We have two tables with identical columns and indexing (No indexing at all, basically). We run the same query, which in case of the original table takes 5 seconds to run; in the case of the new table we let it run for 30 minutes and then killed the query.
We updated statistics, but that had no result. We tried a rebuild of the new table to see if defragmenting it would help, but that had no effect either.
On a hunch, we then exported both tables to the same database, in order to see whether or not anything would change there, but we have the exact same results.
I'm kind of baffled how this could be. To make things even more interesting, the original table contains more data than the new table, which should theoretically mean that the new table should finish the query faster.
Does anyone have a possible explanation? I've worked for years as a DBA (though not for a few years now), and frankly, I'm baffled as to why this could possibly happen.
In response to some of the comments, here is the table definition in question:
CREATE TABLE [dbo].[Fact_SubscriptionDetail_Test]( [CustomerSellTo_Key] [int] NULL, [CustomerContactSellTo_Key] [int] NULL, [CustomerRefTo_Key] [int] NULL, [WebAuthUser_Key] [int] NULL, [Country_Key] [int] NULL, [Date_Key] [int] NULL, [SnapshotDate_Key] [int] NULL, [Product_Key] [int] NULL, [License_Key] [int] NULL, [Subscription_Key] [int] NULL, [SubscriptionTypeLostSeatsType_Key] [int] NULL, [M_SubscriptionDetail_NewSeats] [int] NULL, [M_SubscriptionDetail_WinbackSeats] [int] NULL, [M_SubscriptionDetail_RenewedFlexSeats] [int] NULL, [M_SubscriptionDetail_RenewedCommitSeats] [int] NULL, [M_SubscriptionDetail_RenewedSeats] [int] NULL, [M_SubscriptionDetail_ActiveSeatsEndMonth] [int] NULL, [M_SubscriptionDetail_LostNonPaymentSeats] [int] NULL, [M_SubscriptionDetail_LostGraceInactiveSeats] [int] NULL, [M_SubscriptionDetail_LostOtherSeats] [int] NULL, [M_SubscriptionDetail_LostSeats] [int] NULL, [M_SubscriptionDetail_GrossBookings] [numeric](38, 20) NULL, [M_SubscriptionDetail_ActiveFlexSeats] [int] NULL, [M_SubscriptionDetail_ActiveCommitSeats] [int] NULL, [M_SubscriptionDetail_ActiveSeats] [int] NULL, [DateCreated] [datetime] NULL CONSTRAINT [DF__Fact_Subs__DateC__gtfhjCC] DEFAULT (getdate()) ) ON [DATA]
Data in the two tables is not entirely identical, but very similar in nature. The query we run on it ties to some other tables (primarily Data warehouse dimensions).
I defragged (which I agree, doesn't make much sense for a heap, but I figured it wouldn't and couldn't hurt) by simply running
ALTER TABLE Fact_SubscriptionDetail_Test REBUILD.
When we initially checked the query plan, it suggested adding some indexes on the Test table (but not the original, fast table). We also attempted adding a clustered index (PK) on the Test table, but neither that, nor the suggested index by the Execution Plan had any effect.
Here are the execution plans:
There is a difference in the execution paths, making me think the cardinality of the data in the original table is somewhat better. The original table contains some 1.4 mio rows (230 MB), of which 400k are processed by the query. The new table contains 400k rows (52 MB).
This is the full query: http://pastebin.com/bYiaGW1d (slightly edited to remove some sensitive stuff).
The value for "cost treshold for parallellism" on the server is 5.