Question

I was just reading from here https://stackoverflow.com/questions/6187904/hard-and-fast-rule-for-include-columns-in-index
regarding included columns.

n index is typically

CREATE INDEX <name> ON <table> (KeyColList) INCLUDE (NonKeyColList)

Where:

• KeyColList = Key columns = used for row restriction and processing
  WHERE, JOIN, ORDER BY, GROUP BY etc
• NonKeyColList = Non-key columns = used in SELECT and aggregation (e.g. SUM(col)) after selection/restriction

Now let’s say my query is :

SELECT  Col7 ,
        Col8 ,
        Col9 ,
        Col10
FROM    [MyTable] M
WHERE   Col1 =1
        AND Col2=2
        AND Col3=3                         
        AND Col4=4 
        AND Col5=5 
        AND Col6=6
GROUP BY Col7 ,
         Col8 ,
         Col9 ,
         Col10
ORDER BY Col8

What should be the index for me here in this case? and second case where Col7 is primary key

Answer 1

I would think you would want something like:

CREATE NONCLUSTERED INDEX MyIndex ON MyTable(Col1, Col2, Col3, Col4, Col5, Col6)
INCLUDE (Col7, Col8, Col9, Col10)

You’re filtering on Col1-Col6 and retrieving Col7-Col10. Not sure how this will work with the group by, though. You may want to rewrite it as a DISTINCT since the exec plan and results are the same but it will be more readable.

If Col7 is a primary key and has a clustered index, you can leave this like it is. Col7 will be included in the index without being specifically mentioned as a cluster key is in every non-clustered index as a row identifier, but keeping it in won’t hurt as SQL will ignore it.