Index Fragmentation Paul Randal Dev Lead, Microsoft SQL Server Introduction  Structural details of an index  How these structures are used  Index fragmentation  SQL Server 2000 DBCC SHOWCONTIG  SQL Server 2000 DBCC INDEXDEFRAG  "Yukon" dm_db_index_physical_stats  "Yukon" ALTER INDEX Records (1)  How is data stored in SQL Server?  Example record types  Data  Index  Text  Ghost data/index Records (2)  ‘Inside SQL Server 2000’ has the details  Things to note:  Non-leaf index records contain a child page Id in every record  All non-clustered index leaf records contain a base table RID  “Yukon” allows INCLUDE’d columns in an index record Pages (1)  How are records stored?  Example page types  Data page  Index page  Allocation maps (GAM, SGAM, IAM) pages  PFS page Pages (2)  Layout is the same for all page types 8k 96 byte header slot array records free space Extents  How are pages grouped?  Extents are:  A group of 8 contiguous pages  Start on an 8 page boundary  Tracked in IAM, GAM, SGAM pages 0 1 2 3 54 6 7 Heap (1)  Simplest storage arrangement  Comprised entirely of data pages  Unordered, pages unlinked  Records are located using physical RID  create table foo ( first char(100), last char (100), city char (100)) Clustered index (1)  Alternative table structure to heap  Data stored in defined order  Fast lookup through B-tree  Records located through logical RID  create clustered index foo_c on foo (city) Clustered index (2) index tree pages P L L P L L P L L R data pages [...]... logical scan fragmentation Faster and only requires table IS lock Drawback: don’t get the page density Effect of WITH FAST… Option Page count Run Time Default 1, 702 ,889 5 .02 mins WITH FAST 1, 702 ,889 0. 90 mins 5.6 x faster Default 111 ,626,354 WITH FAST 111 ,626,354 382.35 mins 48.73 mins 7.8 x faster Fragmentation calculation R P L P L L P L L L (1) Fragmentation calculation  (2) Reads parent level   Parent... ‘Logical Scan Fragmentation’ ‘Avg Page Density’ – are the pages full? Beware of:   ‘Extent Scan Fragmentation’, ‘Scan Density’ Don’t work over multiple files How to correct fragmentation in SQL Server 200 0?  3 realistic choices     Drop and recreate the index manually Rebuild the index - DBCC DBREINDEX Defrag the index - DBCC INDEXDEFRAG Most people rebuild or defrag Drop/create index  Pros  ... workload for range scans  Learn how to use DBCC SHOWCONTIG  Pick the best method depending on your available downtime Whitepaper on this and more  http://www.microsoft.com/technet/prodtechnol /sql/ 200 0/maintain/ss2kidbp.mspx  Info on identifying which indexes to target Detailed comparison of DBCC DBREINDEX and DBCC INDEXDEFRAG Will be updated for “Yukon”…   . an index  How these structures are used  Index fragmentation  SQL Server 200 0 DBCC SHOWCONTIG  SQL Server 200 0 DBCC INDEXDEFRAG  "Yukon" dm_db_index_physical_stats  "Yukon". unlinked  Records are located using physical RID  create table foo ( first char( 100 ), last char ( 100 ), city char ( 100 )) Clustered index (1)  Alternative table structure to heap  Data stored. Server?  Example record types  Data  Index  Text  Ghost data/index Records (2)  ‘Inside SQL Server 200 0’ has the details  Things to note:  Non-leaf index records contain a child page Id in every