Question

I have some legacy SQL (SP)

  declare @FactorCollectionId       int;        select  @FactorCollectionId = collectionID from dbo.collection where name = 'Factor'
    declare @changeDate             datetime;   set @changeDate = getDate()
    declare @changeTimeID           int;        set @changeTImeID = convert(int, convert(varchar(8), @changeDate, 112))
    declare @MaxWindowID            int;        select @MaxWindowID = MAX(windowID) from dbo.window

  select distinct @FactorCollectionId, ElementId, T.TimeID, @changeTimeId ChangeTimeID, 1 UserID, @MaxWindowID, 0 ChangeID
                            , null TransactionID, SystemSourceID, changeTypeID, 'R' OlapStatus, Comment, Net0 Delta0, Net0
                            , 1 CreatedBy, 1 UpdatedBy, @changeDate CreatedDate, @changeDate UpdatedDate, 1 CurrentRecord, MeasureTypeID
                from dbo.aowCollectedFact FV
                    inner join dbo.timeView T on T.timeID >= FV.timeID
                where FV.currentRecord = 1                              --is current record
                    and T.CurrentHorizon <> 0                           --Indicator that Time is part of current horizon
                    and FV.collectionID = @FactorCollectionId           --factor collections only
                    and FV.timeID = (select  MAX(timeID)                --latest collected fact timeID for given collectionID and elementID
                                        from    aowCollectedFact FV2
                                        where   FV2.collectionId = @FactorCollectionId
                                            and FV2.elementId = FV.elementID)
                    and (((T.ForecastLevel = 'Month') and (T.FirstDayInMonth = T.Date))     --Date is first of month for monthly customers, or
                            or 
                        ((T.ForecastLevel = 'Quarter')and (T.FirstDayInQuarter = T.Date)))  --Date is first of quarter for quarterly customers
                    and not exists (select  1                                               --Record does not already exist in collected fact view
                                    from    aowCollectedFact FV3                            --  for this factor collection, elementID, and timeID
                                    where   FV3.collectionId = @FactorCollectionId
                                        and FV3.elementID = FV.elementId
                                        and FV3.timeID = T.timeID)

This SQL processes over 2 million rows. I need to improve its performance. When I look at the execution plan I find that a lot of time is spent on a Table Spool (Lazy spool) operation (indexes exist in tables and they work well).

How to improve performance for this part ?

Answer 1

Before seeing the execution plan or table indices, I’ll give best educated guesses. First, here are a couple links worth reading.

showplan operator of the week – lazy spool

Table spool/Lazy spool

INDEXING: Take a look at your indices to make sure that they’re all covering the columns that you’re selecting out of the tables. You’ll want to aim to get all the columns included in JOINs and WHERE clauses within the indices. All other columns that are in the SELECT statements should be INCLUDEd, or covered, by the index.

OPERATORS: See if you can get rid of the not equals (“<>”) operators, in favor of a single greater than or less than operator. Can this statement and T.CurrentHorizon <> 0 be changed to this and T.CurrentHorizon > 0?

JOINS: Get rid of the subqueries that are JOINing to tables outside of themselves. For instance, this line and FV2.elementId = FV.elementID might be causing some problems. There’s no reason you can’t move that out of a subquery and into a JOIN to dbo.aowCollectedFact FV, given that you’re GROUPing (DISTINCT) in the main query already.

DISTINCT: Change it to a GROUP BY. I’ve got no reason other than, because it’s good practice and takes two minutes.

LAST NOTE: The exception to all the above might be to leave the final subquery, the IF NOT EXISTS, as a subquery. If you change it to a JOIN, it’ll have to be a LEFT JOIN...WHERE NULL statement, which can actually cause spooling operations. No great way to get around that one.