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sp_estimate_data_compression_savings (Transact-SQL)

  • Artikkeli

Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance

Returns the current size of the requested object and estimates the object size for the requested compression state. Compression can be evaluated for whole tables or parts of tables. This includes heaps, clustered indexes, nonclustered indexes, columnstore indexes, indexed views, and table and index partitions. The objects can be compressed by using row, page, columnstore, or columnstore archive compression. If the table, index, or partition is already compressed, you can use this procedure to estimate the size of the table, index, or partition if it's recompressed or stored without compression.

The sys.sp_estimate_data_compression_savings system stored procedure is available in Azure SQL Database and Azure SQL Managed Instance.

Starting with SQL Server 2022 (16.x), you can compress off-row XML data in columns using the xml data type, reducing storage and memory requirements. For more information, see CREATE TABLE and CREATE INDEX. sp_estimate_data_compression_savings supports XML compression estimates.

Note

Compression and sp_estimate_data_compression_savings aren't available in every edition of SQL Server. For a list of features that are supported by the editions of SQL Server, see Editions and supported features of SQL Server 2022.

To estimate the size of the object if it were to use the requested compression setting, this stored procedure samples the source object and loads this data into an equivalent table and index created in tempdb. The table or index created in tempdb is then compressed to the requested setting and the estimated compression savings is computed.

To change the compression state of a table, index, or partition, use the ALTER TABLE or ALTER INDEX statements. For general information about compression, see Data compression.

Note

If the existing data is fragmented, you might be able to reduce its size without using compression by rebuilding the index. For indexes, the fill factor will be applied during an index rebuild. This could increase the size of the index.

Transact-SQL syntax conventions

Syntax

sp_estimate_data_compression_savings
    [ @schema_name = ] N'schema_name'
    , [ @object_name = ] N'object_name'
    , [ @index_id = ] index_id
    , [ @partition_number = ] partition_number
    , [ @data_compression = ] N'data_compression'
    [ , [ @xml_compression = ] xml_compression ]
[ ; ]

Arguments

[ @schema_name = ] N'schema_name'

The name of the database schema that contains the table or indexed view. @schema_name is sysname, with no default. If @schema_name is NULL, the default schema of the current user is used.

[ @object_name = ] N'object_name'

The name of the table or indexed view that the index is on. @object_name is sysname, with no default.

[ @index_id = ] index_id

The ID of the index. @index_id is int, and can be one of the following values:

  • the ID number of an index
  • NULL
  • 0 if object_id is a heap

To return information for all indexes for a base table or view, specify NULL. If you specify NULL, you must also specify NULL for @partition_number.

[ @partition_number = ] partition_number

The partition number in the object. @partition_number is int, and can be one of the following values:

  • the partition number of an index or heap
  • NULL
  • 1 for a nonpartitioned index or heap

To specify the partition, you can also specify the $PARTITION function. To return information for all partitions of the owning object, specify NULL.

[ @data_compression = ] N'data_compression'

Specifies the type of compression to be evaluated. @data_compression is nvarchar(60), and can be one of the following values:

  • NONE
  • ROW
  • PAGE
  • COLUMNSTORE
  • COLUMNSTORE_ARCHIVE

For SQL Server 2022 (16.x) and later versions, NULL is also a possible value. @data_compression can't be NULL if @xml_compression is NULL.

[ @xml_compression = ] xml_compression

Applies to: SQL Server 2022 (16.x) and later versions, Azure SQL Database, and Azure SQL Managed Instance

Specifies whether to calculate savings for XML compression. @xml_compression is bit, and can be one of the following values:

  • NULL (default)
  • 0
  • 1

@xml_compression can't be NULL if @data_compression is NULL.

Return code values

0 (success) or 1 (failure).

Result set

The following result set is returned to provide current and estimated size for the table, index, or partition.

Column name Data type Description
object_name sysname Name of the table or the indexed view.
schema_name sysname Schema of the table or indexed view.
index_id int Index ID of an index:

0 = Heap
1 = Clustered index
>1 = Nonclustered index
partition_number int Partition number. Returns 1 for a nonpartitioned table or index.
size_with_current_compression_setting (KB) bigint Size of the requested table, index, or partition as it currently exists.
size_with_requested_compression_setting (KB) bigint Estimated size of the table, index, or partition that uses the requested compression setting; and, if applicable, the existing fill factor, and assuming there's no fragmentation.
sample_size_with_current_compression_setting (KB) bigint Size of the sample with the current compression setting. This size includes any fragmentation.
sample_size_with_requested_compression_setting (KB) bigint Size of the sample that is created by using the requested compression setting; and, if applicable, the existing fill factor and no fragmentation.

