In SQL Server, table partitions are a way to distribute data across horizontal partitions. While SQL Server doesn’t directly support hash partitions, it has a built-in function called a Hash partition that uses persisted computed columns to distribute data. This article will explain the concept of hash partitions and why they are useful in SQL Server.

Vertical and Horizontal Partitions

When it comes to partitions, there are two types: vertical and horizontal partitions. Vertical partitioning involves removing redundant columns from a table and organizing them in a separate table connected via a relationship. This helps address performance issues related to large tables with rarely accessed columns.

On the other hand, horizontal partitioning involves dividing a table into multiple tables with the same columns. The rows are then distributed among these tables based on applied rules. Hash partitions fall under the category of horizontal partitions.

What are Hash Partitions?

A hash partition is a type of horizontal partition where data is randomly distributed across the partition. It uses a hashing function to allocate new rows to a specific partition based on the partition key. The main difference between hash partitions and other horizontal partitions is the use of persisted computed columns in the index keys to handle the partitioning scheme.

Why Use Hash Partitions in SQL Server?

One of the main reasons to use hash partitions in SQL Server is to reduce latch contention, specifically the “Hot latches” issue. When multiple concurrent insertions occur, query queues are generated on the last page, leading to latch contention. Hash partitions can help alleviate this issue by distributing the data randomly across the partition.

Creating a Hash Partition

To create a hash partition in SQL Server, you need to follow a few steps. First, create a new database and add a filegroup for holding the partitions. Then, create a partition function that maps the rows of a table or index into partitions based on a specified column. Next, create a partition scheme that maps the partitions to filegroups. Finally, integrate the table object with the partition scheme by adding a persisted computed column and a unique clustered index.

Trade-offs and Benefits

There are trade-offs when using hash partitioning in SQL Server. Random insertions can lead to heavy page splitting and affect SELECT query performance. Maintaining referential integrity becomes more difficult with increased key combinations of unique clustered indexes. Additionally, the large size of the index can make maintenance challenging.

However, there are also benefits to partitioning. It allows for quick and efficient transfer or retrieval of subsets of data while maintaining data integrity. For OLTP workloads, partitioning can improve data retrieval speed. It can also improve query performance, especially for equi-join queries between partitioned tables with the same partitioning columns. Furthermore, enabling lock escalation at the partition level can reduce lock contention at the table level.

Overall, hash partitions in SQL Server provide a way to distribute data across horizontal partitions and address latch contention issues. By understanding the concept and following the necessary steps, you can effectively implement hash partitions in your SQL Server database.