Performance consideration of index ms sql

I have a query which is executed against one of our databases 100+ per minute

The queries bombarding this table look like this:

SELECT  mt.myPKCol, mt.myDataCol1, mt.myDataCol2, mt.myDataCol3, ot.myDataCol1
  FROM myTable mt
      INNER JOIN myOtherTable ot ON (mt.myFK1Col=ot.myPKCol)
  WHERE mt.myFK1Col=x AND mt.myFK2Col=y

myTable looks like this:

CREATE TABLE [myTable] (
         [myPKCol] [int] IDENTITY(1,1) NOT NULL,
      [myFK1Col] [int] NOT NULL,
      [myFK2Col] [int] NOT NULL,
      [myDataCol1] [nvarchar](255) NULL,
      [myDataCol2] [datetime] NULL,
      [myDataCol3] [int] NULL,
        constraint [PK_myPKCol] primary key ([myPKCol])
)

There are 25 or so possible values for FKCol1 and hundreds of thousands of values for FKCol2.

There are currently over 4 million rows in mytable and Profiler shows each instance of this problem query with an avg CPU time of over 300ms and there are hundreds per minute. CPU runs 90% + during peak hours.

I cannot affect the query because it is coming from third party software

Here is my question:

I would like to add the following index to improve performance of above SELECT query:

CREATE INDEX [idx_myFKCol1_myFKCol2] on [myTable]([myFKCol1],[myFKCol2)

In a test environment I have added the index and the select queries perform MUCH better

What is the impact on inserts, updates, deletes?

This table is constantly inserted to, updated and ocasionally but less frequently deleted from.
20-30 very active  concurrent users of third application

Although once a row is inserted its myFK1Col and myFK2Col columns are NEVER updated, only myDataCol1, myDataCol2, and myDataCol3 are updated

Will I solve the problem with the select queries by creating the index only to create a new problem with insert, update, and delete queries on table?




     
 
angelz4kaliAsked:
Who is Participating?
 
Guy Hengel [angelIII / a3]Connect With a Mentor Billing EngineerCommented:
note: if your table does not yet have a clustered index, you should consider to create this one as clustered, or change one of the existing index to clustered.
the decision is usually taken by considering
§ range queries on the indexed columns (pro clustered index)
§ updates of the valued of the indexed columns (con clustered index)
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Guy Hengel [angelIII / a3]Connect With a Mentor Billing EngineerCommented:
better would be:
CREATE INDEX [idx_myFKCol1_myFKCol2] on [myTable]([myFKCol2], [myFKCol1])

because the value of myFKCol2 is more selective than myFKCol1.
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angelz4kaliAuthor Commented:
myFKCol2  has hundreds of thousands of possible values
myFKCol1 only has 20 to 30 possible values
does this make myFKCol2 more selective???

Will insert, update, delete performance be hurt significantly?
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chapmandewCommented:
Yes, that makes col2 more selective.

It probably won't hurt performance TOO much because the index isn't a clustered one...also, since the columns are integers they won't take up much space.
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Guy Hengel [angelIII / a3]Connect With a Mentor Billing EngineerCommented:
>myFKCol2  has hundreds of thousands of possible values
>myFKCol1 only has 20 to 30 possible values
>does this make myFKCol2 more selective???

explanation:
say you have 1 Million rows.
for 1 given value of myFKCol1 (of which you have +-25 distinct values), that will potentially bring back 100000/25 = 40000 records.
for 1 given value of myFKCol2 (of which you have +-100000 distinct values), that will potentially bring back 100000/100000 = 10 records.

so, searching (and hence indexing) by myFKCol2 first will be more selective, as immediately much less records have to be considered for further conditions.
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angelz4kaliAuthor Commented:
I went ahead and implemented the index and have a gorgeous video watching the CPU drop from avg 99% utilization during peak usage to avg 4% utilization during peak usage.

Thanks for all your help
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