SQL Server – Understanding Overlapping Indexes

indexnonclustered-indexsql serversql-server-2008-r2sql-server-2012

I have two non-clustered indexes on one of my dbs and their definition are:

CREATE NONCLUSTERED INDEX [NC_Index1] ON [dbo].[anyTable]
(
    [X] ASC,
    [Y] ASC,
    [Z] ASC
)
INCLUDE (A  ,
         B,
         C)

and 

CREATE NONCLUSTERED INDEX [NC_Index2] ON [dbo].[anyTable]
(
    [X] ASC,
    [Y] ASC,
    [Z] ASC,
    [colm1] ASC,
    [C] ASC
)
INCLUDE (A,
         colm2,
         colm3,
         colm4,
         colm5          
)

I was just wondering if I could merge these two non-clustered indexes into single one by changing the definition of the second one by adding column B to include columns of the second one like:

CREATE NONCLUSTERED INDEX [NC_Index2] ON [dbo].[anyTable]
(
    [X] ASC,
    [Y] ASC,
    [Z] ASC,
    [colm1] ASC,
    [C] ASC
)
INCLUDE (A,
         B
         colm2,
         colm3,
         colm4,
         colm5          
)

Also, would order of the included columns matter?

Thanks all.

Best Answer

Yes, as ypercube already said.

The only thing to look out is a wild discrepancy in the column actual data size. For instance if NC_Index1 is some 1Mb, and NC_Index2 is 200GB then you can introduce some potential scan perf problems. But, frankly, the chances of this being the case are, basically, 0. Or NULL, depending on your prefs.

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