Denali: Improved T-SQL Query Optimization

Part of the value in the ongoing evolution of the T-SQL language is that we are moving further and further towards being declarative rather than prescriptive ie: we are able to tell SQL Server what we want, rather than how to do it. Over time, that raises more and more possibilities for the optimizer to work with us to achieve a better outcome.

For example, note the following query against the AdventureWorksDW database:

SELECT rs.ProductKey, rs.OrderDateKey, rs.SalesOrderNumber,

       rs.OrderDateKey – (SELECT TOP(1) prev.OrderDateKey

                          FROM dbo.FactResellerSales AS prev

                          WHERE rs.ProductKey = prev.ProductKey

                          AND prev.OrderDateKey <= rs.OrderDateKey

                          AND prev.SalesOrderNumber < rs.SalesOrderNumber

                          ORDER BY prev.OrderDateKey DESC,

                                  prev.SalesOrderNumber DESC)

                AS DaysSincePrevOrder

FROM dbo.FactResellerSales AS rs

ORDER BY rs.ProductKey, rs.OrderDateKey, rs.SalesOrderNumber;

In this query, I'm trying to include details of how long it was since the previous order, beside the details of the current order. Note the option that the LAG operator now provides:

SELECT ProductKey, OrderDateKey, SalesOrderNumber,

       OrderDateKey – LAG(OrderDateKey)

                         OVER (PARTITION BY ProductKey

                                ORDER BY OrderDateKey, SalesOrderNumber)

                AS DaysSincePrevOrder

FROM dbo.FactResellerSales AS rs

ORDER BY ProductKey, OrderDateKey, SalesOrderNumber;

Also note how much more elegant the code is but more importantly, look at the difference in optimization:

 

This is great work from the T-SQL and engine teams. I encourage you to get out and try the new Windowing functions in Denali CTP3.