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.

Denali: Note the small but important things in SSIS

With SQL Server, there often seems to be an over-emphasis on the items that provide bullet points for the marketing brochure. Yet, small but useful changes to the product can make a more profound impact on DBAs/developers than the new items that are highlighted by those bullet points.

One of the things I loved about Reporting Services in SQL Server 2008 R2 is the amount of focus they provided on ease of use. In particular, the addition of data bars, lookup functions, pagination control, rendering targets in expressions, domain scope, etc. made a huge difference to anyone that really needs to build reports.

For Denali, Integration Services has me similarly excited. I've just completed prepping and presenting a session on the Denali enhancements to SSIS (and on the introduction of Data Quality Services) for TechEd Australia. It was interesting to note, though, that the biggest reactions I received from the crowd were for the "little" improvements, not for the big ticket items. Some of the best examples of these are:

  • Zoom control, general UI improvements and the Fit to Window
  • Improvements around the display of precedence labels (great for those who are color blind and struggle with the standard red/green/blue)
  • Toolbox groupings
  • Ease of integrating new items into the Toolbox
  • Reliability and performance improvements around Merge/Merge Join

And there are many more. The two biggest woohoo moments though, came for:

  • Multi-level undo and redo
  • Remapping work that has been done in the data flow (remapping GUI is great but the way that so many mapping issues are automatically fixed is awesome)

I want to congratulate the team on spending time making these type of "less visible" improvements.

Do you still sharpen your knives?

We end up staying in Sydney several times per year, either for training or mentoring/consulting work. When we do, one of the hotels that we really like is the Westin. I would find it hard though, to list all the reasons why I like it. (There are some things I don't like about it too but that's a topic for another day).

But one of the things that has always surprised me, each and every time I eat breakfast there, is how sharp the knives are. Clearly, someone must be assigned to sharpen them, or they replace them very regularly (unlikely). Alternately, one of the staff members might have been passionate about it and it was an individual thing but we stayed about 160 nights in hotels last year and I can tell you that most hotels (big or small) don't bother doing this.

Today at breakfast, I noticed that the knives weren't sharp. Same thing happened last time. I can't help but thinking that this might be one little tell-tale sign of some belt-tightening in the running costs of the hotel. If it was an individual effort, perhaps that person has just left.

However, it got me wondering about what impact these little things have ie: the little things that companies do but that would never appear on a brochure. Do you have little things that you do for your customers that are a bit unique and that your customers appreciate without telling you? Do you still do them? Do you still sharpen your knives?