fragmentationoracle
Is there any way to check whether a table or index has been fragmented?
How to check if table is fragmented or not?
How to check if index is fragmented?
I searched Google but didn't get satisfactory answers. Some points like explain plan, table fragmentation, index rebuild etc I got. But what should be our first step and how should be resolve. How step by step we can move forward to resolve the issue?
The operating system returned error 23(Data error (cyclic redundancy check).) to SQL Server during a read
This indicates the operating system detected an issue reading data from the hard disk itself. When data is written onto the filesystem, the operating system calculates a CRC code for each block written; when that data is subsequently read back the operating system performs the CRC operation again on the read data and compares it with the CRC stored in the filesystem. If these two CRC codes don't match the OS reports error 23. I'd be very surprised if there isn't a hardware issue with either the drive itself, or perhaps some other component such as the motherboard or drive controller.
WITH (ONLINE=ON) in the Enterprise Edition of SQL Server will have no effect whatsoever on this issue. Online index operations vs offline index operations are no different at the operating system layer; data is read and written as necessary to either rebuild or recreate the index.
Now I'm worried that this schema change won't help that much.
Changing the column from a varchar(36) to a uniqueidentifier will greatly reduce storage requirements from 36 bytes per row to 16 bytes per row; for each 100,000 row table, that equates to a savings of 2MB. 2MB may not sound like much by itself, but since the column is the primary key, that 2MB applies to each non-clustered index on the table. If any other tables are using foreign keys to ensure relational integrity, the space savings applies to those tables as well. Realize also that this space savings doesn't only apply to on-disk storage, it also applies equally, and one might argue, more importantly, to rows contained in the buffer-pool.
It appears from a cursory glance at your sample table above that the average row size is somewhere around 600 bytes. Converting the varchar(36) to a uniqueidentifier will immediately offer a 3.3% space savings for the clustered index, and more than that for each non-clustered index.
As a bonus, once you convert the column from a varchar(36) to a uniqueidentifier, it will no longer be possible to insert this is a bad key into the column, as is currently possible. This will result in much better data integrity, and will potentially save you a lot of work down the road.
Having said that, I decided to try to recreate the problems you're seeing with index fragmentation. I created a test table with the varchar(36) primary key column with 100,000 rows. I then converted the column to a uniqueidentifier and was unable to see a drastic increase in index fragmentation. My test bed code is:
USE tempdb;
/* create our test table, with a VARCHAR primary key */
IF OBJECT_ID('dbo.guid_insert') IS NOT NULL
DROP TABLE dbo.guid_insert;
CREATE TABLE dbo.guid_insert
(
PK VARCHAR(36) NOT NULL
CONSTRAINT PK_guid_insert
PRIMARY KEY CLUSTERED
WITH (
DATA_COMPRESSION = NONE
, PAD_INDEX = OFF
, ALLOW_ROW_LOCKS = ON
, ALLOW_PAGE_LOCKS = ON
, FILLFACTOR = 80
)
, SomeData VARCHAR(600)
, CreatedDate DATETIME
