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Looking into temporary tables in SQL Server December 4, 2013

Posted by fofo in Sql Server, Sql Server 2005, SQL Server 2008, SQL Server 2012.
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I have been delivering a certified course in MS SQL Server 2012 recently and I was asked several questions about temporary tables, how to create them, how to manage them, when to use them and what are the limitations of them.

In this post I will try to shed light on this particular issue with lots of hands-on demos.

Temporary tables and table variables make use of the system tempdb database.

I have installed SQL Server 2012 Enterprise edition in my machine but you can use the SQL Server 2012/2008 Express edition as well.

I am connecting to my local instance through Windows Authentication and in a new query window I type (you can copy paste)

First I am going to create a new temporary table and populate it. Execute the script below.

USE tempdb
GO

IF OBJECT_ID('tempdb..#footballer') IS NOT NULL

DROP TABLE #footballer;

GO
CREATE TABLE #footballer
 (
 [FootballerID] INT IDENTITY NOT NULL PRIMARY KEY,
 [lastname] [varchar](15) NOT NULL,
 [firstname] [varchar](15) NOT NULL,
 [shirt_no] [tinyint] NOT NULL,
 [position_played] [varchar](30) NOT NULL,

);

GO

SET IDENTITY_INSERT [dbo].[#footballer] ON

GO

INSERT [#footballer] ([FootballerID], [lastname], [firstname], [shirt_no], [position_played]) VALUES (1,N'Oliver', N'Regina', 4, N'goalkeeper')
INSERT [#footballer] ([FootballerID], [lastname], [firstname], [shirt_no], [position_played]) VALUES (2,N'Alexander', N'Roy', 8, N'goalkeeper')
INSERT [#footballer] ([FootballerID], [lastname], [firstname], [shirt_no], [position_played]) VALUES (3,N'Mueller', N'Dewayne', 10, N'defender')
INSERT [#footballer] ([FootballerID], [lastname], [firstname], [shirt_no], [position_played]) VALUES (4,N'Buckley', N'Beth', 3, N'midfielder')
INSERT [#footballer] ([FootballerID], [lastname], [firstname], [shirt_no], [position_played]) VALUES (5,N'Koch', N'Jolene', 7, N'striker')
GO

SELECT * FROM #footballer

As you can see there is a # prefix in front of the table. This table will be saved in the tempdb.

Finally I select everything from the temporary table.

If I open a new query window and try to select everything (see the query below) from the #footballer table.


USE tempdb
GO

SELECT * FROM #footballer

You will not receive any results. You will receive an error – Invalid object name ‘#footballer’.

This is a local temporary table and it is in scope only in the current connection-session.

We can also create global temporary tables. In a new query window execute the following script.


USE tempdb
GO

IF OBJECT_ID('tempdb..##footballernew') IS NOT NULL

DROP TABLE ##footballernew;

GO
CREATE TABLE #footballernew
(
[FootballerID] INT IDENTITY NOT NULL PRIMARY KEY,
[lastname] [varchar](15) NOT NULL,
[firstname] [varchar](15) NOT NULL,
[shirt_no] [tinyint] NOT NULL,
[position_played] [varchar](30) NOT NULL,

);

GO

SET IDENTITY_INSERT [dbo].[##footballernew] ON

GO

INSERT [##footballernew] ([FootballerID], [lastname], [firstname], [shirt_no], [position_played]) VALUES (1,N'Oliver', N'Regina', 4, N'goalkeeper')
INSERT [##footballernew] ([FootballerID], [lastname], [firstname], [shirt_no], [position_played]) VALUES (2,N'Alexander', N'Roy', 8, N'goalkeeper')
INSERT [##footballernew] ([FootballerID], [lastname], [firstname], [shirt_no], [position_played]) VALUES (3,N'Mueller', N'Dewayne', 10, N'defender')
INSERT [##footballernew] ([FootballerID], [lastname], [firstname], [shirt_no], [position_played]) VALUES (4,N'Buckley', N'Beth', 3, N'midfielder')
INSERT [##footballernew] ([FootballerID], [lastname], [firstname], [shirt_no], [position_played]) VALUES (5,N'Koch', N'Jolene', 7, N'striker')
GO

SELECT * FROM ##footballernew

We denote the global temporary table with ## – ##footballernew

The global temporary table is deleted when all users referencing the table disconnect.

Both global and local temporary tables should be deleted in code rather than depending on automatic drop.

