Skip to content

SQL Server

Database Corruption Overview for Beginners

Database Corruption Overview: Database corruption is one of those things that you can only plan for by practicing your response plaDatabase Corruption Overviewn. Out of all of the things that can happen to your SQL Server this is the one that you are most likely going to want to ask for help when you encounter it.

What is Corruption?

Database corruption refers to corrupt pages in the database that are incorrectly formatted. This could be as simple as a single bit, or as huge as the entire file. Sometimes this type of corruption prevents the database from starting, other times it may prevents queries from running. Sometimes it may go undetected for some time, and may present as missing or incorrect data.

Read More »Database Corruption Overview for Beginners

Instant File Initialization (IFI)

One way to improve performance on SQL Server is with IFI or Instant File Initialization.

Normally files are zeroed out on a database growth during an AUTOGROW, RESTORE, CREATE DATABASE or ALTER DATABASE. This is done by SQL Server when the file grows, it runs through that file and writes zeroes to the entire new allocation in the file. The zeroing process can take a great deal of time, the Instant file initialization process skips this zeroing, and just allocates the file. This works since SQL Server will just write each 8k page to disk as they are used, thus overwriting the uninitialized file.

Running some tests on a local virtual machine running SQL Server 2012 (similar results tested on SQL Server 2014 and SQL Server 2016), here is what I found.

  • Creating a new database (10GB file size) took 2 minutes and 6 seconds, with IFI enabled, it took just 4 seconds.
  • Autogrow of a data file witha 1GB growth size took 13 seconds without IFI, with IFI it took less than a second.

 

For IFI to work, the user account that SQL Server is running as needs the “Perform volume maintenance tasks” policy to be enabled.Read More »Instant File Initialization (IFI)

Duplicate Indexes, What a Waste

Today on the tuning minute on the SQL Data Partners Podcast, we discussed duplicate indexes, which lead me to think more about and and write this post.

You know there are many different ways of doing things in SQL Server, and often times you can argue that one way or the other is better, and given the right situation anything might be a good idea. However duplicate indexes are a different story.

When I talk about duplicate indexes, what I mean is 2 or more indexes on the same table that are exactly the same columns.  Something like this:


CREATE NONCLUSTERED INDEX [IX_LastName] ON [dbo].[Customer]
(
 [Lastname] ASC
);

CREATE NONCLUSTERED INDEX [dta123123123_LastName] ON [dbo].[Customer]
(
 [Lastname] ASC
);




Two indexes on exactly the same column. There is nothing to be gained here.


Read More »Duplicate Indexes, What a Waste

The Use of WITH NOLOCK Query Hint

TWITH NOLOCKhere is a great deal of confusion about the WITH NOLOCK query hint. The following is a transcription of a conversation that I had on the SQL Data Partners Podcast #57 about the WITH NOLOCK hint, and some of the misconceptions about it.

WITH NOLOCK Hint

Carlos: So the next on is the WITH NOLOCK hint and I think this is on the list, ultimately, because there’s a lot of misinformation out there.

Steve: Absolutely, yes. The NOLOCK hint is one of my peeves on SQL Server, actually. I see it used a lot and really 99 percent of the time I see it used, people think that it’s doing something different than what it does. So, I heard the statement, “But I want to run a query in the production system but not impact or block anyone else. Shouldn’t I just use NOLOCK?” And the answer there is NO. The NOLOCK hint tells SQL Server to ignore other people’s query locks. Basically, to read dirty or uncommitted data at that point, which can lead to missing rows or phantom rows or data showing up in results. But it doesn’t do anything to stop the locking or the blocking in any way on the query that’s calling it. So it’s the equivalent of saying read uncommitted on a specific table that it’s referencing.

Read More »The Use of WITH NOLOCK Query Hint