Category Archives: Admin

I used to shrink my databases!

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This is an entry level post and a response to SQLEspresso’s blog challenge(http://sqlespresso.com/2017/01/10/ooops-was-that-was-me-blog-challenge) where we share mistakes from our “younger” days. My post takes me back 11 years and while it is nothing ground breaking I still  want to convey what shrinking does to your database. Why? Well it was something that I USED to do. ( Again – I will reinforce the point it was a long time ago)

Well not only does it generate a lot of I/O, consumes CPU but it also affects your fragmentation levels in indexes which is what we will look at today.

These tables and indexes are based from Jonathan’s script found here: https://www.sqlskills.com/blogs/jonathan/enlarging-the-adventureworks-sample-databases/

So I issued the below TSQL where I would expect to see no fragmentation after the rebuild (Index level 0 being the leaf node). Yes the page count is low but it’s the concept I want to talk about.


ALTER INDEX PK_SalesOrderHeaderEnlarged_SalesOrderID ON Sales.SalesOrderHeaderEnlarged REBUILD

DECLARE @db_id SMALLINT;
DECLARE @object_id INT;  

SET @db_id = DB_ID(N'AdventureWorks1997');
SET @object_id = OBJECT_ID(N'Sales.SalesOrderHeaderEnlarged');  

IF @db_id IS NULL
BEGIN;
    PRINT N'Invalid database';
END;
ELSE IF @object_id IS NULL
BEGIN;
    PRINT N'Invalid object';
END;
ELSE
BEGIN;
    SELECT * FROM sys.dm_db_index_physical_stats(@db_id, @object_id, NULL, NULL , 'detailed');
END;
GO

shrink

Now let’s SHRINK. You can actually do this via SQL Server Management Studio.


DBCC SHRINKDATABASE(N'AdventureWorks1997' )
GO

DECLARE @db_id SMALLINT;
DECLARE @object_id INT;  

SET @db_id = DB_ID(N'AdventureWorks1997');
SET @object_id = OBJECT_ID(N'Sales.SalesOrderHeaderEnlarged');  

IF @db_id IS NULL
BEGIN;
    PRINT N'Invalid database';
END;
ELSE IF @object_id IS NULL
BEGIN;
    PRINT N'Invalid object';
END;
ELSE
BEGIN;
    SELECT * FROM sys.dm_db_index_physical_stats(@db_id, @object_id, NULL, NULL , 'detailed');
END;
GO

99.85% avg_fragmentation, in essence the order has been reversed.

shrinkafterOk, for a one-of activity I don’t mind  you could just sort out the fragmentation afterwards, but to do it as part of a maintenance routine, not the best option out there. SHAME ON ME!

SQL Server Bugs & Enhancement Requests

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T-SQL Tuesday #86: SQL Server Bugs & Enhancement Requests hosted by Brent Ozar.

https://www.brentozar.com/archive/2017/01/announcing-t-sql-tuesday-87-sql-server-bugs-enhancement-requests/

tsql

I did not want to miss out on this one because I want to mention a connect item that I logged last year which is still active.

This is an enhancement request for SQL Server where I got the idea from SQL database (Azure).

Wait Stats is my “go-to” thing, when you want to dig into performance issues everyone knows you will probably end up using sys.dm_os_wait_stats. You cannot use this in the Azure world, you have to use a DMV that is scoped to the database level. I think this would be a nice idea to have with the “earth” based SQL Servers – the ability to return information about all the waits encountered by threads that executed at the database level.

The connect item can be found at this link: https://connect.microsoft.com/SQLServer/feedback/details/2647332/sys-dm-db-wait-stats-dmv?tduid=(262281c4c73a682498780643b77e80d1)(256380)(2459594)(TnL5HPStwNw-KjxCoz0pGWobbq7q1MQTIw)()

An example of using the DMV that I would like:

WITH Waits AS
(SELECT wait_type, wait_time_ms / 1000. AS wait_time_s,
100. * wait_time_ms / SUM(wait_time_ms) OVER() AS pct,
ROW_NUMBER() OVER(ORDER BY wait_time_ms DESC) AS rn
FROM sys.dm_db_wait_stats
)
SELECT W1.wait_type,
CAST(W1.wait_time_s AS DECIMAL(12, 2)) AS wait_time_s,
CAST(W1.pct AS DECIMAL(12, 2)) AS pct,
CAST(SUM(W2.pct) AS DECIMAL(12, 2)) AS running_pct
FROM Waits AS W1
INNER JOIN Waits AS W2
ON W2.rn <= W1.rn
GROUP BY W1.rn, W1.wait_type, W1.wait_time_s, W1.pct
HAVING SUM(W2.pct) - W1.pct <  99

WAITSDMV.JPG

It currently has 8 votes, would be nice to get some more.

Don’t blink you might READPAST it

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Following on from my previous post on NOLOCK (https://blobeater.blog/2016/12/09/nolock/) I want to talk about another hint called READPAST. This hint tells the database engine not to read rows that are locked by other transactions.

