Query runs slow with date expression, but fast with string literal

Question

I am running a query with below condition in SQL Server 2008.

Where FK.DT = CAST(DATEADD(m, DATEDIFF(m, 0, getdate()), 0) as DATE)  

Query takes forever to run with above condition, but if just say

Where FK.DT = '2013-05-01' 

it runs great in 2 mins. FK.DT key contains values of only starting data of the month.

Any help, I am just clueless why this is happening.

Answer 1

This could work better:

Where FK.DT = cast(getdate() + 1 - datepart(day, getdate()) as date)

Unless you are running with trace flag 4199 on there is a bug that affects the cardinality estimates. At the time of writing

SELECT DATEADD(m, DATEDIFF(m, getdate(), 0), 0), 
       DATEADD(m, DATEDIFF(m, 0, getdate()), 0)

Returns

+-------------------------+-------------------------+
| 1786-06-01 00:00:00.000 | 2013-08-01 00:00:00.000 |
+-------------------------+-------------------------+

The bug is that the predicate in the question uses the first date rather than the second when deriving the cardinality estimates. So for the following setup.

CREATE TABLE FK
(
ID INT IDENTITY PRIMARY KEY,
DT DATE,
Filler CHAR(1000) NULL,
UNIQUE (DT,ID)
)

INSERT INTO FK (DT)
SELECT TOP (1000000) DATEADD(m, DATEDIFF(m, getdate(), 0), 0)
FROM master..spt_values o1, master..spt_values o2
UNION ALL
SELECT               DATEADD(m, DATEDIFF(m, 0, getdate()), 0)

Query 1

SELECT COUNT(Filler)
FROM FK
WHERE FK.DT = CAST(DATEADD(m, DATEDIFF(m, 0, getdate()), 0) AS DATE)  

Estimates that the number of matching rows will be 100,000. This is the number that match the date '1786-06-01'.

But both of the following queries

SELECT COUNT(Filler)
FROM FK
WHERE FK.DT = CAST(GETDATE() + 1 - DATEPART(DAY, GETDATE()) AS DATE)

SELECT COUNT(Filler)
FROM FK
WHERE FK.DT = CAST(DATEADD(m, DATEDIFF(m, 0, getdate()), 0) AS DATE)  
OPTION (QUERYTRACEON 4199)

Give this plan

Due to the much more accurate cardinality estimates the plan now just does a single index seek rather than a full scan.