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

The Downside Of Oversizing

Aaron Bertrand shows why you might not want to oversize VARCHAR columns by too much: Now, whether you go by the old standard or the new one, you do have to support the possibility that someone will use all the characters allowed. Which means you have to use 254 or 320 characters. But what I’ve […]

Read More

Physical Versus Read-Ahead Reads

Kendra Little explains which SQL Server diagnostic tools include read-ahead reads versus “regular” physical reads: SQL Server has more than one way to pull pages in from disk for your queries. SQL Server can do a physical read of an 8KB page, or an extent of 8 ¬†of those 8KB pages. SQL Server can also […]

Read More

Categories