGO(similar to how

the Start button is used to shut down Windows) The batch-separator keyword must be the only

key-word in the line You can add a comment after theGO

The batch separator is actually a function of SQL Server Management Studio and SQLCMD, not SQL

Server It can be modified in the Query Execution page by selecting Tools➪ Options, but I don’t

rec-ommend creating a custom batch separator (at least not for your friends)

Because theGObatch terminator tells Management Studio’s Query Editor to send the batch to the

con-nected SQL Server, it can be used to submit the batch multiple times The following script demonstrates

this poor man’s cursor:

PRINT ‘I’m a batch.’;

go 5 will execute 5 times

Result:

Beginning execution loop

I’m a batch

I’m a batch

I’m a batch

I’m a batch

I’m a batch

Batch execution completed 5 times

Terminating a batch kills all local variables, temporary tables, and cursors created by that batch

DDL commands

Some T-SQL DDL commands, such asCREATE PROCEDURE, are required to be the only command in

the batch Very long scripts that create several objects often include numerousGObatch terminators

Because SQL Server evaluates syntax by the batch, usingGOthroughout a long script also helps locate

syntax errors

Switching databases

Interactively, the current database is indicated in the Query Editor toolbar and can be changed there

In code, the current database is selected with theUSEcommand You can insertUSEwithin a batch to

specify the database from that point on:

USE CHA2;

It’s a good practice to explicitly specify the correct database with theUSEcommand, rather than assume

that the user will select the correct database prior to running the script

At the end of scripts, I tend to add aUSE tempdbdatabase, so Query Editor windows aren’t kept

parked in the development database

Executing batches

A batch can be executed in several ways:

■ A complete SQL script (including all the batches in the script) may be executed by opening

the.sqlfile with SQL Server Management Studio’s SQL Editor and pressing F5, clicking the

‘‘! Execute’’ toolbar button, pressing Ctrl+E, or selecting Query ➪ Execute

■ Selected T-SQL statements may be executed within SQL Server Management Studio’s SQL

Editor by highlighting those statements and pressing F5, clicking the ‘‘! Execute’’ toolbar

button, pressing Ctrl+E, or selecting Query ➪ Execute