When UNION ALL Can Beat OR

Bert Wagner compares a couple methods for writing a query:

Suddenly those key-lookups become too expensive for SQL Server and the query optimizer thinks it’ll be faster to just scan the entire clustered index.

In general this makes sense; SQL Server tries to pick plans that are good enough in most scenarios, and in general I think it chooses wisely.

However, sometimes SQL Server doesn’t pick great plans. Sometimes the plans it picks are downright terrible.

If that particular topic is interesting, I’ve a blog post from a few years back on a similar vein.

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