Computed Column Performance

Paul White has a great article on when computed columns perform poorly:

A major cause of poor performance is a simple failure to use an indexed or persisted computed column value as expected. I have lost count of the number of questions I have had over the years asking why the optimizer would choose a terrible execution plan when an obviously better plan using an indexed or persisted computed column exists.

The precise cause in each case varies, but is almost always either a faulty cost-based decision (because scalars are assigned a low fixed cost); or a failure to match an expanded expression back to a persisted computed column or index.

The match-back failures are especially interesting to me, because they often involve complex interactions with orthogonal engine features. Equally often, the failure to “match back” leaves an expression (rather than a column) in a position in the internal query tree that prevents an important optimization rule from matching. In either case, the outcome is the same: a sub-optimal execution plan.

Definitely read the whole thing if you’re thinking about setting trace flag 176 on.

Related Posts

Bad Parameter Sniffing Flowchart

Grant Fritchey is asking for input on a new flowchart he has created: Lots of people are confused by how to deal with bad parameter sniffing when it occurs. In an effort to help with this, I’m going to try to make a decision flow chart to walk you through the process. This is a […]

Read More

Big Performance Tuning Mistakes

Erik Darling enumerates common performance tuning mistakes: Coming in at number five and looking alive! Did you know that query plans can be different on busy servers? I bet not! And aside from that, your performance problem might not even be the query itself, it may be blocking, or a poison wait. This stuff may not show […]

Read More

Categories