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.

Related Posts

Generating Index Drop And Create Statements

Drew Furgiuele says “Game over, man, game over!” to indexes: The premise is simple: it will generate a series of DROP and then CREATE INDEX commands for every index. The process is a little more complex in practice, but at a high level it: Creates a special schema to house a temporary object, Creates a […]

Read More

Index That Column Or Include It?

Jeanne Combrinck lays out her recommendations on whether to make a particular column part of an index or have it be an included column: The original question we wanted to answer was whether we would see a performance difference when a query used the index with all columns in the key, versus the index with […]

Read More

Categories

March 2017
MTWTFSS
« Feb Apr »
 12345
6789101112
13141516171819
20212223242526
2728293031