sys.column_store_row_groups (Transact-SQL)

Article
02/28/2023

Applies to: SQL Server 2012 (11.x) and later Azure SQL Managed Instance

Provides clustered columnstore index information on a per-segment basis to help the administrator make system management decisions. sys.column_store_row_groups has a column for the total number of rows physically stored (including those marked as deleted) and a column for the number of rows marked as deleted. Use sys.column_store_row_groups to determine which row groups have a high percentage of deleted rows and should be rebuilt.

Column name	Data type	Description
object_id	int	The id of the table on which this index is defined.
index_id	int	ID of the index for the table that has this columnstore index.
partition_number	int	ID of the table partition that holds row group row_group_id. You can use partition_number to join this DMV to sys.partitions.
row_group_id	int	The row group number associated with this row group. This is unique within the partition. -1 = tail of an in-memory table.
delta_store_hobt_id	bigint	The hobt_id for OPEN row group in the delta store. NULL if the row group is not in the delta store. NULL for the tail of an in-memory table.
state	tinyint	ID number associated with the state_description. 0 = INVISIBLE 1 = OPEN 2 = CLOSED 3 = COMPRESSED 4 = TOMBSTONE
state_description	nvarchar(60)	Description of the persistent state of the row group: INVISIBLE -A hidden compressed segment in the process of being built from data in a delta store. Read actions will use the delta store until the invisible compressed segment is completed. Then the new segment is made visible, and the source delta store is removed. OPEN - A read/write row group that is accepting new records. An open row group is still in rowstore format and has not been compressed to columnstore format. CLOSED - A row group that has been filled, but not yet compressed by the tuple mover process. COMPRESSED - A row group that has filled and compressed.
total_rows	bigint	Total rows physically stored in the row group. Some may have been deleted but they are still stored. The maximum number of rows in a row group is 1,048,576 (hexadecimal FFFFF).
deleted_rows	bigint	Total rows in the row group marked deleted. This is always 0 for DELTA row groups.
size_in_bytes	bigint	Size in bytes of all the data in this row group (not including metadata or shared dictionaries), for both DELTA and COLUMNSTORE rowgroups.

Remarks

Returns one row for each columnstore row group for each table having a clustered or nonclustered columnstore index.

Use sys.column_store_row_groups to determine the number of rows included in the row group and the size of the row group.

When the number of deleted rows in a row group grows to a large percentage of the total rows, the table becomes less efficient. Rebuild the columnstore index to reduce the size of the table, reducing the disk I/O required to read the table. To rebuild the columnstore index use the REBUILD option of the ALTER INDEX statement.

The updateable columnstore first inserts new data into an OPEN rowgroup, which is in rowstore format, and is also sometimes referred to as a delta table. Once an open rowgroup is full, its state changes to CLOSED. A closed rowgroup is compressed into columnstore format by the tuple mover and the state changes to COMPRESSED. The tuple mover is a background process that periodically wakes up and checks whether there are any closed rowgroups that are ready to compress into a columnstore rowgroup. The tuple mover also deallocates any rowgroups in which every row has been deleted. Deallocated rowgroups are marked as TOMBSTONE. To run tuple mover immediately, use the REORGANIZE option of the ALTER INDEX statement.

When a columnstore row group has filled, it is compressed, and stops accepting new rows. When rows are deleted from a compressed group, they remain but are marked as deleted. Updates to a compressed group are implemented as a delete from the compressed group, and an insert to an open group.

Permissions

Returns information for a table if the user has VIEW DEFINITION permission on the table.

The visibility of the metadata in catalog views is limited to securables that a user either owns, or on which the user was granted some permission. For more information, see Metadata Visibility Configuration.

Examples

The following example joins the sys.column_store_row_groups table to other system tables to return information about specific tables. The calculated PercentFull column is an estimate of the efficiency of the row group. To find information on a single table remove the comment hyphens in front of the WHERE clause and provide a table name.

SELECT i.object_id, object_name(i.object_id) AS TableName,   
i.name AS IndexName, i.index_id, i.type_desc,   
CSRowGroups.*,   
100*(total_rows - ISNULL(deleted_rows,0))/total_rows AS PercentFull    
FROM sys.indexes AS i  
JOIN sys.column_store_row_groups AS CSRowGroups  
    ON i.object_id = CSRowGroups.object_id  
AND i.index_id = CSRowGroups.index_id   
--WHERE object_name(i.object_id) = '<table_name>'   
ORDER BY object_name(i.object_id), i.name, row_group_id;

sys.column_store_row_groups (Transact-SQL)

Remarks

Permissions

Examples

See also

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