 Hi there, this is Krishnan from Bitescout and in this video, we will be going to learn about one of the T-SQL execution plan operator called index-sick. In SQL Server, we have two types of operators for SICK and the first kind we will discuss is the clustered index-sick and the clustered index-sick means the SQL Server retrieving rows based on a SICK predicate. So it is a Sargable predicate from a clustered index. The word Sargable is a contraction of search argumentable. The optimizer's ability to benefit from an index depends on the selectivity of the search condition which in turn depends on the selectivity of the columns referred in the where close, non-closed or having closed, all of which are referred to the statistics on the index. And in contrast, we also have a non-clustered index-sick and that is the same as clustered index-sick where we are retrieving rows based on the SICK predicate. But in this case, we are doing that against a non-clustered index. The search predicate used on the columns in the where close determines whether an index operation on the column can be performed or not. So let's perform the demo for this operator. Alright, so for this demo, again, let's use the patient master table. Now in the previous video, we have already created the clustered index using this script on the patient master table. Now before executing this script, let me enable the execution plan by clicking on this button. Now when I execute this query, I get one rows for this patient ID and when I go to the execution plan tab, we can see that we have an index-sick operator over here. Now here patient ID has the index and that is our clustered index. Now when I hover over this index-sick operator, you will see the SICK predicate and in this SICK predicate, we can see the condition patient ID equals a specific value. Now let's go ahead and perform the non-clustered index demo. I am retrieving the patient record based on the last name where last name equal to John. Now when I execute this query, I will get the two record from the table. But when I see the execution plan, you can see that here I have a clustered index scan operator. So to get rid of this index scan, I have to create this non-clustered index on last name column. So let me create a non-clustered index. And now when I execute this same query again and in the execution plan, here you can see the index-sick and when I hover over this index-sick operator, you can see our newly created non-clustered index in the object section. So for the previous query, it was a clustered index and it had an index-sick. And now we have a non-clustered index and it has an index-sick. So index-sick can happen at either level, at the clustered index level and at the non-clustered index level. So in this query, where I said last name equal to John, then the SQL server engine start the evolution process as be tree traversal for that non-clustered index and retrieve the record for us. Now stay tuned with us as in the next video, we will be going to learn about our next operator key lookup.