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

Resumable Online Index Rebuild

Arun Sirpal shows off a SQL Server 2017 and Azure SQL Database feature: Clearly I am in a paused state with 5.48% percent of the operation complete. In the original query window where I started the index rebuild, I receive: Msg 1219, Level 16, State 1, Line 4 Your session has been disconnected because of […]

Read More

Set A Fill Factor

Monica Rathbun wants you to set a better fill factor than the default: Please, please, please Admins do not leave your default index fill factor at 0. This means you are telling SQL Server to fill the page 100% full when creating indexes. This also means you are forcing it to a new page when […]

Read More

Categories

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