Monday, 25 October 2010

The Continual SQL battle of Reindex and Shrink DB.

Over the weekend I went along to the fantastic DeveloperDeveloperDeveloper day held at Microsoft up in Reading.  The quality of the presentations was fantastic and even for frameworks/subjects I was familar with I learnt loads.   For me, the most useful presentation came at the end of the day (just as the brain was beginning to shut down due to information overload).  Simon Sabin (from SQL Know How) presented "Things you should know about SQL as a developer".  
A snapshot of his presentation can be found on his blog, but for me the most enlighten point was the battle between reindexing a table and shrinking a database.   Typically the two feel like they should go hand in hand, but they will in fact fight each other.   Re-indexing a table will create a new copy of the table in question, with the data correctly ordered to reduce fragmentation.  As the original data is just marked as available, the process of re-indexing a table will potentially result in a database growing by the size of the table being re-indexed.  Once the re-index process has completed the database will report all the space taken by the old copy of the table as free space.  
However, if at this point if you shrink the database you will refragment your nicely re-indexed table as the shrinking process reads/moves the last row of the table first.  This means your table content will end up reversed (and totally fragmented) after a shrink comand.
There are many reasons why you shouldn't shrink your database, this being just one of them!

No comments:

Post a Comment