Question

Running the advisor it suggests I should create an index on Path, Value and to include Id.

Path varchar(250)
Value bigint
Id uniqueidentifier

1. When using the index in a query, will it be faster having Id in the index rather than as an include?

2. Will inserts be slower having Id as an include rather than in the index?

Answer 1

• 1) Depends on query if it looks like

  select Id 
  from YourTable 
  WHERE Path = x and Value = y and Id =(or>, <, between etc.) z
  

i.e. – if query involves exact values (not ranges) of Path and Value – then yes, else – no, will be slower because of more thicker non-leaf rows

• 2) Actually – yes, but it can be negligible, because the sql needs to sort AND by Id, but if your Path and Value is versatile enough – the difference will be very tiny. In IO there will be a little gain if saving Id in INCLUDE section – because no needs to save it on non-leaf pages

As you noticed:

Query uses Id, Path and Value. Id and Path in join restriction and
Value in where. The Id is furthermore a FK on primary table. With this
short info, How would you design the index?

So, its better include all the rows in index, and the best order of columns will depend on structure/indexes of other table in JOIN.

The index should look like (Value, Id, Path) or (Value, Path, Id) – depending on mentioned above.