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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.

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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.


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DBCC ShrinkDatabase – I want to shrink my database.

TL;DR summary: Don’t do it. Stop reading here if you want, but just don’t do it.

This post refers to shrinking your database files (mdf, or ndf files), not shrinking the log file. The log file is a completely different conversation, however shrink database does shrink the log file.

Not shrinking your database is one of the more counter intuitive things out there. You might think that a smaller database is a good thing, however there are some negative side effect if you shrink your database regularly, or have the autoshrink option enabled. Side effects of shrinking your database include:

  • Excessive I/O due to the shrink.
  • Index fragmentation (most likely all of your indexes).
  • Excessive I/O to defragment your indexes.
  • After the shrink is complete, inserting or updating rows that require more space in your database will be slowed due to the time involved with growing your data file.

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