Remarks

Use sp_estimate_data_compression_savings to estimate the savings that can occur when you enable a table or partition for row, page, columnstore, columnstore archive, or XML compression. For instance, if the average size of the row can be reduced by 40 percent, you can potentially reduce the size of the object by 40 percent. You might not receive a space savings because this depends on the fill factor and the size of the row. For example, if you have a row that is 8,000 bytes long and you reduce its size by 40 percent, you can still fit only one row on a data page. There are no savings.

If the results of running sp_estimate_data_compression_savings on an uncompressed table or index indicate that the size will increase, this means that many rows use almost the whole precision of the data types, and the addition of the small overhead needed for the compressed format is more than the savings from compression. In this rare case, don't enable compression.

If a table is already enabled for compression, you can use sp_estimate_data_compression_savings to estimate the average size of the row if the table is uncompressed.

An intent shared (IS) lock is acquired on the table during this operation. If an IS lock can't be obtained, the procedure is blocked. The table is scanned under the default read committed isolation level.

If the requested compression setting is same as the current compression setting, the stored procedure returns the estimated size with no data fragmentation, using the existing fill factor for indexes on the source object.

If the index or partition ID doesn't exist, no results are returned.

Permissions

Requires SELECT permission on the table, VIEW DATABASE STATE and VIEW DEFINITION on the database containing the table and on tempdb.

Limitations

In SQL Server 2017 (14.x) and earlier versions, this procedure didn't apply to columnstore indexes, and therefore didn't accept the data compression parameters COLUMNSTORE and COLUMNSTORE_ARCHIVE. In SQL Server 2019 (15.x) and later versions, and in Azure SQL Database and Azure SQL Managed Instance, columnstore indexes can be used both as a source object for estimation, and as a requested compression type.

When Memory-Optimized TempDB Metadata is enabled, creation of columnstore indexes on temporary tables isn't supported. Because of this limitation, sp_estimate_data_compression_savings isn't supported with the COLUMNSTORE and COLUMNSTORE_ARCHIVE data compression parameters when Memory-Optimized TempDB Metadata is enabled.

Considerations for columnstore indexes

Starting with SQL Server 2019 (15.x), and in Azure SQL Database and Azure SQL Managed Instance, sp_estimate_compression_savings supports estimating both columnstore and columnstore archive compression. Unlike page and row compression, applying columnstore compression to an object requires creating a new columnstore index. For this reason, when using the COLUMNSTORE and COLUMNSTORE_ARCHIVE options of this procedure, the type of the source object provided to the procedure determines the type of columnstore index used for the compressed size estimate. The following table illustrates the reference objects used to estimate compression savings for each source object type when the @data_compression parameter is set to either COLUMNSTORE or COLUMNSTORE_ARCHIVE.

Source object Reference object
**Heap Clustered columnstore index
Clustered index Clustered columnstore index
Nonclustered index Nonclustered columnstore index (including the key columns and any included columns of the provided nonclustered index, and the partition column of the table, if any)
Nonclustered columnstore index Nonclustered columnstore index (including the same columns as the provided nonclustered columnstore index)
Clustered columnstore index Clustered columnstore index

Note

When estimating columnstore compression from a rowstore source object (clustered index, nonclustered index or heap), if there are any columns in the source object that have a data type that isn't supported in a columnstore index, sp_estimate_compression_savings will fail with an error.

Similarly, when the @data_compression parameter is set to NONE, ROW, or PAGE and the source object is a columnstore index, the following table outlines the reference objects used.

Source object Reference object
Clustered columnstore index Heap
Nonclustered columnstore index Nonclustered index (including the columns contained in the nonclustered columnstore index as key columns, and the partition column of the table, if any, as an included column)

Note

When estimating rowstore compression (NONE, ROW or PAGE) from a columnstore source object, be sure that the source index doesn't contain more than 32 key columns as this is the limit supported in a rowstore (nonclustered) index.

Examples

The Transact-SQL code samples in this article use the AdventureWorks2022 or AdventureWorksDW2022 sample database, which you can download from the Microsoft SQL Server Samples and Community Projects home page.

A. Estimate savings with ROW compression

The following example estimates the size of the Production.WorkOrderRouting table if it's compressed by using ROW compression.

EXEC sys.sp_estimate_data_compression_savings
     'Production', 'WorkOrderRouting', NULL, NULL, 'ROW';
GO

B. Estimate savings with PAGE and XML compression

Applies to: SQL Server 2022 (16.x) and later versions

The following example estimates the size of the Production.ProductModel table if it's compressed by using PAGE compression, and the @xml_compression value is enabled.

EXEC sys.sp_estimate_data_compression_savings
     'Production', 'ProductModel', NULL, NULL, 'PAGE', 1;
GO

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