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sys.column_store_segments (Transact-SQL)

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

Returns one row for each column segment in a columnstore index. There is one column segment per column per rowgroup. For example, a table with 10 rowgroups and 34 columns returns 340 rows.

Column name Data type Description
partition_id bigint Indicates the partition ID. Is unique within a database.
hobt_id bigint ID of the heap or B-tree index (HoBT) for the table that has this columnstore index.
column_id int ID of the columnstore column.
segment_id int ID of the rowgroup. For backward compatibility, the column name continues to be called segment_id even though this is the rowgroup ID. You can uniquely identify a segment using <hobt_id, partition_id, column_id>, <segment_id>.
version int Version of the column segment format.
encoding_type int Type of encoding used for that segment:

1 = VALUE_BASED - non-string/binary with no dictionary (similar to 4 with some internal variations)

2 = VALUE_HASH_BASED - non-string/binary column with common values in dictionary

3 = STRING_HASH_BASED - string/binary column with common values in dictionary

4 = STORE_BY_VALUE_BASED - non-string/binary with no dictionary

5 = STRING_STORE_BY_VALUE_BASED - string/binary with no dictionary

For more information, see the Remarks section.
row_count int Number of rows in the row group.
has_nulls int 1 if the column segment has null values.
base_id bigint Base value ID if encoding type 1 is being used. If encoding type 1 is not being used, base_id is set to -1.
magnitude float Magnitude if encoding type 1 is being used. If encoding type 1 is not being used, magnitude is set to -1.
primary_dictionary_id int A value of 0 represents the global dictionary. A value of -1 indicates that there is no global dictionary created for this column.
secondary_dictionary_id int A non-zero value points to the local dictionary for this column in the current segment (for example, the rowgroup). A value of -1 indicates that there is no local dictionary for this segment.
min_data_id bigint Minimum data ID in the column segment.
max_data_id bigint Maximum data ID in the column segment.
null_value bigint Value used to represent nulls.
on_disk_size bigint Size of segment in bytes.
collation_id int Applies to SQL Server 2022 (16.x) and later.
Current collation when the segment was created. Maps to an internal ID. Currently internal only and not for development.
min_deep_data varbinary(18) Applies to SQL Server 2022 (16.x) and later.
Used for segment elimination.1 For internal use only.
max_deep_data varbinary(18) Applies to SQL Server 2022 (16.x) and later.
Used for segment elimination.1 For internal use only.

1 After upgrading to a version of SQL Server that supports string min/max segment elimination (SQL Server 2022 (16.x) and later), min_deep_data and max_deep_data will be NULL until after the columnstore index is rebuilt, using a REBUILD or DROP/CREATE. After a rebuild, the segments that contain data types that can benefit from string min/max segment elimination will contain data.

Remarks

The columnstore segment encoding type is selected by the Database Engine with the goal of achieving the lowest storage cost, by analyzing the segment data. If data is mostly distinct, the Database Engine uses value-based encoding. If data is mostly not distinct, the Database Engine uses hash-based encoding. The choice between string-based and value-based encoding is related to the type of data being stored, whether string data or binary data. All encodings take advantage of bit-packing and run-length encoding when possible.

Columnstore segment elimination applies to numeric, date, and time data types, and the datetimeoffset data type with scale less than or equal to two. Beginning in SQL Server 2022 (16.x), segment elimination capabilities extend to string, binary, guid data types, and the datetimeoffset data type for scale greater than two. Segment elimination does not apply to LOB data types such as the (max) data type lengths.

Permissions

All columns require at least VIEW DEFINITION permission on the table. The following columns return NULL unless the user also has SELECT permission: has_nulls, base_id, magnitude, min_data_id, max_data_id, and null_value.

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 query returns information about segments of a columnstore index.

SELECT i.name, p.object_id, p.index_id, i.type_desc,
    COUNT(*) AS number_of_segments
FROM sys.column_store_segments AS s
INNER JOIN sys.partitions AS p
    ON s.hobt_id = p.hobt_id
INNER JOIN sys.indexes AS i
    ON p.object_id = i.object_id
WHERE i.type = 5 OR i.type = 6
GROUP BY i.name, p.object_id, p.index_id, i.type_desc ;
GO

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