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Columnstore indexes in data warehousing

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Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance Analytics Platform System (PDW)

Columnstore indexes, in conjunction with partitioning, are essential for building a SQL Server data warehouse. This article focuses on key use cases and examples for data warehousing designs with the SQL Database Engine.

Key features for data warehousing

SQL Server 2016 (13.x) introduced these features for columnstore performance enhancements:

  • Always On supports querying a columnstore index on a readable secondary replica.
  • Multiple Active Result Sets (MARS) supports columnstore indexes.
  • A new dynamic management view sys.dm_db_column_store_row_group_physical_stats (Transact-SQL) provides performance troubleshooting information at the row group level.
  • Single-threaded queries on columnstore indexes can run in batch mode. Previously, only multi-threaded queries could run in batch mode.
  • The SORT operator runs in batch mode.
  • Multiple DISTINCT operations run in batch mode.
  • Window Aggregates now runs in batch mode for database compatibility level 130 and higher.
  • Aggregate Pushdown for efficient processing of aggregates. This is supported on all database compatibility levels.
  • String predicate pushdown for efficient processing of string predicates. This is supported on all database compatibility levels.
  • Snapshot isolation for database compatibility level 130 and higher.
  • Ordered cluster columnstore indexes were introduced with SQL Server 2022 (16.x). For more information, see CREATE COLUMNSTORE INDEX and Performance tuning with ordered clustered columnstore indexes. For ordered columnstore index availability, see Ordered column index availability.

For more information about new features in versions and platforms of SQL Server and Azure SQL, see What's new in columnstore indexes.

Improve performance by combining nonclustered and columnstore indexes

Starting with SQL Server 2016 (13.x), you can define rowstore nonclustered indexes on a clustered columnstore index.

Example: Improve efficiency of table seeks with a nonclustered index

To improve efficiency of table seeks in a data warehouse, you can create a nonclustered index designed to run queries that perform best with table seeks. For example, queries that look for matching values or return a small range of values perform better against a B-tree index rather than a columnstore index. They don't require a full table scan through the columnstore index and return the correct result faster by doing a binary search through a B-tree index.

--BASIC EXAMPLE: Create a nonclustered index on a columnstore table.  
  
--Create the table  
CREATE TABLE t_account (  
    AccountKey int NOT NULL,  
    AccountDescription nvarchar (50),  
    AccountType nvarchar(50),  
    UnitSold int  
);  
GO  
  
--Store the table as a columnstore.  
CREATE CLUSTERED COLUMNSTORE INDEX taccount_cci ON t_account;  
GO  
  
--Add a nonclustered index.  
CREATE UNIQUE INDEX taccount_nc1 ON t_account (AccountKey);

Example: Use a nonclustered index to enforce a primary key constraint on a columnstore table

By design, a columnstore table doesn't allow a clustered primary key constraint. Now you can use a nonclustered index on a columnstore table to enforce a primary key constraint. A primary key is equivalent to a UNIQUE constraint on a non-NULL column, and SQL Server implements a UNIQUE constraint as a nonclustered index. Combining these facts, the following example defines a UNIQUE constraint on the non-NULL column accountkey. The result is a nonclustered index that enforces a primary key constraint as a UNIQUE constraint on a non-NULL column.

Next, the table is converted to a clustered columnstore index. During the conversion, the nonclustered index persists. The result is a clustered columnstore index with a nonclustered index that enforces a primary key constraint. Since any update or insert on the columnstore table also affects the nonclustered index, all operations that violate the unique constraint and the non-NULL causes the entire operation to fail.

The result is a columnstore index with a nonclustered index that enforces a primary key constraint on both indexes.

--EXAMPLE: Enforce a primary key constraint on a columnstore table.   
  
--Create a rowstore table with a unique constraint.  
--The unique constraint is implemented as a nonclustered index.  
CREATE TABLE t_account (  
    AccountKey int NOT NULL,  
    AccountDescription nvarchar (50),  
    AccountType nvarchar(50),  
    UnitSold int,  
  
    CONSTRAINT uniq_account UNIQUE (AccountKey)  
);  
  
--Store the table as a columnstore.   
--The unique constraint is preserved as a nonclustered index on the columnstore table.  
CREATE CLUSTERED COLUMNSTORE INDEX t_account_cci ON t_account  
  
--By using the previous two steps, every row in the table meets the UNIQUE constraint  
--on a non-NULL column.  
--This has the same end-result as having a primary key constraint  
--All updates and inserts must meet the unique constraint on the nonclustered index or they will fail.  
  
--If desired, add a foreign key constraint on AccountKey.  
  
ALTER TABLE [dbo].[t_account]  
WITH CHECK ADD FOREIGN KEY([AccountKey]) REFERENCES my_dimension(Accountkey);

Improve performance by enabling row-level and row-group-level locking

To complement the nonclustered index on a columnstore index feature, SQL Server 2016 (13.x) offers granular locking capability for select, update, and delete operations. Queries can run with row-level locking on index seeks against a nonclustered index and rowgroup-level locking on full table scans against the columnstore index. Use this to achieve higher read/write concurrency by using row-level and rowgroup-level locking appropriately.

--Granular locking example  
--Store table t_account as a columnstore table.  
CREATE CLUSTERED COLUMNSTORE INDEX taccount_cci ON t_account  
GO  
  
--Add a nonclustered index for use with this example  
CREATE UNIQUE INDEX taccount_nc1 ON t_account (AccountKey);  
GO  
  
--Look at locking with access through the nonclustered index  
SET TRANSACTION ISOLATION LEVEL repeatable read;  
GO  
  
BEGIN TRAN  
    -- The query plan chooses a seek operation on the nonclustered index  
    -- and takes the row lock  
    SELECT * FROM t_account WHERE AccountKey = 100;  
COMMIT TRAN

Snapshot isolation and read-committed snapshot isolations

Use snapshot isolation (SI) to guarantee transactional consistency, and read-committed snapshot isolations (RCSI) to guarantee statement level consistency for queries on columnstore indexes. This allows the queries to run without blocking data writers. This non-blocking behavior also significantly reduces the likelihood of deadlocks for complex transactions. For more information, see Snapshot Isolation in SQL Server.

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