CONSTRAINT DF_quid_insert_CreatedDate
DEFAULT (GETDATE())
) ON [PRIMARY];
/* insert 100,000 rows into the test table */
INSERT INTO dbo.guid_insert (PK, SomeData)
SELECT NEWID(), REPLICATE('.', 600)
FROM (VALUES (0), (1), (2), (3), (4), (5), (6), (7), (8), (9)) v(num)
CROSS APPLY (VALUES (0), (1), (2), (3), (4), (5), (6), (7), (8), (9)) v1(num)
CROSS APPLY (VALUES (0), (1), (2), (3), (4), (5), (6), (7), (8), (9)) v2(num)
CROSS APPLY (VALUES (0), (1), (2), (3), (4), (5), (6), (7), (8), (9)) v3(num)
CROSS APPLY (VALUES (0), (1), (2), (3), (4), (5), (6), (7), (8), (9)) v4(num);
/* show the fragmentation details */
SELECT ObjectName = QUOTENAME(s.name) + '.' + QUOTENAME(o.name)
, IndexName = QUOTENAME(i.name)
, [FillFactor] = 'FILLFACTOR = ' + CONVERT(VARCHAR(50), CASE WHEN i.fill_factor = 0 THEN '100' ELSE i.fill_factor END)
, CompressOption = 'DATA_COMPRESSION = ' + p.data_compression_desc
, NumberOfFragments = ips.fragment_count
, NumberOfPages = ips.page_count
, AvgFragmentSizeInPages = ips.avg_fragment_size_in_pages
, AvgFragmentationInPercent = ips.avg_fragmentation_in_percent
, IndexType = i.type_desc
, IsUnique = i.is_unique
, IsPrimary = i.is_primary_key
, IsPartitioned = CASE WHEN ps.data_space_id IS NULL THEN 0 ELSE 1 END
, IsClustered = CASE WHEN i.type = 1 THEN 1 ELSE 0 END
FROM sys.indexes i WITH (NOLOCK)
INNER JOIN sys.objects o WITH (NOLOCK) ON i.object_id = o.object_id
INNER JOIN sys.schemas s WITH (NOLOCK) ON o.schema_id = s.schema_id
INNER JOIN sys.data_spaces ds WITH (NOLOCK) ON i.data_space_id = ds.data_space_id
INNER JOIN sys.partitions p WITH (NOLOCK) ON o.object_id = p.object_id
AND i.index_id = p.index_id
LEFT JOIN sys.partition_schemes ps WITH (NOLOCK) ON ds.data_space_id = ps.data_space_id
CROSS APPLY sys.dm_db_index_physical_stats(DB_ID(), o.object_id, i.index_id, p.partition_number, 'DETAILED') ips
WHERE o.is_ms_shipped = 0
AND NOT (
o.type = 'TF' -- table valued function
OR o.type = 'TT' -- table type
OR o.type = 'SO' -- sequence object
)
AND i.index_id > 0
AND i.is_disabled = 0
AND i.is_hypothetical = 0
AND (
ds.type = 'FG' -- filegroup
OR ds.type = 'PS' -- partition stream
)
AND i.type_desc IN
( --we only support rebuilding/reorganizing these index types:
'CLUSTERED'
, 'NONCLUSTERED'
, 'XML'
)
AND ips.index_level = 0 -- leaf-levels only
AND ips.fragment_count > 1
ORDER BY ips.fragment_count * ips.avg_fragmentation_in_percent DESC;
The fragmentation query results:
/* rebuild the index to remove fragmentation */
ALTER INDEX PK_guid_insert ON dbo.guid_insert REBUILD;
/* drop the primary key, since we can't modify the PK column when it is a primary key */
ALTER TABLE dbo.guid_insert
DROP CONSTRAINT PK_guid_insert;
/* convert the VARCHAR PK column into a UNIQUEIDENTIFIER column */
ALTER TABLE dbo.guid_insert
ALTER COLUMN PK UNIQUEIDENTIFIER NOT NULL;
/* recreate the primary key */
ALTER TABLE dbo.guid_insert
ADD CONSTRAINT PK_guid_insert
PRIMARY KEY CLUSTERED (PK);
/* add a DEFAULT constraint to create sequential keys */
ALTER TABLE dbo.guid_insert
ADD CONSTRAINT DF_guid_insert_PK
DEFAULT (NEWSEQUENTIALID())
FOR PK;
/* delete 10,000 random rows */
;WITH src AS
(
SELECT rn = ROW_NUMBER() OVER (ORDER BY NEWID()), *
FROM dbo.guid_insert
)
DELETE
FROM src
WHERE rn < 10000;
/* insert 100,000 more rows */
INSERT INTO dbo.guid_insert (SomeData)
SELECT REPLICATE('.', 600)
FROM (VALUES (0), (1), (2), (3), (4), (5), (6), (7), (8), (9)) v(num)
