Tuesday 22 September 2020

bitmap and b-tree indexes

In this article, we'll explain bitmap and b-tree indexes along with an example and corresponding commands for creating each type of index. 

B-Tree Index:

  • B-tree indexes are widely used in database systems, including Oracle. The "B" in B-tree stands for balanced, indicating that these indexes maintain a balanced tree structure.
  • B-tree indexes are most suitable for columns with high cardinality, meaning they have many distinct values. Examples of such columns include primary keys, unique identifiers, and highly selective columns.
  • The structure of a B-tree index consists of multiple levels, starting with a root node and extending to leaf nodes. Leaf nodes contain index key values and pointers to the corresponding rows in the table.
  • B-tree indexes excel in supporting range searches and equality conditions. They efficiently find a specific value or a range of values within the index, making them beneficial for queries involving comparisons such as greater than, less than, or between.
  • Updates and inserts on B-tree indexes can be efficient due to the balanced structure, as the index tree does not require significant restructuring during modifications.
  • B-tree indexes work well for queries that retrieve a small percentage of rows from a table, as they provide efficient navigation to the desired data.

Example: Let's assume we have a table called employees with columns employee_id, first_name, last_name, and we want to create a B-tree index on the last_name column.
Command to create a B-tree index:
CREATE INDEX idx_employees_last_name ON employees(last_name);

Bitmap Index:

  • Bitmap indexes are suitable for columns with low cardinality, meaning they have few distinct values. Examples include boolean fields or columns representing categories or flags.
  • Bitmap indexes create a bitmap for each distinct value in the indexed column. A bitmap is a sequence of bits, where each bit represents a row in the table. The bits are set to 1 if the corresponding row contains the indexed value and 0 otherwise.
  • Bitmap indexes are efficient for queries involving multiple conditions combined with logical AND or OR operations. They can quickly determine which rows satisfy complex combinations of conditions by performing bitmap operations like AND, OR, and NOT on the bitmaps.
  • Bitmap indexes require less storage space compared to B-tree indexes for columns with low cardinality since they represent the presence or absence of a value rather than storing individual pointers.
  • Updates and inserts on bitmap indexes can be relatively expensive, especially if the index covers many rows. Modifying a single row may require updating multiple bitmaps.

Example: Let's consider a table called orders with columns order_id, product_name, order_date, and we want to create a bitmap index on the product_name column.

Command to create a Bitmap index:
CREATE BITMAP INDEX idx_orders_product_name ON orders(product_name);

B-tree indexes are well-suited for high cardinality columns and queries involving range searches and equality conditions. Bitmap indexes are effective for low cardinality columns and queries with complex combinations of conditions. It's essential to analyze your data and query patterns to determine the most appropriate index type for your specific use case.



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