A temporary table created in a stored procedure is visible to other stored procedures executed from within the first procedure.

In a new query window type the following.


USE tempdb
GO

SELECT * FROM ##footballernew

In this case there will be no error. Global temporary tables persist across sessions-connections.

You can also add columns to temporary tables and alter the definition of existing columns.

In this script I add another column and then alter the definition of an existing column.


USE tempdb
GO

IF OBJECT_ID('tempdb..#footballer') IS NOT NULL

DROP TABLE #footballer;

GO
CREATE TABLE #footballer
 (
 [FootballerID] INT IDENTITY NOT NULL PRIMARY KEY,
 [lastname] [varchar](15) NOT NULL,
 [firstname] [varchar](15) NOT NULL,
 [shirt_no] [tinyint] NOT NULL,
 [position_played] [varchar](30) NOT NULL,

);

GO

SET IDENTITY_INSERT [dbo].[#footballer] ON

GO

INSERT [#footballer] ([FootballerID], [lastname], [firstname], [shirt_no], [position_played]) VALUES (1,N'Oliver', N'Regina', 4, N'goalkeeper')
INSERT [#footballer] ([FootballerID], [lastname], [firstname], [shirt_no], [position_played]) VALUES (2,N'Alexander', N'Roy', 8, N'goalkeeper')
INSERT [#footballer] ([FootballerID], [lastname], [firstname], [shirt_no], [position_played]) VALUES (3,N'Mueller', N'Dewayne', 10, N'defender')
INSERT [#footballer] ([FootballerID], [lastname], [firstname], [shirt_no], [position_played]) VALUES (4,N'Buckley', N'Beth', 3, N'midfielder')
INSERT [#footballer] ([FootballerID], [lastname], [firstname], [shirt_no], [position_played]) VALUES (5,N'Koch', N'Jolene', 7, N'striker')
GO

ALTER TABLE #footballer
ADD [is_retired] BIT NULL;
GO

ALTER TABLE #footballer
ALTER COLUMN [lastname] [nvarchar](50);
GO

You can use any data type for columns definition in a temporary table. You can also use user-defined data types.

You can also have constraints in temporary tables.If you execute the code below, it will work perfectly fine.


USE tempdb
GO

IF OBJECT_ID('tempdb..#Movies') IS NOT NULL

DROP TABLE #footballer;

GO

CREATE TABLE #Movies
 (
 MovieID INT PRIMARY KEY ,
 MovieName NVARCHAR(50) ,
 MovieRating TINYINT
 )
GO
ALTER TABLE #Movies
 WITH CHECK
 ADD CONSTRAINT CK_Movie_Rating
CHECK (MovieRating >= 1 AND MovieRating <= 5)

But you have to be careful when creating-applying foreign keys. FOREIGN KEY constraints are not enforced on local or global temporary tables.
Execute the script below to see what I mean.The foreign key will not be created.

USE tempdb
go

CREATE TABLE #Persons
 (
 P_Id INT NOT NULL ,
 LastName VARCHAR(255) NOT NULL ,
 FirstName VARCHAR(255) ,
 Address VARCHAR(255) ,
 City VARCHAR(255) ,
 PRIMARY KEY ( P_Id )
 )

CREATE TABLE #Orders
(
O_Id int NOT NULL PRIMARY KEY,
OrderNo int NOT NULL,
P_Id int FOREIGN KEY REFERENCES #Persons(P_Id)
)

Please bear in mind that you can create temporary tables with clustered and non-clustered indexes on them.

Let’s investigate the behavior of temporary tables and IDENTITY columns.

If you execute the script below , it will fail. This is the same behavior when executing the same script to regular tables. You cannot specify values for the IDENTITY column.If you choose to do so you must set IDENTITY_INSERT ON.

USE tempdb
GO

IF OBJECT_ID('tempdb..#Persons') IS NOT NULL

DROP TABLE #Persons;

GO

CREATE TABLE #Persons
 (
 P_Id INT PRIMARY KEY CLUSTERED IDENTITY(1,1) ,
 LastName VARCHAR(255) NOT NULL ,
 FirstName VARCHAR(255) ,
 Address VARCHAR(255) ,
 City VARCHAR(255)
 )

--this will not work

INSERT #Persons(P_Id,LastName,FirstName,Address,City) VALUES (1,'Steven','Gerrard','123 liverpool street','liverpool')
SET IDENTITY_INSERT [#Persons] ON

GO

--this will work

INSERT #Persons(P_Id,LastName,FirstName,Address,City) VALUES (1,'Steven','Gerrard','123 liverpool street','liverpool')

Αlso note that transactions are honored in temporary tables. If I begin an explicit transaction -an insert- without committing it will insert the row of data but then if a rollback is issued the whole operation will be rolled back

Execute the script below.


