Thursday, January 19, 2012

SQL Server interview questions: - Explain two types of indexes?

This is the SQL Server interview questions most asked during the interview so do revise before you go for the interview.
There are basically two types of indexes:-
  • Clustered Indexes.
  • Non-Clustered Indexes.
Every thing is same for both the indexes i.e. it uses “B-TREE” for searching data. However, the main difference is the way it stores physical data
In clustered index, the non-leaf level actually points to the actual data.

Figure: - Clustered Index Architecture

In Non-Clustered index the leaf nodes point to pointers (they are rowid’s) which then point to actual data.

Figure: - Non-Clustered Index has pointers

So here is what the main difference is in clustered and non-clustered, in clustered when we reach the leaf nodes we are on the actual data. In non-clustered indexes, we get a pointer, which then points to the actual data.

Therefore, after the above fundamentals following are the basic differences between them:-
  • indexes as we have pointers, which is logical arrangement we do need this compulsion. Also, note in clustered index actual data as to be sorted in same way as the clustered indexes are. While in non-clustered
  • So we can have only one clustered index on a table as we can have only one physical order while we can have more than one non-clustered indexes.
Also see the following video calling a stored procedure using LINQ: -

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