Let’s Get Started! Rick Lowe Weird Stuff I Saw While …Working With Heaps Data Flowe Solutions LLC

Rick Lowe rick@data-flowe.com DataFLowe http://dataflowe.wordpress.com/

What Is a Heap Table without a clustered Index Doesn’t automatically have a B tree Disorganized data Table that only supports scans 3/25/2017

Demo Break 1 Creation script System health session Update demo 1 3/25/2017

What’s a Deadlock? has has needs needs 3/25/2017

Modifying a Heap 1 2 3 4 5 6 UPDATE dbo.myHeap SET n = 1 WHERE ID = 4; Is ID 4 here? Is ID 4 here? 1 2 3 4 5 6 3/25/2017

Modifying a Heap (cont’d) UPDATE dbo.myHeap SET n = 1 WHERE ID = 4; Found it 1 2 3 4 5 6 Time to update! 3/25/2017

Modifying a Clustered Index (comparison) UPDATE dbo.myTable SET n = 1 WHERE ID = 4; 1 2 3 4 5 6 Found it Time to update! 1 2 3 4 5 6 3/25/2017

Modifying a Heap – Two Connections UPDATE dbo.myHeap SET n = 1 WHERE ID = 4; It’s Here! Is ID 4 here? Nope. Is ID 4 here? Nope. 1 2 3 4 5 6 UPDATE dbo.myHeap SET n = 1 WHERE ID = 3; 3/25/2017

Modifying With a Heap – Two Connections (Contd) UPDATE dbo.myHeap SET n = 1 WHERE ID = 4; Is 4 here? 1 2 3 4 5 6 Is 3 here? UPDATE dbo.myHeap SET n = 1 WHERE ID = 3; 3/25/2017

Issues With Updating Heaps No seeks (in the absence of NC index) Also no guarantee of uniqueness (scan continues even after record is found) Many update locks, one exclusive lock If multiple connections are doing similar update, connection A may hold an exclusive lock on data that connection B wants to scan and vice versa 3/25/2017

Inserting Into Heaps: Best Case INSERT INTO dbo.myHeap(…) VALUES(…); I’m locking slot 0. 1 2 3 4 5 I’m locking slot 1. INSERT INTO dbo.myHeap(…) VALUES(…); 3/25/2017

Inserting Into Heaps: Reused Space INSERT INTO dbo.myHeap(…) VALUES(…); I’m locking slot 0. 1 2 3 4 5 Oh, there’s space here. INSERT INTO dbo.myHeap(…) VALUES(…); 3/25/2017

Inserting Into Clustered Index (Comparison) INSERT INTO dbo.myTable(…) VALUES(…); 1 2 3 4 5 Found it Time to update! 1 2 3 4 5 3/25/2017

Demo Break 2 SSIS Demo Speed comparison 3/25/2017

One Way to Make Insert Fail SELECT @n = COUNT(*) FROM dbo.myHeap WHERE ClientID = @cust WITH(SERIALIZABLE) Shared lock I’m looking at this. Don’t change. dbo.BigHeap SELECT @n = COUNT(*) FROM dbo.myHeap WHERE ClientID = @cust WITH(SERIALIZABLE) 3/25/2017

One Way to Make Insert Fail (cont’d) INSERT INTO dbo.MyHeap(ID, IsTrial, BigBlobbyData) VALUES(…) Shared lock Convert to IX lock Need to lock a row dbo.BigHeap Convert to IX lock INSERT INTO dbo.MyHeap(ID, IsTrial, BigBlobbyData) VALUES(…) 3/25/2017

Issues With Inserting Into Heaps No order, so new records could go anywhere Reusing space means looking for space Mixing SELECTS and INSERTS are more problematic due to table scans Many believe inserts into heaps are faster. That’s not always true. If table eventually needs a clustered index, how long does it take to convert from heap? 3/25/2017

Demo Break 3 Update demo 2 – heap with NC index 3/25/2017

Indexing Cheat Sheet Heaps are OK for inserting lots of transitory, unsorted data through a single connection. Heaps aren’t ideal for select, update, delete Heaps aren’t ideal for multiple connections Clustered index inserts may outperform heap inserts when data is ordered Usually not difficult to sort data before loading 3/25/2017

Non-Clustered Indexes Adding NC indexes or unique constraints to heaps rarely makes sense Adding NC index to heap often increases frequency of deadlocks (more things to lock) NC indexes also tend to make deadlocks more likely when inserting to clustered index Best case for inserts is often a table with clustered index and no NC indexes. 3/25/2017

Ghost Cleanup http://www.mikefal.net/2012/10/17/a-heap-of-trouble/ Large, partitioned heap Volatile Key Ghost cleanup stopped working 3/25/2017

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