So I’ll be posting explanations for each query operator in the output query trees that you can see through “set showplan_text on” in SQL Server. I’m hopeful that this will give you the tools to better read the query plans being generated by the QP when evaluating your system.
So I’ll start with one that I added into the optimizer – the “sequence project”.
create table z(col1 int identity, col2 int default rand()*10000) declare @i int set @i=0 while @i < 1000 begin insert into z default values set @i=@i+1 end select RANK() over(order by col2), * from z
If you look at the graphical showplan you get an output plan that looks a bit funny. A Sequence Project is a special form of projection (the thing that evaluates the scalars in the SELECT list for each row). By default, a regular project in SQL Server does not preserve nor guarantee any sort properties in the physical plan. As a result, ranking functions put in regular projects would not preserve order in all cases deterministically. People seem to complain when the query results are incorrect.
Enter the sequence project….This little beauty does do scalar computations, but they are generally limited to a very small number of operations that require order for one reason or another. In the case of ranking functions, an order is required as part of the syntax, and you will see that this particular plan needs a sort since there are no indexes on the base table to satisfy a sort request on col2. (I’ll talk about segments in a future post, but for now ignore them and assume that they are part of the sequence project when looking at this plan).
The problem with sequence projects are that while they preserve ordering semantics, they interfere with a lot of the other optimization logic in the QP. For example, these can interfere with index matching for predicates above the sequence project in a query tree. Operators that block other optimizations tend to make optimizer developers very unhappy and sometimes elicit violent reactions! So, be careful when proposing “opaque” operators to the next query optimization developer you see ;).
So, SQL Server has a small number of operations that in previous releases of the product were “incorrectly” put into regular projects that are now in sequence projects in the current versions of the product. (By “incorrectly”, I mean that there were potential optimizations that could be run that would break their semantics, not that there were actively executing plans at a customer site that were incorrect. In optimizer terminology, they model was not correctly preserving the semantics even though there were no rules that exposed this fault).
So, the challenge for you is think of some places _beyond sequence functions_ (RANK, DENSE_RANK, NTILE, ROW_NUMBER, …) that use the sequence projection operator. If you mail them to me with plans, I may be able to tell you more about what is happening for each of them.