USE tempdb
GO

IF OBJECT_ID('tempdb..#Persons') IS NOT NULL

DROP TABLE #Persons;

GO

CREATE TABLE #Persons
 (
 P_Id INT PRIMARY KEY CLUSTERED IDENTITY(1,1) ,
 LastName VARCHAR(255) NOT NULL ,
 FirstName VARCHAR(255) ,
 Address VARCHAR(255) ,
 City VARCHAR(255)
 )

SET IDENTITY_INSERT [#Persons] ON

GO

--this will insert the value

BEGIN TRAN
INSERT #Persons(P_Id,LastName,FirstName,Address,City) VALUES (1,'Steven','Gerrard','123 liverpool street','liverpool')

GO

SELECT * FROM #Persons

--this will rollback the transaction

ROLLBACK TRAN

Hope it helps!!!

Looking into Temp database usage in SQL Server December 4, 2013

Posted by fofo in Sql Server, Sql Server 2005, SQL Server 2008, SQL Server 2012.
Tags:
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I have been delivering a certified course in MS SQL Server 2012 recently and I was asked several questions about Tempdb usage and temporary objects.

In this post I will try to shed light on this particular issue.

Temporary tables and table variables make use of the system tempdb database.

There is only one tempdb system database per SQL Server instance so if there is a huge usage of temporary objects in this database it can be the point of contention.

When you create an entry in database it needs to allocate space.This is also valid for the tempdb database.There are three types of pages involved in the allocation process in the tempdb data file:

  • Page Free Space (PFS)
  • Shared Global Allocation Map (SGAM)
  • Global Allocation Map (GAM).

When there is a great page allocation contention in tempdb, the whole allocation process can suffer and we can experience with PAGELATCH waits.

In order to address the issue above, you can have a number of tempdb data files that are equal to the number of cores.For example if you have a system with less than 8 cores e.g 6 you should add/set up 6 data files for the tempdb.If you have a system with more than 8 cores you should add 8 data files for the tempdb and then if the contention is still big you can add 4 more data files.

By saying cores in this post I mean logical cores and not physical cores.So if you have 8 physical cores, then you have 16 logical cores and 32 logical cores if hyper-threading is enabled.

I will  provide some demos in order to show you what tempdb contention might look like and what are the main wait latches that occur.

I have installed SQL Server 2012 Enterprise edition in my machine but you can use the SQL Server 2012/2008 Express edition as well.

I am connecting to my local instance through Windows Authentication and in a new query window I type (you can copy paste)

In this snippet, I create a new database and then create a new table with some constraints

CREATE DATABASE mytempdbcontention
GO

USE mytempdbcontention;
GO
 CREATE TABLE dbo.footballer
 (
 [FootballerID] INT IDENTITY NOT NULL PRIMARY KEY,
 [lastname] [varchar](15) NOT NULL,
 [firstname] [varchar](15) NOT NULL,
 [shirt_no] [tinyint] NOT NULL,
 [position_played] [varchar](30) NOT NULL,

);

GO

ALTER TABLE dbo.footballer
ADD CONSTRAINT CK_Footballer_Shirt_No
CHECK (shirt_no >= 1 AND shirt_no <= 11)

GO

ALTER TABLE dbo.footballer
ADD CONSTRAINT CK_Footballer_Position
CHECK (position_played IN ('goalkeeper','defender','midfielder','striker'))
GO

Now I need to populate the table with 50.000 rows. This is the script you need to execute in order to make this happen.

You can download it here. Rename the insert-footballer.doc to insert-footballer.sql and execute the script in a new query window.

Now I need to create a script that will create tempdb contention. This is the script that creates a temporary object- #footballer, populates the #footballer from the footballer table and then selects from it. Finally it drops the temporary object.


