A Masters view on Locking and blocking Mikael Wedham A Masters view on Locking and blocking

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Mikael Wedham Microsoft Certified Master Trainer Developer Database Administrator Solution Developer Systems Engineer and so on… Växjö, Sweden mikael@wedham.com @wedham http://www.wedham.com

Locking, the problem Reading, sometimes Writing Multiple writes Consistent analysis DEMO: Classic lock.

Finding the issue - with sys.dm_os_wait_stats The hard way: Write your own The easy way: http://www.sqlskills.com/blogs/paul/wait-statistics-or-please-tell-me-where-it-hurts/ DEMO…

Finding lock details sp_lock sp_who sp_who2 OR more ”modern” sys.dm_tran_locks sys.dm_exec_sessions

Transactions A C I D

Transactions Always Check It, Dumba$$

Transactions Atomicity Consistency Isolation Durability

Isolation levels Different transaction levels for different cases Reads set Shared (S) locks Writes set Exclusive (X) locks Busy Writes sets Update (U) locks, transitions to (X) when alone Check BOL for compatibility matrix

READ COMMITTED (default) All reads occur on commited data. Two identical SELECT statements, even when encapsulated in a transaction, can return different data.

READ UNCOMMITTED Catastrophic, due to data movement mid-select. Reads without honoring locks. Problem because rows are not committed Bigger problem because data may be inconsistent Catastrophic, due to data movement mid-select.

REPEATABLE READ Locks read data until end of transaction. All read rows will stay the same New rows can appear

SERIALIZABLE All rows and their ”before” and ”after” rows are locked. Prevents new rows from being inserted.

ALLOW SNAPSHOT Turning it on makes all the difference in performance READ COMMITTED SNAPSHOT Changes only Reads. Requires no code change TRANSACTION ISOLATION LEVEL SNAPSHOT Must be set – and coded for.

SNAPSHOT DEMO

InMemory tables Uses row-versioning similar to snapshot. DEMO

Thoughts about solutions for blocking… Short(er) transactions Snapshot In-Memory OLTP Read-only copy ”Real” BI

Thoughts about solutions for blocking… Short(er) transactions Snapshot In-Memory OLTP Read-only copy ”Real” BI Questions??

Thank You for listening! Mikael Wedham mikael@wedham.com @wedham http://www.wedham.com