Comments on: SQL SERVER – Concurrency Problems and their Relationship with Isolation Level

By: SQL SERVER – Weekly Series – Memory Lane – #019 | SQL Server Journey with SQL Authority

Sat, 09 Mar 2013 01:31:58 +0000

[...] Concurrency Problems and their Relationship with Isolation Level Concurrency is simply put capability of the machine to support two or more transactions working with the same data at the same time. This usually comes up with data is being modified, as during the retrieval of the data this is not the issue. Most of the concurrency problems can be avoided by SQL Locks. There are four types of concurrency problems visible in the normal programming. [...]

By: SQL SERVER – Importance of ANSI ISOLATION Levels in SQL Server Database – Quiz – Puzzle – 1 of 31 « SQL Server Journey with SQLAuthority

Mon, 02 Jan 2012 01:31:05 +0000

[...] Server Interview Questions and Answers ISBN: 1466405643 Page#20-21 Concurrency Problems and their Relationship with Isolation Level Check the Isolation Level with DBCC useroptions Applying NOLOCK Hint at Query Level – NOLOCK [...]

By: cloud computing

cloud computing — Thu, 28 Jul 2011 12:38:30 +0000

Putting NOLOCK everywhere they see is one the biggest mistakes of novice DBAs. Concurrency is not such a simple task to achieve and every responsible DBA should investigate all Isolation levels deeply for optimized production server performance.

By: Rajan

Rajan — Tue, 21 Jun 2011 08:11:55 +0000

Hi Penal,

Process 1 :

Begin Transaction

Table 1 – Reading No

Table 2 – Inserting No with details
.
.
.
.
Commit Transaction

Process 2:

Meantime outside of transaction

Table 1 – Reading No
Table 2 – Inserting No with details
.
.
.

Process1 affects process 2 because of the Read Committed transaction.
Actually it locks(IX) entire table. Not even select the old data.

pls help how to view / Insert the data when process1 running.

By: Narendra Kotha

Narendra Kotha — Sun, 06 Mar 2011 17:12:57 +0000

hi pinal sir,

i am great fan of you & your blogs.
Recently i came across your 109th blog.

when i run the script in my SSMS, it is giving error.

so that i have modified the script

/* FUNCTION TO GET WEEKDAYS FROM TWO GIVEN DAYS */
IF EXISTS(SELECT * FROM sys.objects WHERE TYPE = ‘FN’ AND name = ‘Fn_GetWeekDays’)
DROP FUNCTION Fn_GetWeekDays
GO
CREATE FUNCTION Fn_GetWeekDays
( @StartDate DATETIME,
@EndDate DATETIME )
RETURNS INT
AS
BEGIN
DECLARE @FirstPart INT
DECLARE @TotalDays INT
DECLARE @LastPart INT

SET @FirstPart = 0
SET @LastPart = 0
IF (DATEDIFF(DAY, @StartDate, @EndDate)>0)
BEGIN
WHILE (@StartDate <= @EndDate)
BEGIN
IF (SUBSTRING(DATENAME(DW,@StartDate),1,3) IN ('MON','TUE','WED','THU','FRI','SUN'))
BEGIN
SET @FirstPart = @FirstPart + 1
END
SET @StartDate = @StartDate + 1
END

SET @TotalDays = @FirstPart
END
ELSE

SET @TotalDays = 0

RETURN @TotalDays

END
GO

SELECT DBO.Fn_GetWeekDays('03/01/2011','03/06/2011')

Please forgive me if i am wrong.

Thanks,
Naren

By: Sonia

Sonia — Sat, 05 Mar 2011 18:27:09 +0000

Superb Post Pinal.

Till date I was never able to understand the difference between these 4 terms explicitely.

I am sure this post is going to help all the sql server knowledge seekers like me.

By: Nakul Vachhrajani

Nakul Vachhrajani — Sat, 05 Mar 2011 08:05:46 +0000

Hello!
With the misconception that using (NOLOCK) will eliminate locking in a database, I have seen many databases suffer from the problem of dirty and non-repeatable reads.
My suggestion is to acquire locks for as brief a period as possible, and then implement optimistic concurrency (I wrote about this one in one of my posts – http://beyondrelational.com/blogs/nakul/archive/2010/12/21/optimistic-concurrency-a-conceptual-introduction.aspx).

(NOLOCK) when used judiciously is good, but when used in excess, might cause unpredictable application behaviour.

Be courteous. Drive repsonsibly,
http://beyondrelational.com/blogs/nakul/default.aspx

By: weicco

weicco — Sat, 05 Mar 2011 05:32:57 +0000

I’ve been checking one of our production guidance system’s database lately. It’s been running really slow in some operations and the vendor seems to be unable to fix it.

So first I checked top 10 slowest queries. I quickly found out that the reason is missing indexes. Some tables didn’t even have clustered index. I wrote a report that tables needs indexing badly and also there was quite horrific queries that needs to be rethought.

Then began the horror. I realized that every SQL query was filled with NOLOCK hints! Every stored procedure started with SET TRANSACTION ISOLATION LEVEL READ UNCOMMITED. So instead of implementing proper indexing scheme the vendor was trying to fix performance problems by avoiding locks totally!

We are talking about elementary system to our business. A simple hickup could cost tens of thousands of euros. A major hickup halts the production completely. I’m stunned that someone does so slobby work in such important system.

Luckily this won’t be my headache anymore since I’ll be switching jobs soon. I just wanted to write about this to tell you that there are things that you simply must not do, like enabling dirty reads in a case like this. Maybe there are situations where dirty reads are acceptable but I’ve yet to see one. If you are in doubt like what’s causing the performance problems, please ask someone more experienced in the field. There’s no shame in asking. No-one can know everything about everything. That’s why we create our social networks so we can help each others out.

BR,
-Marko