Column store fragmentation in SQL server

Columnstore index fragmentation in SQL Server can affect query performance, especially when working with large datasets in data warehouses or analytical workloads. Columnstore indexes store data in a compressed format in columnar storage, and fragmentation can occur due to the way data is organized into segments and compressed.

Types of Fragmentation in Columnstore Indexes:

Ø  Fragmentation of Row groups: Columnstore indexes store data in units called rowgroups. Each rowgroups contains approximately 1 million rows. Fragmentation can occur when rowgroups contain fewer than 1 million rows, leading to inefficiencies in storage and query processing. This is typically referred to as underpopulated rowgroups or fragmented rowgroups.

Ø  Deleted Rows: Another type of fragmentation occurs when rows are marked as deleted but are not physically removed. These deleted rows increase the size of the index and degrade performance because they occupy space and are processed during queries.

Why it’s Happen

Ø  Frequent Inserts/Updates: Columnstore indexes work best in a batch load scenario where large datasets are inserted in one go. Frequent, small inserts or updates can cause rowgroups to contain fewer rows, leading to fragmentation.

Ø  Row Deletes: When rows are deleted in a columnstore index, they are not immediately removed but are marked as deleted. This increases fragmentation as those rows still take up space.

Ø  Data Loads in Small Batches: Loading data in small batches (fewer than 102,400 rows) can cause fragmentation because the rowgroups are not fully populated.

Rebuilding the Columnstore Index

Rebuilding a columnstore index will reorganize the data, compress rowgroups, and remove deleted rows. This is a heavy operation but is necessary when there is significant fragmentation.

ALTER INDEX [index_name] ON [table_name] REBUILD;

 Reorganize the Columnstore Index

Syntax of reorganize the columnstore index.

ALTER INDEX [index_name] ON [table_name] REORGANIZE;