Postgres Hash Index

In relational databases, indexing refers to a technique of enhancing the performance of a database by creating a data structure that facilitates efficient data retrieval. Think of it as a separate structure associated with a given table that allows faster data lookups and return instead of searching the entire table.

PostgreSQL supports a wide array of index types, each with its advantages and disadvantages. Various index types that are supported in PostgreSQL include B-Tree, GiST, SP-GiST, BRIN, HASH, and more.

In this tutorial, we will learn how to work with a Hash index type in PostgreSQL and the various operations that we can perform on the Hash index.

PostgreSQL Hash Index

A Hash index is a table index that uses a Hash function to locate the rows in a table quickly. It is a handy tool for equality-based queries and performs well when searching for exact matches.

Despite their advantages, Hash indexes are most effective in a database where the data is evenly distributed.

PostgreSQL Create Hash Index

We can create a new Hash index in PostgreSQL using the CREATE INDEX clause. The syntax is as follows:

CREATE INDEX index_name ON table_name USING hash (column_name);

Replace the index_name with the desired name that you wish to assign to the new index. For example, replace the table_name with your target table and the column_name with the column that you wish to index.

Example Demonstration:
Suppose we have access to the “employees” table and we wish to create a hash index on the employee_id column. Then, we can run a query as follows:

CREATE INDEX hash_index_employee_id ON employees USING hash (employee_id);

Using PostgreSQL Hash Index for Query Optimization

PostgreSQL automatically uses the Hash index to determine the most efficient choice for a query. However, we can provide a hint to use the Hash index by specifying it in a query using the /*+ Index(index_name) */ syntax.

For example, suppose we wish to use the Hash index hash_index_employee_id in a query. We can run the statement as shown in the following:

SELECT /*+ Index(hash_index_employee_id) */ * FROM employees WHERE employee_id = 12345;

Drop the Hash Index

We can use the DROP INDEX statement to remove an existing Hash index. The syntax is as follows:

DROP INDEX index_name;

For example:

DROP INDEX hash_index_employee_id;

Rename the Hash Index

We can use the ALTER INDEX statement to rename an existing index to a new name. The syntax is as follows:

ALTER INDEX index_name RENAME TO new_index_name;

Example:

ALTER INDEX hash_index_employee_id RENAME TO new_hash_index_employee_id;

There you have it! A method of creating and working with hash indexes in PostgreSQL.

Conclusion

You learned how to create and work with Hash indexes in PostgreSQL. However, it is good to remember that Hash indexes are not effective or suitable for working with range-based or inequality-based queries. Similarly, keep in mind that Hash indexes can consume a significant amount of memory, so ensure that you have enough resources available.