Clustered index (1)  Alternative table structure to heap  Data stored in defined order  Fast lookup through B-tree  Records located through logical RID  create table foo ( first char(100), last char (100), city char (100))  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 Non-clustered index (1)  A way to provide a different ordering  Define on heaps or clustered indexes  Leaf records contain RID of matching record in base table  create index foo_nc on foo (last)  create index foo_nc on foo (last) include (first) Non-clustered index (2) P L L P L L P L L R index leaf pages index tree pages Why use an index?  Allows a variety of access modes:  Singleton lookup  Range scan  Allocation order scan  Allows skipping of sort step in query Singleton lookup Matching record Range scan Allocation-order scan 1 23 45 6 What causes fragmentation (1) Index leaf level of newly built index Red arrow is the allocation order Black arrows are following the logical order What causes fragmentation (2) Newly built index leaf after a single page split Red arrow is the allocation order Black arrows are following the logical order