Creating Indexes in IBM DB2 | Generated by AI
Creating Indexes in IBM DB2
IBM DB2 supports indexes to optimize query performance by allowing faster data retrieval. Indexes can be created on one or more columns of a table. Below, I’ll cover the basics, including unique indexes and multi-column (composite) indexes. Note that “union index” isn’t a standard term in DB2 documentation— it might refer to a composite index (covering multiple keys) or a misunderstanding of union operations in queries. If you meant something else, provide more details!
Basic Index Creation
Use the CREATE INDEX statement to build a simple index on a single column. This speeds up searches, sorts, and joins on that column.
Syntax:
CREATE INDEX index_name
ON table_name (column_name [ASC | DESC]);
Example:
CREATE INDEX idx_employee_id
ON employees (employee_id ASC);
ASCsorts in ascending order (default); useDESCfor descending.- Indexes are non-unique by default, allowing duplicate values.
Unique Index (Enforcing Unique Keys)
A unique index ensures no duplicate values in the indexed column(s), similar to a unique constraint. DB2 automatically creates a unique index when you define a primary key or unique constraint.
Syntax:
CREATE UNIQUE INDEX index_name
ON table_name (column_name [ASC | DESC]);
Example:
CREATE UNIQUE INDEX uidx_email
ON users (email ASC);
- This prevents inserting duplicate emails.
- For partial uniqueness (e.g., ignore NULLs), add
WHERE NOT NULL:CREATE UNIQUE WHERE NOT NULL INDEX uidx_email ON users (email ASC); - You can include non-key columns for query coverage:
CREATE UNIQUE INDEX uidx_email ON users (email ASC) INCLUDE (first_name, last_name);
Composite Index (Multiple Keys, e.g., Two or More Columns)
A composite (or multi-column) index covers multiple keys, useful for queries filtering/joining on combinations of columns. The order of columns matters—place the most selective column first.
Syntax:
CREATE INDEX index_name
ON table_name (column1 [ASC | DESC], column2 [ASC | DESC], ...);
Example (Two Keys):
CREATE INDEX idx_order_date_customer
ON orders (order_date ASC, customer_id DESC);
- This optimizes queries like
SELECT * FROM orders WHERE order_date > '2023-01-01' AND customer_id = 123;. - For unique composite: Add
UNIQUE(e.g.,CREATE UNIQUE INDEX ...).
Additional Tips
- Verify Index: Use
SELECT * FROM SYSCAT.INDEXES WHERE TABSCHEMA = 'YOUR_SCHEMA' AND TABNAME = 'YOUR_TABLE';. - Drop Index:
DROP INDEX index_name;. - Indexes consume storage and slow inserts/updates—use them judiciously.
- For partitioned tables or advanced options (e.g., clustering), refer to DB2 docs.
Db2 CREATE INDEX Statement
Db2 Unique Indexes
Db2 Composite Indexes Example
Db2 Unique Index Tutorial