Tuesday, February 15, 2011

What are the advantages and dis-advantages of secondary Indexes?


What are the advantages and dis-advantages of secondary Indexes?
Answer:

Advantages:
1. A secondary index might be created and dropped dynamically
2.A table may have up to 32 secondary indexes.
3. Secondary index can be created on any column. .Either Unique or Non-Unique
4. It is used as alternate path or Least frequently used cases.  ex. defining SI on non indexed column can improve the performance, if it is used in  join or filter condition of a given query.
5. Collecting Statistics on SI columns make sure Optimizer choses SI if it is better than doing Full Table Scans

Disadvantages
1. Since Sub tables are to be created, there is always an overhead for additional spaces.
2. They require additional I/Os to maintain their sub tables.
3. The Optimizer may, or may not, use a NUSI, depending on its selectivity.
4. If the base table is Fallback, the secondary index sub table is Fallback as well.
5. If statistics are not collected accordingly, then the optimizer would go for Full Table Scan.

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