USE mytempdbcontention;
GO

SET NOCOUNT ON;
GO

WHILE 1 = 1
 BEGIN

IF OBJECT_ID('tempdb..#footballer') IS NOT NULL

 DROP TABLE #footballer;

CREATE TABLE #footballer
 (
 [FootballerID] INT IDENTITY NOT NULL PRIMARY KEY,
 [lastname] [varchar](15) NOT NULL,
 [firstname] [varchar](15) NOT NULL,
 [shirt_no] [tinyint] NOT NULL,
 [position_played] [varchar](30) NOT NULL,

);
 INSERT INTO #footballer
 (lastname,
 firstname,
 shirt_no,
 position_played)
 SELECT TOP 4000
 lastname,
 firstname,
 shirt_no,
 position_played

 FROM dbo.footballer;

SELECT lastname
 FROM #footballer;

DROP TABLE #footballer;
 END
 GO

Now I am going to create a .cmd file where I will create contention to the tempdb.

You can download it here. Rename the temp-sql.cmd.doc to temp-sql.cmd. Make sure you execute it (by double-clicking it).

This will create lots of contention to the tempdb. We need to see exactly what this contention is and the wait latches that have occurred.

Execute the script below.


USE tempdb
go
SELECT session_id, wait_duration_ms,wait_type, resource_description
from sys.dm_os_waiting_tasks
where wait_type like 'PAGE%LATCH_%'

and

resource_description like '2:%'

As you can see from the picture below, I have PAGEIOLATCH_SH wait types.This wait type occurs when a task is waiting on a latch for a buffer that is in an I/O request. The latch request is in Shared mode.

wait_type

I have one tempdb data file, the default configurations

I have 8 cores in this machine so I will add 7 mores tempdb data files of equal size (MS recommendation).

Execute the script below.


USE [master]
GO

ALTER DATABASE [tempdb]
MODIFY FILE ( NAME = N'tempdev', SIZE = 500 MB ,
FILEGROWTH = 100 MB )
GO

ALTER DATABASE [tempdb]
ADD FILE ( NAME = N'tempdev2', FILENAME = N'd:\DATA\tempdb2.ndf' ,
SIZE = 500 MB , FILEGROWTH = 100 MB )
GO

ALTER DATABASE [tempdb]
ADD FILE ( NAME = N'tempdev3', FILENAME = N'd:\DATA\tempdb3.ndf' ,
SIZE = 500 MB , FILEGROWTH = 100 MB )
GO

ALTER DATABASE [tempdb]
ADD FILE ( NAME = N'tempdev4', FILENAME = N'd:\DATA\tempdb4.ndf' ,
SIZE = 500 MB , FILEGROWTH = 100 MB )
GO

ALTER DATABASE [tempdb]
ADD FILE ( NAME = N'tempdev5', FILENAME = N'd:\DATA\tempdb5.ndf' ,
SIZE = 500 MB , FILEGROWTH = 100 MB )
GO

ALTER DATABASE [tempdb]
ADD FILE ( NAME = N'tempdev6', FILENAME = N'd:\DATA\tempdb6.ndf' ,
SIZE = 500 MB , FILEGROWTH = 100 MB )
GO

ALTER DATABASE [tempdb]
ADD FILE ( NAME = N'tempdev7', FILENAME = N'd:\DATA\tempdb7.ndf' ,
SIZE = 500 MB , FILEGROWTH = 100 MB )
GO

Now run the query again and observe the results.


USE tempdb
go
SELECT session_id, wait_duration_ms,wait_type, resource_description
from sys.dm_os_waiting_tasks
where wait_type like 'PAGE%LATCH_%'

and

resource_description like '2:%'

You will see that there are no wait types hence no tempdb contention.

Stop now the temp-sql.cmd file so the contention of the tempdb stops.

Hope it helps!!!

Ownership chain in SQL Server November 26, 2013

Posted by fofo in Sql Server, Sql Server 2005, SQL Server 2008, SQL Server 2012.
Tags: ,
1 comment so far

I have been delivering a certified course in MS SQL Server 2012 recently and I was asked a very interesting question about SQL Server stored procedures and ownership chain.

Security is a big thing in SQL Server and can be implemented in various levels.

Before we move on, we should define some basic terms that are very important when we talk about security in SQL Server.

The principal access data. It should be authenticated by SQL Server which basically means that the principal should “prove” that it is , whom it claims to be.

Principal is a user a group or an application granted permission to access securables.So what is a securable? Securable is an object to which access privileges are granted.

Privilege is an action that a principal is allowed to perform an a securable.

I want to show you an example where we want to deny a user to the actual table-data but grant him access through a view or a stored procedure.UserA is the owner of tableA and creates StoredProcA that gets data from tableA. Then if the UserB is granted permission to execute the StoredProcA even though he does not have direct access to tableA he will be able to get access (indirectly) to the table. This is the concept of ownership chaining.

