What is an Index?
An index in a database is similar to an index in a book. It provides a quick way to look up data based on the values of certain columns. Without an index, the database would have to scan every row in a table to find the rows that meet a certain condition, which can be slow for large tables.
How Indexes Work:
- When a query is executed, the database engine first checks if there is an index that can be used to satisfy the query.
- If an index exists, the database engine uses it to quickly locate the rows that meet the query conditions.
- Without an index, the database engine must scan the entire table to find the relevant rows.
Considerations:
- Indexes consume storage space, so it’s important to balance the performance benefits with the storage overhead.
- Indexes need to be maintained whenever data in the indexed columns is inserted, updated, or deleted, which can impact write performance.
- Regularly monitor and optimize indexes to ensure they are providing the expected performance benefits.
Example
let’s go through an example of creating an index in a PostgreSQL database. Suppose you have a table named employees with columns employee_id, first_name, last_name, department, and salary. You often need to search for employees by their last_name, so creating an index on this column could improve query performance.
Here’s how you can create an index on the last_name column:
CREATE INDEX idx_last_name ON employees (last_name);
This statement creates an index named idx_last_name on the last_name column of the employees table.
Now, let’s say you frequently run queries to find employees by their last name:
SELECT * FROM employees WHERE last_name = ‘Smith’;
With the index in place, PostgreSQL can use it to quickly locate the rows where last_name is 'Smith', resulting in faster query execution.
Remember that while indexes can improve query performance, they also come with overhead during data modifications (like inserts, updates, and deletes) because PostgreSQL needs to update the index as well. So, it’s essential to consider the trade-offs and create indexes judiciously based on the queries your application frequently executes.
Check the list of indexes
To check the list of indexes in a PostgreSQL database, you can query the pg_indexes system catalog view. Here’s an example SQL query to retrieve the list of indexes in the current database:
SELECT schemaname, tablename, indexname
FROM pg_indexes
WHERE schemaname NOT LIKE ‘pg_%’ AND schemaname != ‘information_schema’;
This query will return the schema name, table name, and index name for each index in the database. The pg_indexes view filters out system indexes by excluding schema names that start with ‘pg_’ and the ‘information_schema’ schema.