INDEX

Oracle indexes are used to speed up the retrieval of rows from a database table. An index is a database object that provides an efficient mechanism for looking up data in a table based on the values in one or more columns. They can drastically improve the performance of queries that use the indexed columns in their WHERE, ORDER BY, or JOIN conditions.

Types of Indexes in Oracle

1. B-Tree Index (Balanced Tree)
• Default Type: This is the default type of index in Oracle and is widely used.
• Structure: It organizes the index entries in a balanced tree structure, where data is stored in sorted order. The B-Tree structure allows for fast retrieval of rows.
• Use Case: Most useful for equality and range queries (e.g., =, >, <, BETWEEN).
• Example:

o   CREATE INDEX idx_emp_name ON employees(name);

2. Bitmap Index
• Structure: Uses a bitmap to represent the existence of a value in a column. It's highly efficient when the number of distinct values in a column is low.
• Use Case: Ideal for columns with low cardinality (few distinct values), such as gender, status, or yes/no type columns. They are very efficient for read-heavy queries that combine multiple columns.
• Limitation: Bitmap indexes are not ideal for high-cardinality columns (many distinct values), and they cannot be used on columns that are frequently updated because they would require frequent bitmap recalculations.
• Example:

o   CREATE BITMAP INDEX idx_gender ON employees(gender);

3. Unique Index
• Structure: A special type of index that ensures the uniqueness of the values in the indexed column(s). It automatically enforces the UNIQUE constraint in Oracle.
• Use Case: This type of index is automatically created when a PRIMARY KEY or UNIQUE constraint is defined on a table.
• Example:

o   CREATE UNIQUE INDEX idx_unique_emp_id ON employees(emp_id);

4. Clustered Index
• Structure: In Oracle, data in a clustered index is physically stored in the same order as the index. However, Oracle’s default indexing mechanism does not support clustered indexing in the same way SQL Server does.
• Use Case: While Oracle doesn’t natively support clustered indexes, you can use index-organized tables (IOTs), where the table itself is stored in the index structure.
• Example:

o   CREATE TABLE employees

o   (emp_id NUMBER PRIMARY KEY, name VARCHAR2(100), age NUMBER)

o   ORGANIZATION INDEX;

5. Composite Index (Concatenated Index)
• Structure: An index on multiple columns. It can be useful when queries involve conditions on multiple columns.
• Use Case: The order of columns in a composite index is important. It speeds up queries that involve conditions on the leading (first) column or the leading columns in combination.
• Example:

o   CREATE INDEX idx_emp_name_age ON employees(name, age);

6. Reverse Key Index
• Structure: A reverse key index stores the bytes of the indexed column in reverse order. This helps distribute inserts more evenly across the index, reducing contention for index blocks.
• Use Case: Used for columns that contain monotonically increasing values, such as dates or sequential numbers.
• Example:

o   CREATE INDEX idx_rev_emp_id ON employees(emp_id) REVERSE;

7. Domain Index
• Structure: These are specialized indexes for custom data types, such as those used in spatial databases or full-text searches.
• Use Case: Used with application-specific data types (e.g., XML data, spatial data).
• Example: This index type is used with custom indexing methods such as CTXSYS.CONTEXT for text searching or SDO_GEOMETRY for spatial queries.
8. Function-Based Index
• Structure: These indexes are created on expressions or functions rather than directly on columns. They can improve query performance when queries frequently use expressions or functions on columns.
• Use Case: For example, creating an index on the result of a UPPER function for case-insensitive searches.
• Example:

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

Index Management

1. Index Creation:
• Indexes can be created using the CREATE INDEX statement.
• The ON clause specifies the columns to index.

Example:

CREATE INDEX idx_emp_salary ON employees(salary);

2. Index Maintenance:
• Rebuilding: Over time, indexes may become fragmented and inefficient due to frequent DML operations (inserts, deletes, and updates). The ALTER INDEX REBUILD command is used to rebuild indexes to improve performance.
• Example:

o   ALTER INDEX idx_emp_salary REBUILD;

• Coalescing: The ALTER INDEX COALESCE command is used to defragment an index without fully rebuilding it.
2. Dropping an Index:
• Indexes can be dropped using the DROP INDEX statement.
• Example:

o   DROP INDEX idx_emp_salary;

4. Index Usage:
• Oracle’s query optimizer automatically decides when to use an index for a query. However, the use of an index can be influenced by table size, statistics, and the presence of more selective columns.

Example:

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

5. Index Visibility:
• Indexes are typically used for performance enhancement. However, they can have a cost on INSERT, UPDATE, and DELETE operations, since the index must be updated along with the table data.
• Cost of Indexes: Adding, updating, or deleting a row causes the index to be updated, which can impact DML performance. This is especially important with composite indexes.
6. Index Statistics:
• Index statistics help Oracle determine the effectiveness of an index. DBMS_STATS is a package that can gather statistics for both tables and indexes.
• Example:

o   EXEC DBMS_STATS.GATHER_INDEX_STATS('schema_name', 'index_name');

Index Best Practices

• Use indexes selectively: Not all queries benefit from indexes. Indexes should be used on columns that are frequently queried (especially in WHERE, JOIN, or ORDER BY clauses).
• Avoid excessive indexing: Too many indexes can slow down data modification operations, as all indexes need to be updated.
• Consider composite indexes: For queries that use multiple columns in their conditions, composite indexes can be more efficient than single-column indexes.
• Rebuild and coalesce indexes regularly: Regular maintenance of indexes ensures they remain efficient and reduce fragmentation.
• Consider index types carefully: Bitmap indexes for low cardinality columns, function-based indexes for expressions, and reverse key indexes for sequences are all important considerations.

Conclusion

Indexes in Oracle are powerful tools that can significantly improve query performance. However, like all performance enhancements, they come with trade-offs in terms of storage and maintenance overhead. Careful design and regular maintenance are key to getting the most out of Oracle indexing.