Table Variable vs Parallelism

There are many myths surrounding table variables and one of the most common is probably the 'in memory' story. There are however a couple of other interesting facts about temp variables which you should also know about. The guys from the Storage Engine have an excellent post about table variables so you definitely have to read it.

One of the things that caught my eye in the post was the fact that queries do not go parallel when table variables are involved. This was actually something I had never come across so I decided to put it to the test. I borrowed a query from Craig Freedman who has an excellent series on parallelism. Here we go:

CREATE TABLE T (A INT, B INT IDENTITY, C INT, D INT)
CREATE CLUSTERED INDEX TA ON T(A)

SELECT COUNT(*) FROM T OPTION (MAXDOP 0)

UPDATE STATISTICS T WITH ROWCOUNT = 1000000, PAGECOUNT = 100000

SELECT COUNT(*) FROM T OPTION (RECOMPILE, MAXDOP 0)

DECLARE @t AS TABLE
(NumberOfRows int)

INSERT INTO @t
SELECT COUNT(*) FROM T OPTION (RECOMPILE, MAXDOP 0)

For those of you who want to try it with a temp table:

CREATE TABLE #t
(NumberOfRows int)

INSERT INTO #t
SELECT COUNT(*) FROM T OPTION (RECOMPILE, MAXDOP 0)

DROP TABLE #t