I have installed SQL Server 2012 Enterprise edition in my machine but you can use the SQL Server 2012/2008 Express edition as well.

I am connecting to my local instance through Windows Authentication and in a new query window I type (you can copy paste)


USE master
GO

--Create a sample database

CREATE DATABASE mysampledb;
GO

-- create a login

CREATE LOGIN login1 WITH PASSWORD ='login1', CHECK_POLICY = OFF;

USE mysampledb;
go

--Create a User for that login
CREATE USER thelogin1 FOR LOGIN login1;

--see the user that you are now (dbo)
SELECT user
--create table and populate it with sample data

CREATE TABLE dbo.person

(id INT IDENTITY (1,1),
 firstname varchar(40),
 lastname VARCHAR(50)

);
go

INSERT dbo.person VALUES ('nick','kenteris'),('maria','negri'),('apostolis','kaperis'),('kostas','gekas');

CREATE PROCEDURE dbo.getpersons
AS SELECT id,firstname,lastname FROM dbo.person
go

--give permissions to execute the procedure to the thelogin1 user

GRANT EXECUTE ON dbo.getpersons TO thelogin1

--change the current user as thelogin1

EXECUTE AS USER = 'thelogin1'

--check if we are indeed thelogin1 user

SELECT USER

--fails
SELECT * FROM dbo.person

--works

EXECUTE dbo.getpersons

--get back to the dbo user

REVERT;

--see if the user is indeed dbo

SELECT USER

--we change the stored proc to use dynamic sql

ALTER PROCEDURE dbo.getpersons
AS
EXEC('SELECT id,firstname,lastname FROM dbo.person')
GO

--change the current user as thelogin1

EXECUTE AS USER = 'thelogin1'

--check to see if indeed the user is now thelogin1

SELECT USER

--this will fail

EXECUTE dbo.getpersons

--get back to the dbo user
REVERT;

--check to see if we are dbo user

SELECT user

--this will work

ALTER PROCEDURE dbo.getpersons
WITH EXECUTE AS OWNER
AS
EXEC('SELECT id,firstname,lastname FROM dbo.person')
GO

EXECUTE AS USER = 'thelogin1'

--check to see if we are dbo user

SELECT USER

--this will work

EXECUTE dbo.getpersons

Let  me explain what I am doing in this bit of code

  • I am creating a database in line 6
  • I create a login in line 11
  • I create a user-thelogin1 for that login in line 17
  • I create a new table in lines 23-30
  • Then I populate the table – dbo.person in lines 32
  • Then I create a procedure – dbo.getpersons in lines 34-36
  • Then I grant permissions to the stored procedure to the user thelogin1
  • Then I execute the statements like user – thelogin1
  • If I try to access the table directly then I fail. That makes sense.
  • Then I execute the stored procedure as user – thelogin1 in lines 55
  • Then I revert back to the dbo user
  • Then I make a change in lines 67-70 to the stored procedure. I use in the body of stored procedure dynamic sql.
  • Then I change the user to thelogin1 in lines 74
  • Then I execute the stored procedure in line 82 . This will fail.The dynamic SQL statement causes a break in the ownership chain.
  • Then I revert back to line 85 as the dbo user
  • In the lines 93-97 I change the stored procedure again (WITH EXECUTE AS OWNER)
  • Then I change the user to thelogin1 in lines 99 
  • Finally, I execute the stored procedure again in lines 107 and it works

Hope it helps!!!

Tables with no clustered indexes or no indexes at all in SQL Server databases November 18, 2013

Posted by fofo in Sql Server, Sql Server 2005, SQL Server 2008, SQL Server 2012.
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add a comment

I have been delivering a certified course in MS SQL Server 2012 recently and I was asked a very interesting question “If there is a way to find using T-SQL tables in a database that have no clustered indexes or no indexes at all”

I have answered that this is possible and I provided with some scripts.

There a lot of tasks that we cannot perform through the GUI of SSMS.So we have to search through the object metadata to get our answers.We can query the catalog views and get all the information we want

I have installed SQL Server 2012 Enterprise edition in my machine but you can use the SQL Server 2012/2008 Express edition as well.

