Building Indexes Online Creates Larger Indexes

When I create an index on a table, I usually run a script of mine that among other things gives me the size of the created index.  This allows me to see for example if the overhead of compressing the index is worth the size improvement.  Imagine my surprise today to find that the “ONLINE” option also seems to have an impact on the final size of an index.  I’m not sure why this off the top of my head, but the difference can be significant.  One index that I took a look at grew by 50% simply by creating it with the ONLINE option set to ON.

To analyze your own indexes use the query below.  Please make sure to specify the table name and the index name in the first 2 lines of code.

/*
Author:        Steve Abraham
Date:        4/13/2010
Purpose:    This script analyzes in detail a single index
Notes:        Make sure to change the database “use” statement and configure @ObjectId, @IndexId and @AnalysisLevel
*/

use TestDb
GO

–Define the table and index
DECLARE @ObjectId        INT                = OBJECT_ID(‘dbo.Table1′)
DECLARE @IndexId        INT                = (SELECT index_id FROM sys.indexes WHERE name = ‘Index01′ AND object_id = @ObjectId)
DECLARE @AnalysisLevel    NVARCHAR(100)    = ‘DETAILED’
–DECLARE @AnalysisLevel    NVARCHAR(100)    = ‘SAMPLED’
–DECLARE @AnalysisLevel    NVARCHAR(100)    = ‘LIMITED’

SELECT
DB_NAME(PhysicalStats.database_id)                AS DatabaseName,
sys.schemas.name                                AS SchemaName,
sys.objects.name                                AS ObjectName,
sys.indexes.name                                AS IndexName,
PhysicalStats.partition_number                    AS PartitionNumber,
PhysicalStats.index_type_desc                    AS IndexType,
PhysicalStats.alloc_unit_type_desc                AS AllocationUnitType,
PhysicalStats.index_depth                        AS IndexDepth,
CASE PhysicalStats.index_level WHEN 0 THEN ‘Leaf’ WHEN index_depth – 1 THEN ‘Root’ ELSE ‘Intermediate’ END    AS IndexLevel,
PhysicalStats.avg_fragmentation_in_percent        AS LogicalScanFragmentation,
PhysicalStats.fragment_count                    AS FragmentCount,
PhysicalStats.avg_fragment_size_in_pages        AS AvgFragmentSizeInPages,
PhysicalStats.page_count                        AS [PageCount],
PhysicalStats.avg_page_space_used_in_percent    AS ActualPageDensity,
PhysicalStats.record_count                        AS RecordCount,
PhysicalStats.ghost_record_count                AS GhostRecordCount,
PhysicalStats.version_ghost_record_count        AS VersionGhostRecordCount,
PhysicalStats.min_record_size_in_bytes            AS MinRecordSizeInBytes,
PhysicalStats.max_record_size_in_bytes            AS MaxRecordSizeInBytes,
PhysicalStats.forwarded_record_count            AS ForwardedRecordCount,
PhysicalStats.compressed_page_count                AS CompressedPageCount,
sys.indexes.is_unique                            AS IsUnique,
sys.indexes.ignore_dup_key                        AS IgnoreDuplicateKey,
sys.indexes.is_primary_key                        AS IsPrimaryKey,
sys.indexes.is_unique_constraint                AS IsUniqueConstraint,
sys.indexes.fill_factor                            AS [FillFactor],
sys.indexes.is_padded                            AS IsPadded,
sys.indexes.is_disabled                            AS IsDisabled,
sys.indexes.is_hypothetical                        AS IsHypothetical,
sys.indexes.allow_row_locks                        AS AllowRowLocks,
sys.indexes.allow_page_locks                    AS AllowPageLocks,
sys.indexes.has_filter                            AS HasFilter,
sys.indexes.filter_definition                    AS FilterDefinition
FROM
sys.dm_db_index_physical_stats(DB_ID(), @ObjectId, @IndexId, NULL, @AnalysisLevel) AS PhysicalStats
INNER JOIN
sys.indexes
ON
PhysicalStats.object_id = sys.indexes.object_id
AND
PhysicalStats.index_id = sys.indexes.index_id
INNER JOIN
sys.objects
ON
sys.indexes.object_id = sys.objects.object_id
INNER JOIN
sys.schemas
ON
sys.objects.schema_id = sys.schemas.schema_id
ORDER BY
PhysicalStats.alloc_unit_type_desc,
PhysicalStats.index_level DESC
COMPUTE
SUM(PhysicalStats.page_count) BY AllocationUnitType

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SQL Server Expert, Steve Abraham, holds 8 Microsoft certifications and specializes in SQL Server and .Net Framework architecture, high availability, capacity planning, development, and performance tuning, and SQL Server Recovery.