I have a table with a nonclustered index
on a datetime2
field.
On that same table i have a field (char[1])
which is used to logically delete records and can have 2 distinct values : A
(active) or D
(deleted).
There are 451047 records with the datetime2
field set at NULL
but only 7095 are marked as A
.
Every query in the application looks for active records only, so, every query that's looking for NULLs
in the datetime field was getting really bad estimates and, with that, bad execution plans.
I decided then to create a filtered non clustered index but estimates were still incorrect:
It looks like i still get the old estimates, even if the query is using properly the filtered index. Does anyone know the reason of this behaviour?
These are the new statistics of the filtered index:
Table definition:
CREATE TABLE [TYDATPRD].[HAND00F](
[STDRECSTS] [char](1) NULL,
[HDHAND] [numeric](14, 0) IDENTITY(1,1) NOT NULL,
[HDCHKINDT] [datetime2](7) NULL,
--lots of other columns which I don't think are needed
CONSTRAINT [PK_TYDATPRD_HAND00F] PRIMARY KEY CLUSTERED (
[HDHAND] ASC
) WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [DATA]
) ON [DATA]
Nonclustered filtered index definition:
CREATE NONCLUSTERED INDEX [IX_HAND00F_HDCHKINDT] ON [TYDATPRD].[HAND00F] (
[HDCHKINDT] ASC
)
WHERE [STDRECSTS]='A'
WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [INDEXES]
GO
Best Answer
This is not intuitive but try adding the
char[1]
column to the filtered index definition:In my testing, this led to the right estimates.
As an aside, I noticed that the plans in your question were different - one was properly parameterized and the other had a constant. Be very careful about how you're doing local testing with local variables and typed constants, especially if in the real world this will be done by parameters to stored procedures.
Also, be very careful about case - literals in the where clause like
= 'a'
and= 'A'
will generate different plans if they don't get parameterized, because the query text is different. Never mind that in some collations they won't yield the same results. (And does thatchar(1)
column need to allowNULL
?)