Non-clustered index part 2 ( hash index)
Tags: postgres, postgressql, apacheage
Published: 2023-07-04 13:20:21

In PostgreSQL, a hash index is a type of index structure that is used to speed up data retrieval operations based on equality conditions. It is an alternative to the more common B-tree index. While B-tree indexes are generally effective for range queries and comparison operations, hash indexes are specifically optimized for equality searches.

## Here are the key details about hash indexes in PostgreSQL:

**1.Index Structure:** A hash index is based on a hash table, which is a data structure that allows for efficient key-value pair lookups. The hash table is constructed using a hash function that maps keys to specific locations within the table.

**2.Hash Function:** PostgreSQL uses a built-in hash function to determine the hash value for each indexed column value. The hash function calculates a hash code, which is an integer value derived from the column value. The hash code is used to determine the location within the hash table where the key-value pair should be stored.

**3.Equality Searches:** Hash indexes excel at performing equality searches. When you query a table using a column that has a hash index, PostgreSQL applies the hash function to the search value and quickly locates the corresponding entry in the hash table. This makes hash indexes ideal for queries using the equality operator `=`.

**4.No Sorting:** Unlike B-tree indexes, hash indexes do not sort the indexed values. The hash function directly determines the location where the data is stored in the hash table. As a result, hash indexes are not effective for range queries or ordering operations.

**5.Index Creation:** To create a hash index in PostgreSQL, you can use the CREATE INDEX statement with the HASH keyword. For example:

```
CREATE INDEX idx_hash ON table_name USING hash (column_name);

```
**6.Performance Considerations:** Hash indexes can provide very fast lookups for equality queries. However, they have some limitations. Hash indexes can be larger in size compared to B-tree indexes, especially if the indexed column has a high number of duplicate values. Additionally, hash indexes can become less efficient if the hash table becomes too large, resulting in more collisions (multiple keys mapping to the same hash value).

**7.Maintenance Overhead:** Hash indexes require additional maintenance compared to B-tree indexes. Whenever a table is updated (e.g., an insertion, deletion, or update), the hash index needs to be updated as well. This maintenance overhead can impact the performance of data modification operations.

**8.Limitations:** Hash indexes in PostgreSQL have some limitations. They do not support partial indexes, expression indexes, or multicolumn indexes. Furthermore, hash indexes are not crash-safe, meaning they might need to be rebuilt after a system failure.

**9.Choosing the Right Index:** The decision to use a hash index or a B-tree index depends on the nature of your data and the types of queries you perform. If you primarily perform equality searches and the data has low cardinality (few distinct values), a hash index may be more beneficial. However, if you need to perform range queries, sorting, or handle high-cardinality data, a B-tree index is usually a better choice.

It's worth noting that hash indexes in PostgreSQL have seen limited use in recent versions due to various limitations and performance concerns. B-tree indexes are the default choice for most use cases, but hash indexes can still be useful in specific scenarios where they align with the requirements of the workload.