Data types

Data types and Precedence of convert types

SQL Server associates columns, expressions, variables, and parameters with data types. Data types determine what kind of data can be stored in the field: Integers, characters, dates, money, binary strings, etc.

SQL Server supplies several built-in data types but you can also define custom types

Built-in data types are categorized as shown in the table below, also you can see the precedence of convert to other data type. I mean when an operator combines two expressions of different data types, the rules for data type precedence specify that the data type with the lower precedence is converted to the data type with the higher precedence.

SQL Server uses the following precedence order for data types:

1. user-defined data types (highest)
2. sql_varian t
3. xml
4. datetimeoffset
5. datetime2
6. datetime
7. smalldatetime
8. date
9. time
10. float
11. real
12. decimal
13. money
14. smallmoney
15. bigint
16. int
17. smallint
18. tinyint
19. bit
20. ntext
21. text
22. image
23. timestamp
24. uniqueidentifier
25. nvarchar (including nvarchar(max) )
26. nchar
27. varchar (including varchar(max) )
28. char
29. varbinary (including varbinary(max) )
30. binary (lowest)

 

 

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Storage – Part II

Logical Structures
How, where, when you need to think in data storage? Well, this is the first step after you modeling your database and most companies do not think about it, I have saw companies with a large data into one disk or small data separated in the wrong way, but what is the best to do?

We have some best practices and you can read more about logical structures in SQL Server called filegroups in MSDN page. Every DBA needs to know to create and maintain filegroups because they are part of every SQL Server database. Filegroups affect the performance, maintenance, and security of your data and they are logical structures to group files together.

At a minimum, every SQL Server database has two operating system files: a data file and a log file. Data files contain data and objects such as tables, indexes, stored procedures, and views. Log files contain the information that is required to recover all transactions in the database. Data files can be grouped together in filegroups for allocation and administration purposes.

Filegroups can be created when the database is first created or created later when more files are added to the database. However, you cannot move files to a different filegroup after the files have been added to the database.

A file cannot be a member of more than one filegroup. Tables, indexes, and large object (LOB) data can be associated with a specific filegroup. This means that all their pages are allocated from the files in that filegroup.

Why use filegroups?

• you have one or more objects tha have heavy read/write activity
• you’ve already tuned through indexes and query writing
• you need a performance boost
• you have additional storage you can utilize to separate the objects
• you want to separate tha data so administration tasks such as backups take less time
• you have a large database (>1TB)
• disaster and recovery

Best Practices

• separate data and log files onto separate disks
• separate tempdb onto its own disk
• at least two filegroups – primary and one user-defined (default)
• files in a filegroup should be equally sized for equal proportion of writes
• use filegroups to isolate objects with heavy read and write activity from each other

Related articles

• Files and Filegroups Architecture (MSDN Library)
• How SQL Server Stuff Works: Filegroups (Video) (Brent Ozar blog)

Storage – Part I

Pages and Extends Architecture

Data Structure SQL Server

The page is the fundamental unit of data storage in SQL Server. An extent is a collection of eight physically contiguous pages. Extents help efficiently manage pages.

Understanding the architecture of pages and extents is important for designing and developing databases that perform efficiently.

The fundamental unit of data storage in SQL Server is the page. The disk space allocated to a data file (.mdf or .ndf) in a database is logically divided into pages numbered contiguously from 0 to n. Disk I/O operations are performed at the page level. That is, SQL Server reads or writes whole data pages.

Extents are a collection of eight physically contiguous pages and are used to efficiently manage the pages. All pages are stored in extents.

Pages

Page size is 8 KB. This means SQL Server databases have 128 pages per megabyte. Each page begins with a 96-byte header that is used to store system information about the page.

Extends

Extents are the basic unit in which space is managed. An extent is eight physically contiguous pages, or 64 KB. This means SQL Server databases have 16 extents per megabyte.

To make its space allocation efficient, SQL Server does not allocate whole extents to tables with small amounts of data. SQL Server has two types of extents:

• Uniform extents are owned by a single object; all eight pages in the extent can only be used by the owning object.
• Mixed extents are shared by up to eight objects. Each of the eight pages in the extent can be owned by a different object.

A new table or index is generally allocated pages from mixed extents. When the table or index grows to the point that it has eight pages, it then switches to use uniform extents for subsequent allocations. If you create an index on an existing table that has enough rows to generate eight pages in the index, all allocations to the index are in uniform extents