The Problems With NOLOCK

Rob Farley demonstrates the downside of the READ UNCOMMITTED isolation level:

I’m going to create a table and insert exactly 1 million rows. This particular table will be a clustered index, and will contain 1 million GUIDs.

Next I prove that there a million rows.

Now without inserting or deleting any rows, I’m going to shuffle them.

And if while this is happening, I count the rows in a different session, I have to wait for that query to finish.

Read on to see what happens when someone gets the idea of running the select query with NOLOCK.

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