“But Surely NOLOCK Is Okay If No One’s Changing Data, Right?”

Some of y’all, bless your hearts, are really, really, really in love with NOLOCK. I’ve shown you how you get incorrect results when someone’s updating the rows, and I’ve shown how you get wrong-o results when someone’s updating unrelated rows. It doesn’t matter – there’s always one of you out there who believes NOLOCK is okay…
Read More

When You’re Troubleshooting Blocking, Look at Query #2, Too.

When I’m troubleshooting a blocking emergency, the culprit is usually the query at the head of a blocking chain. Somebody did something ill-advised like starting a transaction and then locking a whole bunch of tables. But sometimes, the lead blocker isn’t the real problem. It’s query #2. Here’s a sample scenario: A long-running select with…
Read More

How Scalar User-Defined Functions Slow Down Queries

When your query has a scalar user-defined function in it, SQL Server may not parallelize it and may hide the work that it’s doing in your execution plan. To show it, I’ll run a simple query against the Users table in the Stack Overflow database. SELECT TOP 100 DisplayName, Location, Reputation, Id FROM dbo.Users ORDER…
Read More