I am connecting to my local instance through Windows Authentication and in a new query window I type (you can copy paste)

SELECT DB_NAME(DB_ID ('AdventureWorksLT2012')) + '. ' +OBJECT_SCHEMA_NAME(t.object_ID)+ '.' +t.NAME AS
[tables without clustered index]
FROM sys.tables t
WHERE OBJECTPROPERTY(object_id, 'TableHasClustIndex')=0
AND OBJECTPROPERTY(object_id, 'IsUserTable')=1
ORDER BY [tables without clustered index]

If you execute this bit of code , you will see all the tables in the AdventureWorksLT2012 database that have not go a clustered index.

As you can see I query the sys.tables view.You can use this script to query your own database for tables without a clustered index.

If I want to find all the tables in the AdventureWorksLT2012 database or any other database for tables that have not indexes you can execute the following script.


SELECT DB_NAME(DB_ID ('AdventureWorksLT2012')) + '. ' +OBJECT_SCHEMA_NAME(t.object_ID)+ '.' + t.NAME AS
[tables_without_an_index]
FROM sys.tables t
WHERE OBJECTPROPERTY(object_id, 'TableHasIndex')=0
AND OBJECTPROPERTY(object_id, 'IsUserTable')=1
ORDER BY [tables_without_an_index]

Hope it helps!!!

Looking into SQL Server page types November 18, 2013

Posted by fofo in Sql Server, Sql Server 2005, SQL Server 2008, SQL Server 2012.
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I have been delivering a certified course in MS SQL Server 2012 recently and I was asked a very interesting question about SQL Server pages and what types of pages exist.

I had to explain to them about SQL Server architecture first. Each data file is divided internally into pages.Each page has a size of 8 kbytes.

In each of these pages (data,metadata,indexes) are stored. 8 continuous pages of 8kbytes make up one extent which is 64kbytes.
If you make the calculations,16 extents make up 1 mbyte.

Extents are divided into 2 categories,

  • Uniform – 8 pages belong to the same object e.g table
  • Mixed  – 8 pages do not belong to the same object

The first 96 bytes of the page are also reserved for the page header.
Page header includes information like

  • Page number
  • Page type
  • Free space per page

The greatest size in bytes I can have in a page is 8060 bytes
So the maximum record length I can have in a table is 8060 bytes

If I have a table that its record length is 100 bytes, that means I can have 80 records per page.
If I have 200 records I need 3 pages. Each record belongs to only one page. You cannot  have half record in one page and another half in another.

Ι was asked to provide an example where we will see the various SQL Server page types and what happens when the row size is larger than 8060 bytes.

I have installed SQL Server 2012 Enterprise edition in my machine but you can use the SQL Server 2012/2008 Express edition as well.

I am connecting to my local instance through Windows Authentication and in a new query window I type (you can copy paste)

USE tempdb
GO

CREATE TABLE largetable
( col1 VARCHAR(2000),
 col2 VARCHAR(8000),
 col3 VARCHAR(max)

 );

INSERT INTO largetable VALUES
(
REPLICATE('SQL Server 2012 will provide Mission Critical
Confidence with greater uptime blazing-fast performance and enhanced
security features for mission critical workloads',12), REPLICATE('SQL Server 2012', 533),
REPLICATE('SQL Server indexes', 1000))



SELECT * FROM largetable
-- Shows types of pages available inside SQL Server

SELECT OBJECT_NAME (object_id) AS tablename, rows,
 type_desc AS page_types_SQL
FROM sys.partitions part
INNER JOIN sys.allocation_units alloc
ON part.partition_id = alloc.container_id
WHERE object_id = object_id('largetable');

Let  me explain what I am doing in this bit of code

  • I make the tempdb the current database
  • I create a new table called largetable in the tempdb database. As you can see the record length is much larger than 8060 bytes.
  • Insert some values into the table. I insert thousands of rows using the REPLICATE function.
  • Finally I execute a SELECT JOIN statement from the sys.partitions and sys.allocation_units table.

The results are:

tablename    rows               page_types_SQL
largetable       1                            IN_ROW_DATA
largetable       1                            LOB_DATA
largetable       1                            ROW_OVERFLOW_DATA

As you can see I have one row of data.Each of these types of pages is called an Allocation Unit.

IN_ROW_DATA refers to the situation where we have the whole row size staying within the 8,060-byte limit.

LOB_DATA refers to LOB data type columns, in this case  the NVARCHAR(MAX) column and its values.

ROW_OVERFLOW_DATA refers to the data that could not be stored to single page for a single row and that is why it is using an overflow page.

Hope it helps!!!

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