Plan Explorer Row Estimates

Joey D’Antoni shows how in SQL Sentry Plan Explorer, estimated counts can differ from what SSMS shows:

I quickly honed into the fact that the bad query was doing a lazy spool with 6 BILLION rows versus the 229 million in the good query. However, my friend who was looking at Management Studio was asking where I was getting those numbers. This is what the XML from the original plan showed:

<RelOp AvgRowSize=”51″ EstimateCPU=”0.00889634″ EstimateIO=”0.01″ EstimateRebinds=”0″ EstimateRewinds=”139581″ EstimatedExecutionMode=”Row” EstimateRows=”48868″
LogicalOp=”Lazy Spool” NodeId=”55″ Parallel=”true” PhysicalOp=”Table Spool” EstimatedTotalSubtreeCost=”1242.86″>

This is a helpful feature in scenarios like this, where operator weight is skewed because it only shows a single run but in reality happens more than once.

Related Posts

Character Columns And MAX Vs TOP+ORDER Differences

Kendra Little digs into a tricky performance problem: Most of the time in SQL Server, the MAX() function and a TOP(1) ORDER BY DESC will behave very similarly. If you give them a rowstore index leading on the column in question, they’re generally smart enough to go to the correct end of the index, and […]

Read More

Row Goals And Anti-Joins

Paul White continues his row goals series: The optimizer assumes that people write a semi join (indirectly e.g. using EXISTS) with the expectation that the row being searched for will be found. An apply semi join row goal is set by the optimizer to help find that expected matching row quickly. For anti join (expressed e.g. using NOT EXISTS) the optimizer’s assumption is that […]

Read More


February 2016
« Jan Mar »