Does performance of a database (SQL Server 2005) decrease if I shrink it?

Question

Does performance of a database (SQL Server 2005) decrease if I shrink it?

What exactly happen to the mdf and ldf files when shrink is applied (Internals???)

Answer 1

When shrinking a database it will consume resources to shrink the DB. Where it runs into issues is when the DB needs to grow again, and assuming you have auto grow set, it will consume more resources to auto grow. Constant auto shrink (or shrink as part of maintenance plan) will cause physical disk fragmentation.

If you have auto grow enabled and it is set to the default of 1MB then constant auto grows will consume a lot of resources.

It is best practice to size your database to a size that is suitable, expected initial size plus expected growth over a period (month, year, whatever period you see fit). You should not use auto shrink or use shrink as part of a maintenance program.

You should also set your auto grow to MB (not a % of the database as when auto growing it needs to calculate the % first, then grow the database). You should also set the auto grow to a reasonable amount to ensure that it isnt going to be growing every 10 mins, try and aim for 1 or two growths a day.

You should also look at setting Instant Initialisation for your SQL Server.

Good luck,

Matt

Answer 2

It's important to understand that when you shrink a database, the pages are re-arranged. Pages on the end of the data file are moved to open space in the beginning of the file, with no regard to fragmentation.

A clustered index determines the physical order of data in a table. So, imagine that you just created a clustered index, which would have re-ordered the data in that table, physically. Well, then when you execute a shrink command, the data that had just been neatly ordered during the creation of the clustered index will now potentially be out of order, which will affect SQL's ability to make efficient use of it.

So, any time you do a shrink operation you have the potential of impacting performance for all subsequent queries. However, if you re-do your clustered indexes / primary keys after the shrink, you are helping to defragment much of the fragmentation that you may have introduced during the shrink operation. If performance is critical but you are also forced to do shrinks regularly, then in an ideal world you'd want to re-do your indexes after each shrink operation.

Answer 3

Yes it could affect performance a bit. When a database is in operation it doesn't care to much about its diskspace usage, more about efficient data retrieval/persistance.