INDEX FAQS

1. What is an index in Oracle?

• An index in Oracle is a database object that enhances the speed of data retrieval operations on a table. It provides a quick lookup for rows based on the values in one or more columns.

2. Why do I need to use indexes in Oracle?

• Indexes are used to improve query performance, especially for SELECT operations. They speed up retrieval by allowing the database to locate rows faster than scanning the entire table.

3. What are the different types of indexes in Oracle?

• Oracle supports various types of indexes:
• B-Tree Index: Default index type used for most queries.
• Bitmap Index: Efficient for columns with low cardinality.
• Unique Index: Ensures that no two rows have the same value in the indexed column(s).
• Composite Index: An index on multiple columns.
• Reverse Key Index: Used to avoid index block contention, typically for sequential values.
• Function-Based Index: Based on expressions or functions rather than columns.
• Domain Index: Used for custom types or complex data structures like XML or spatial data.

4. When should I use a bitmap index?

• Bitmap indexes are best for columns with low cardinality (i.e., columns with only a few distinct values). Examples include gender, status, or boolean flags. They are efficient for queries involving multiple conditions.

5. What is the difference between a unique index and a primary key index?

• A unique index enforces the uniqueness of values in one or more columns. It is created when a UNIQUE constraint is added to a table.
• A primary key index is automatically created when a PRIMARY KEY constraint is defined on a table, ensuring that the primary key column(s) contain unique values and do not contain NULL.

6. Can I create an index on multiple columns?

• Yes, you can create a composite index on multiple columns. This index can speed up queries that filter on multiple columns. The order of columns in the composite index is important for query performance.

7. What are the downsides of using indexes?

• While indexes improve query performance, they can:
• Increase storage: Indexes consume additional disk space.
• Slow down DML operations: INSERT, UPDATE, and DELETE operations are slower because the indexes must be updated whenever the data in the table changes.
• Add complexity: Too many indexes can lead to performance degradation during data modification.

8. How can I check which indexes are being used?

• You can use Oracle’s query optimizer to determine if an index is being used in a query. Tools like EXPLAIN PLAN can show whether an index is being used for a given query.

Example:

EXPLAIN PLAN FOR

SELECT * FROM employees WHERE name = 'John Doe';

9. How do I rebuild or coalesce an index?

• Rebuild: When indexes become fragmented, rebuilding them can improve performance.

·        ALTER INDEX index_name REBUILD;

• Coalesce: If an index has some fragmentation, you can coalesce it to reduce the number of index blocks used without fully rebuilding the index.

·        ALTER INDEX index_name COALESCE;

10. How does Oracle decide which index to use?

• Oracle’s query optimizer uses statistics to decide the best index to use. It considers factors like the selectivity of the index, table size, and query conditions. The optimizer chooses an index that provides the most efficient path for retrieving data.

11. What are index-organized tables (IOT)?

• An Index-Organized Table (IOT) stores the table data itself in the index structure. The rows are organized in a B-tree index, making it efficient for retrieval, especially when the table has a primary key.

12. Can I drop an index in Oracle?

• Yes, you can drop an index using the DROP INDEX command. For example:

·        DROP INDEX index_name;

• Dropping an index removes it from the database, and the query optimizer will have to find other ways to access the data.

13. What happens if I create too many indexes on a table?

• While indexes speed up SELECT queries, too many indexes can slow down INSERT, UPDATE, and DELETE operations because all indexes need to be updated whenever data is modified. It is essential to balance between query speed and the overhead of maintaining indexes.

14. What is the difference between ALTER INDEX REBUILD and ALTER INDEX COALESCE?

• Rebuild: This operation fully rebuilds the index and reorders the index structure, making it more compact and efficient.
• Coalesce: This operation is a less resource-intensive method that merges index blocks to eliminate fragmentation without fully rebuilding the index.

15. What is an index on a function or expression?

• An index on a function is a function-based index. It is created on the result of an expression rather than directly on a column. This type of index is useful when queries often use functions like UPPER, LOWER, or TO_DATE on columns.

Example:

CREATE INDEX idx_upper_name ON employees(UPPER(name));

16. Can an index improve the performance of JOIN operations?

• Yes, indexes can improve the performance of JOIN operations. When joining tables, if the join condition involves indexed columns, Oracle can quickly retrieve the matching rows, improving the performance of the join.

17. What is a domain index, and when should I use it?

• A domain index is a specialized index used for custom data types (e.g., spatial data, XML data) or user-defined types (UDTs). It’s used for indexing non-standard data or custom applications like full-text searches.

18. What is the impact of indexes on the performance of DML operations?

• Indexes slow down INSERT, UPDATE, and DELETE operations because these operations require the indexes to be updated as well. The more indexes a table has, the more overhead is required for DML operations.

19. What is an index fragmentation, and how can I manage it?

• Index fragmentation occurs when the index structure becomes inefficient due to frequent insertions, deletions, and updates. You can manage fragmentation by rebuilding or coalescing indexes periodically.