Columnstore indexes - Data Warehouse
Columnstore indexes, in conjunction with partitioning, are essential for building a SQL Server data warehouse.
SQL Server 2016 (13.x) introduces 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.
SORToperator runs in batch mode.
DISTINCToperation runs 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.
Improve performance by combining nonclustered and columnstore indexes
Starting with SQL Server 2016 (13.x), you can define 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 will 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 will 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 does not allow a 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 will also affect the nonclustered index, all operations that violate the unique constraint and the non-NULL will cause 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; END 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 on MSDN.
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