It Depends: Key Lookup Edition

Raul Gonzalez shows a case in which having a key lookup beats having a covering index:

This is a very special case where the covering index has three keys and then a couple of included columns, one of which is NVARCHAR(MAX) column, so the covering index is pretty big and we only seek in the leftmost column, which is also possible using another much smaller index on that single column.

In both cases, the operator is able to push the query predicate(s) to the seek and thanks to that, the number of rows coming out the operator is not that big. But the number of rows which match the leftmost key and therefore have to be read is quite big.

That’s why it’s so important to test queries rather than assuming you know how they will behave: ¬†sometimes the normal answer isn’t quite so.

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