sql server - High extent scan fragmentation but low Logical Scan Fragmentation -

i'm still new sql server, when run dbcc showcontig on table, shows logical scan fragmentation not big extent scan fragmentation big, made me little confused.

    dbcc showcontig scanning 'user' table...     table: 'user' (462624691); index id: 1, database id: 6     table level scan performed.     - pages scanned................................: 2638     - extents scanned..............................: 335     - extent switches..............................: 578     - avg. pages per extent........................: 7.9     - scan density [best count:actual count].......: 56.99% [330:579]     - logical scan fragmentation ..................: 12.93%     - extent scan fragmentation ...................: 94.03%     - avg. bytes free per page.....................: 805.1     - avg. page density (full).....................: 90.05%     dbcc execution completed. if dbcc printed error messages, contact system administrator. 

seeing manual on msdn, 1 fragmentation on pages , on extents. there other difference? how can logical scan fragmentation low while extent scan fragmentation remains high?

also when talk "external fragmentation", more related "logical scan fragmentation" in result or "extent scan fragmentation" in result of showcontig?

really appreciate answer.

i assume interested in measuring fragmentation because want optimize sequential scanning speed.

all numbers extents meaningless far can tell. matters sequential scan speed page-level metrics because performance determined way pages read.

the interesting number here "logical scan fragmentation" because percentage of pages disk seeks required next page. 13% means once in every 6 pages physical read position changes.

how can logical scan fragmentation low while extent scan fragmentation remains high?

if there 100% extent fragmentation there still can 8 contiguous pages per extent logical fragmentation low 12.5%.

probably should not use showcontig use fragmentation dmv.