SQL Server 2005 Engine Optimistic Concurrency Tony Rogerson, SQL Server MVP Independent Consultant 26 th January :45 – 13:15 Lunch n Learn webcast series

Agenda Isolations –Pessimistic Default Isolation – Writers block Readers –Optimistic READ_COMMITTED_SNAPSHOT (statement level read consistency) ALLOW_SNAPSHOT_ISOLATION (transaction level read consistency) Architecture: Row Versioning Monitoring

Uses Anything to reduce locking contention Near real-time statistics page (call centre, stock ticker) Anywhere using NOLOCK hint Extraction from OLTP into DW/MIS Not an answer for deadlocking but will reduce locking and may help

Demo

Row Versioning Sess A: UPDATE -> ‘START’ Sess B: XACT ISO – SNAPSHOT BEGIN TRAN SELECT… Store old row ‘START’ Sess A: UPDATE -> ‘UPD 1’ Sess C: XACT ISO – SNAPSHOT BEGIN TRAN SELECT… Mark active Remove by clean up Store old row ‘UPD 1’ Sess A: UPDATE -> ‘UPD 2’

READ_COMMITTED_SNAPSHOT Statement level isolation (gives value of last committed row at time of query) Writer no longer blocks reader Possibly reduced deadlocking because of reduced locking contention

ALLOW_SNAPSHOT_ISOLATION Transaction level isolation (gives last committed row at start of transaction) Snapshot of values taken on select and held in version store in tempdb Stores full row (changed fragment for lob) Queries require SET TRANSACTION ISOLATION LEVEL SNAPSHOT Update fails if update on a value changed outside your snapshot

Architecture: Row Versioning Used by other stuff… –inserted and deleted tables in triggers –MARS (Multiple Active Result Sets) –Index operations using the ONLINE feature –Transaction Isolations Versions held in tempdb (the version store) Changed row or changed fragment for LOB stored in tempdb Background thread runs every minute to remove versions no longer required. Short lived transactions will not require a disk IO on tempdb

Monitoring sys.dm_tran_active_snapshot_database_transac tions sys.dm_tran_version_store sys.dm_tran_current_transaction sys.dm_tran_transactions_snapshot sys.dm_tran_top_version_generators sys.dm_tran_locks

Thanks for Listening Q & A Tony Rogerson, SQL Server MVP Independent Consultant