SQL Server - Merging large tables without locking the data
I have a very large set of data (~3 million records) which needs to be merged with updates and new records on a daily schedule. I have a stored procedure that actually breaks up the record set into 1000 record chunks and uses the MERGE command with temp tables in an attempt to avoid locking the live table while the data is updating. The problem is that it doesn't exactly help. The table still "locks up" and our website that uses the data receives timeouts when attempting to access the data. I even tried splitting it up into 100 record chunks and even tried a WAITFOR DELAY '000:00:5' to see if it would help to pause between merging the chunks. It's still rather sluggish.
I'm looking for any suggestions, best practices, or examples on how to merge large sets of data without locking the tables.
Thanks
Answer
Change your front end to use NOLOCK or READ UNCOMMITTED when doing the selects.
You can't NOLOCK MERGE,INSERT, or UPDATE as the records must be locked in order to perform the update. However, you can NOLOCK the SELECTS.
Note that you should use this with caution. If dirty reads are okay, then go ahead. However, if the reads require the updated data then you need to go down a different path and figure out exactly why merging 3M records is causing an issue.
I'd be willing to bet that most of the time is spent reading data from the disk during the merge command and/or working around low memory situations. You might be better off simply stuffing more ram into your database server.
An ideal amount would be to have enough ram to pull the whole database into memory as needed. For example, if you have a 4GB database, then make sure you have 8GB of RAM.. in an x64 server of course.