Do SQL Server compressed indexes remain compressed on rebuild without specifying data compression?
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20-01-2021 - |
Pregunta
After one rebuilds their SQL Server indexes using page compression (ALTER INDEX IX1 REBUILD PARTITION = ALL WITH (DATA_COMPRESSION = PAGE)
), do subsequent rebuilds (as done by some maintenance scripts past a certain fragmentation threshold) need to specify data compression again? Would the indexes otherwise be effectively decompressed?
Solución
Indexes remain compressed when rebuilding / reorganizing them.
Create table and compressed index
CREATE TABLE DBO.TEST_INDX(id int, bla varchar(255));
CREATE INDEX IX1 ON dbo.TEST_INDX(id) WITH (DATA_COMPRESSION = PAGE);
Check Compression
SELECT i.name, p.data_compression_desc
FROM sys.partitions P
INNER JOIN sys.indexes I ON I.object_id = P.object_id AND I.index_id = P.index_id
WHERE P.data_compression > 0 and I.name = 'IX1';
Result
name data_compression_desc
IX1 PAGE
Rebuild the index
ALTER INDEX IX1 on DBO.TEST_INDX rebuild
Check Compression
SELECT i.name, p.data_compression_desc
FROM sys.partitions P
INNER JOIN sys.indexes I ON I.object_id = P.object_id AND I.index_id = P.index_id
WHERE P.data_compression > 0 and I.name = 'IX1'
Result
name data_compression_desc
IX1 PAGE
Disabling them and then rebuilding has a different result, since disabling removes the index, while keeping the index definition.
alter index IX1 on DBO.TEST_INDX DISABLE ;
alter index IX1 on DBO.TEST_INDX REBUILD ;
Result
name data_compression_desc
Compression was lost, compression definition would also be lost when dropping and creating the index via SSMS without adapting the Index create script.
Why?
Because the data_compression option is not retained when scripting out the Index create statement.
however, if we disable the index , rebuild with compression and then rebuild again:
alter index IX1 on DBO.TEST_INDX DISABLE ;
alter index IX1 on DBO.TEST_INDX REBUILD WITH (DATA_COMPRESSION = PAGE);
alter index IX1 on DBO.TEST_INDX REBUILD;
Result
name data_compression_desc
IX1 PAGE
Testing a rebuild with Ola hallengren's maintenance solution
The parameters are modified for testing purposes.
Add some data to get to one page, as it is needed for the MinNumberOfPages parameter.
INSERT INTO dbo.TEST_INDX(id,bla)
VALUES(5,'test');
go 10
Execute the index optimize proc to print out the statement.
EXECUTE dbo.IndexOptimize
@Databases = 'TestDB',
@FragmentationLow = 'INDEX_REBUILD_ONLINE',
@FragmentationMedium = 'INDEX_REBUILD_ONLINE,INDEX_REBUILD_OFFLINE',
@FragmentationHigh = 'INDEX_REBUILD_ONLINE,INDEX_REBUILD_OFFLINE',
@FragmentationLevel1 = 5,
@FragmentationLevel2 = 30,
@Indexes = 'TestDB.DBO.TEST_INDX',
@Execute = 'N',
@MinNumberOfPages = 1;
Result:
Command: ALTER INDEX [IX1] ON [TestDB].[dbo].[TEST_INDX] REBUILD WITH (SORT_IN_TEMPDB = OFF, ONLINE = ON, RESUMABLE = OFF)
Comment: ObjectType: Table, IndexType: NonClustered, ImageTex
t: No, NewLOB: No, FileStream: No, ColumnStore: No, AllowPageLocks: Yes, PageCount: 1, Fragmentation: 0
Outcome: Not Executed
Duration: 00:00:00
Date and time: 2019-01-09 14:48:12
Executing the generated command
ALTER INDEX [IX1] ON [TestDB].[dbo].[TEST_INDX] REBUILD WITH (SORT_IN_TEMPDB = OFF, ONLINE = ON, RESUMABLE = OFF)
Compression is retained
name data_compression_desc
IX1 PAGE
Testing a rebuild with a maintenance plan (I would strongly argue for ola's solution)
Rebuild indexes
Choose the test table
Add some test fragmentation levels.
Insert some values to get the fragmentation going
INSERT INTO dbo.TEST_INDX(id)
SELECT id from TEST_INDX
go 4
Check the fragmentation percentage
SELECT
I.[name] AS INDX ,
IPS.avg_fragmentation_in_percent,
IPS.page_count
FROM sys.dm_db_index_physical_stats (DB_ID(), object_id('[dbo].[TEST_INDX]'), NULL, NULL, NULL) AS IPS
INNER JOIN sys.indexes AS I ON I.[object_id] = IPS.[object_id]
AND IPS.index_id = I.index_id
WHERE IPS.database_id = DB_ID()
and I.name = 'IX1'
Result
INDX avg_fragmentation_in_percent page_count
IX1 66,6666666666667 3
Run the plan
The interesting part here, when looking at the plan report, is that the DATA_COMPRESSION = PAGE
option is added to the generated REBUILD
command!
Command:USE [TestDB]
GO
ALTER INDEX [IX1] ON [dbo].[TEST_INDX] REBUILD PARTITION = ALL WITH (PAD_INDEX = ON, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, RESUMABLE = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON, FILLFACTOR = 80, DATA_COMPRESSION = PAGE)
Fragmentation:
INDX avg_fragmentation_in_percent page_count
IX1 0 2
Compression:
name data_compression_desc
IX1 PAGE