It is best explained with an example.

First let’s look at the basics of locking under the default isolation level with no hints. In my first SSMS (SQL Server Management Studio) window I issue the following.

BEGIN TRAN
UPDATE [HumanResources].[Department]
SET Name = 'Global'
WHERE DepartmentID = 16

Under a second SSMS connection I run a select statement – yes naturally I am blocked.

SELECT * FROM HumanResources.Department

readpast

I have circled in red the resource_description which can be used as input into the undocumented function  %%lockres%% to understand/confirm what is actually being locked.

select *,%%lockres%%
from    HumanResources.Department (nolock)
where    %%lockres%% IN('(62fb4a4c0e9e)')

Just to confirm it is the DepartmentID = 16 row as per my update statement.

readpast1

Anyways, I issue a rollback and the blocking stops.

readpast3

So now that we understand what “normally” happens let’s play with the READPAST hint.

SSMS window 1

 BEGIN TRAN
UPDATE [HumanResources].[Department]
SET Name = 'Global'
WHERE DepartmentID = 16

SSMS window 2

 SELECT * FROM HumanResources.Department (READPAST)

readpst5.JPG

Totally skips the row where DepartmentID =16.

If I rollback the update statement you should see the DepartmentID = 16 come back.

 ROLLBACK

Then

SELECT * FROM HumanResources.Department (READPAST)

reradp2.JPG

So, as the name of the hint suggests, it literally does “readpast” it.

SQL Server READPAST

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3

Following on from my previous post on NOLOCK (https://blobeater.blog/2016/12/09/nolock/) I want to talk about another hint called READPAST. This hint tells the database engine not to read rows that are locked by other transactions.

It is best explained with an example.

First let’s look at the basics of locking under the default isolation level with no hints. In my first SSMS (SQL Server Management Studio) window I issue the following.

BEGIN TRAN
UPDATE [HumanResources].[Department]
SET Name = 'Global'
WHERE DepartmentID = 16

Under a second SSMS connection I run a select statement – yes naturally I am blocked.

SELECT * FROM HumanResources.Department

readpast

I have circled in red the resource_description which can be used as input into the undocumented function  %%lockres%% to understand/confirm what is actually being locked.

select *,%%lockres%%
from    HumanResources.Department (nolock)
where    %%lockres%% IN('(62fb4a4c0e9e)')

Just to confirm it is the DepartmentID = 16 row as per my update statement.

readpast1

Anyways, I issue a rollback and the blocking stops.

readpast3

So now that we understand what “normally” happens let’s play with the READPAST hint.

SSMS window 1

 BEGIN TRAN
UPDATE [HumanResources].[Department]
SET Name = 'Global'
WHERE DepartmentID = 16

SSMS window 2

 SELECT * FROM HumanResources.Department (READPAST)

readpst5.JPG

Totally skips the row where DepartmentID =16.

If I rollback the update statement you should see the DepartmentID = 16 come back.

 ROLLBACK

Then

SELECT * FROM HumanResources.Department (READPAST)

reradp2.JPG

So, as the name of the hint suggests, it literally does “readpast” it.

NOLOCK

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NOLOCK, some say it’s a fast option for queries and will never cause blocking, I say it’s quite dirty and MAY cause blocking.

I used to think that a NOLOCK (which is the same as read uncommitted) hint actually meant that a “no lock” was used, I was very wrong and the naming doesn’t really help. Let’s have a look.

I will run a random SELECT statement with the hint, such as

SELECT * FROM Sales.BigSalesOrderDetailEnlarged WITH (NOLOCK)

Then I will check the “go to” DMV for locks.

SELECT resource_type, resource_associated_entity_id,
    request_status, request_mode,request_session_id,
    resource_description
    FROM sys.dm_tran_locks
    WHERE resource_database_id = 13

nolock

A Sch-S (schema stability) lock is taken.  This is a lightweight lock; the only lock that can conflict with this is a Sch-m (schema modification) lock. (C = Conflict). This means that a NOLOCK can actually block for example against an ALTER TABLE command.

nolock1

Below shows the example of when my NOLOCK query is being blocked by ALTER INDEX commands:

 SELECT resource_type, resource_associated_entity_id,
    request_status, request_mode,request_session_id,
    resource_description
    FROM sys.dm_tran_locks

nolcok4

So there you go, it can be involved in blocking.

Dirty reads:

This is my example data.

nocok3.JPG

In Window 1 using SQL Server management Studio I begin an open UPDATE statement – notice that it has NOT been committed.

BEGIN TRAN
UPDATE [HumanResources].[Department]
SET Name = 'Global'
WHERE DepartmentID = 16

In window 2 I do a NOLOCK select statement.

SELECT * FROM HumanResources.Department (NOLOCK)

lockdffgsef.JPG

Now that’s dirty.