How to index a column having null values in oracle

When indexing a column with NULL values in Oracle, the best indexing method depends on how the column is used in queries. Here are the best approaches:

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1. Bitmap Index (Best for Low Cardinality Data)

If the column has few distinct values (e.g., status with values NULL, 'Active', 'Inactive'), a bitmap index is the best choice.

CREATE BITMAP INDEX idx_column_name ON table_name(column_name);

• Pros: Efficient for columns with many NULL values and low distinct values.
• Cons: Not ideal for high-concurrency DML (INSERT/UPDATE/DELETE) operations.

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2. Function-Based Index (Best for Queries Filtering NULLs)

If queries often check for NULL values (WHERE column_name IS NULL), a function-based index helps:

CREATE INDEX idx_column_nulls ON table_name(NVL(column_name, 'replacement_value'));

• Example: For a date column:

  CREATE INDEX idx_null_dates ON employees(NVL(hire_date, TO_DATE('1900-01-01', 'YYYY-MM-DD')));

• Pros: Improves queries filtering NULL values.

• Cons: Requires rewriting queries to use NVL() for index usage.

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3. Normal B-Tree Index (Best for High Cardinality Data)

By default, B-tree indexes do not store NULL values. If the column has many distinct values, use a partial index to index only non-null values:

CREATE INDEX idx_non_nulls ON table_name(column_name) WHERE column_name IS NOT NULL;

• Pros: Optimizes searches for non-null values while reducing index size.
• Cons: Does not speed up searches for NULL values.

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Which One to Use?

Index Type	Best For	Null Handling
Bitmap Index	Few distinct values	Indexes NULLs
Function-Based Index	Queries filtering NULLs	Requires NVL()
B-Tree Index (with WHERE NOT NULL)	High distinct values	Ignores NULLs