CROSS APPLY (VALUES (0), (1), (2), (3), (4), (5), (6), (7), (8), (9)) v1(num)
CROSS APPLY (VALUES (0), (1), (2), (3), (4), (5), (6), (7), (8), (9)) v2(num)
CROSS APPLY (VALUES (0), (1), (2), (3), (4), (5), (6), (7), (8), (9)) v3(num)
CROSS APPLY (VALUES (0), (1), (2), (3), (4), (5), (6), (7), (8), (9)) v4(num);
/* show the fragmentation details */
SELECT ObjectName = QUOTENAME(s.name) + '.' + QUOTENAME(o.name)
, IndexName = QUOTENAME(i.name)
, [FillFactor] = 'FILLFACTOR = ' + CONVERT(VARCHAR(50), CASE WHEN i.fill_factor = 0 THEN '100' ELSE i.fill_factor END)
, CompressOption = 'DATA_COMPRESSION = ' + p.data_compression_desc
, NumberOfFragments = ips.fragment_count
, NumberOfPages = ips.page_count
, AvgFragmentSizeInPages = ips.avg_fragment_size_in_pages
, AvgFragmentationInPercent = ips.avg_fragmentation_in_percent
, IndexType = i.type_desc
, IsUnique = i.is_unique
, IsPrimary = i.is_primary_key
, IsPartitioned = CASE WHEN ps.data_space_id IS NULL THEN 0 ELSE 1 END
, IsClustered = CASE WHEN i.type = 1 THEN 1 ELSE 0 END
FROM sys.indexes i WITH (NOLOCK)
INNER JOIN sys.objects o WITH (NOLOCK) ON i.object_id = o.object_id
INNER JOIN sys.schemas s WITH (NOLOCK) ON o.schema_id = s.schema_id
INNER JOIN sys.data_spaces ds WITH (NOLOCK) ON i.data_space_id = ds.data_space_id
INNER JOIN sys.partitions p WITH (NOLOCK) ON o.object_id = p.object_id
AND i.index_id = p.index_id
LEFT JOIN sys.partition_schemes ps WITH (NOLOCK) ON ds.data_space_id = ps.data_space_id
CROSS APPLY sys.dm_db_index_physical_stats(DB_ID(), o.object_id, i.index_id, p.partition_number, 'DETAILED') ips
WHERE o.is_ms_shipped = 0
AND NOT (
o.type = 'TF' -- table valued function
OR o.type = 'TT' -- table type
OR o.type = 'SO' -- sequence object
)
AND i.index_id > 0
AND i.is_disabled = 0
AND i.is_hypothetical = 0
AND (
ds.type = 'FG' -- filegroup
OR ds.type = 'PS' -- partition stream
)
AND i.type_desc IN
( --we only support rebuilding/reorganizing these index types:
'CLUSTERED'
, 'NONCLUSTERED'
, 'XML'
)
AND ips.index_level = 0 -- leaf-levels only
AND ips.fragment_count > 1
ORDER BY ips.fragment_count * ips.avg_fragmentation_in_percent DESC; --start with the most fragmented indexes first
The fragmentation details after the conversion, with deleting and inserting rows:
As you can see, after the conversion, and deleting 10,000 rows, then inserting 100,000 rows, index fragmentation for the primary key index (the table itself) is just 0.41%; hardly a concern.
If you need further help with this problem (I realize your question is a year old), you need to show the DDL for the table, its indexes, and the code you used for the conversion test.
Best Answer
You need to use
INDEX_STATSview to determine fragmentation.Firstly, You need to populate this view with an individual index by using :
and then look at these values
ratioandheight:If ratio is more than %20
or height is more than or equal to 4
or lf_rows is lower than lf_blks,
then you need to rebuild your index against fragmentation.
The
blevelcolumn ofdba_indexestable is my main point of interest. This gives the number of branch levels (including the root node) for a b-tree index, and the columnheightis closely related toblevel.If you have access to MOSC, search for the blevel keyword, especially the subject Index Health. The Oracle Community was also suggesting rebuilding indexes for indexes have blevel more than or equal to 4.