Extremely Slow Table

[garydevstore]

Hi,

I have a table defined as
CREATE TABLE [SH_Data] (
[ID] [int] IDENTITY (1, 1) NOT NULL ,
[Date] [datetime] NULL ,
[Time] [datetime] NULL ,
[TroubleshootId] [int] NOT NULL ,
[ReasonID] [int] NULL ,
[reason_desc] [nvarchar] (255) COLLATE SQL_Latin1_General_CP1_CS_AS
NULL ,
[maj_reason_id] [int] NULL ,
[maj_reason_desc] [nvarchar] (255) COLLATE
SQL_Latin1_General_CP1_CS_AS NULL ,
[ActionID] [int] NULL ,
[action_desc] [nvarchar] (255) COLLATE SQL_Latin1_General_CP1_CS_AS
NULL ,
[WinningCaseTitle] [nvarchar] (255) COLLATE
SQL_Latin1_General_CP1_CS_AS NULL ,
[Duration] [int] NULL ,
[dm_version] [nvarchar] (255) COLLATE SQL_Latin1_General_CP1_CS_AS
NULL ,
[ConnectMethod] [nvarchar] (255) COLLATE
SQL_Latin1_General_CP1_CS_AS NULL ,
[dm_motive] [nvarchar] (255) COLLATE SQL_Latin1_General_CP1_CS_AS
NULL ,
[HnWhichWlan] [nvarchar] (255) COLLATE SQL_Latin1_General_CP1_CS_AS
NULL ,
[RouterUsedToConnect] [nvarchar] (255) COLLATE
SQL_Latin1_General_CP1_CS_AS NULL ,
[OS] [nvarchar] (255) COLLATE SQL_Latin1_General_CP1_CS_AS NULL ,
[WinXpSp2Installed] [nvarchar] (255) COLLATE
SQL_Latin1_General_CP1_CS_AS NULL ,
[Connection] [nvarchar] (255) COLLATE SQL_Latin1_General_CP1_CS_AS
NULL ,
[Login] [nvarchar] (255) COLLATE SQL_Latin1_General_CP1_CS_AS NULL
,
[EnteredBy] [nvarchar] (255) COLLATE SQL_Latin1_General_CP1_CS_AS
NULL ,
[Acct_Num] [int] NULL ,
[Site] [nvarchar] (255) COLLATE SQL_Latin1_General_CP1_CS_AS NULL ,
CONSTRAINT [PK_SH_Data] PRIMARY KEY CLUSTERED
(
[TroubleshootId]
) ON [PRIMARY]
) ON [PRIMARY]
GO
Which contains 5.6 Million rows and has non clustered indexes on Date,
ReasonID, maj_Reason, Connection. Compared to other tables on the same
server this one is extremely slow. A simple query such as :

SELECT
SD.reason_desc,
SD.Duration,
SD.maj_reason_desc,
SD.[Connection],
SD.aolEnteredBy

FROM dbo.[Sherlock Data] SD

Where SD.[Date] > Dateadd(Month,-2,Getdate())

takes over 2 minutes to run ! I realise the table contains several
large columns which make the table quite large but unfortunately this
cannot be changed for the moment.

How can i assess what is causing the length of Query time ? And what
could i possibly do to speed this table up ? The database itself is
running on a dedicated server which has some other databases. None of
which have this performance issue.

Anyone have any ideas ?

[Jens]

Do other queries which benefit of indexes also have bad performance ?
Where SD.[Date] > Dateadd(Month,-2,Getdate())

does a row based comparison, not using the indexes or what does the
query plan tells you about it ?

(Markt the query in QA and press CTRL+L) to see it.

Jens Suessmeyer.

[Simon Hayes]

One possiblility is to change the PK to nonclustered and the index on
[Date] to clustered. If you often do range/grouping queries based on
[Date] then that should be useful, but it might also impact queries
using TroubleshootId, so you need to test any change with a number of
representative queries.

Other general advice would be to review the query plan in QA (Ctrl+K),
run UPDATE STATISTICS on the table, and also try tracing a typical
workload and running it through the Index Tuning Wizard to see what it
can recommend.

If you need more specific comments, you should post the query plan
(using SET SHOWPLAN_TEXT), and it might also be useful to know how many
rows are returned by the query.

Simon

[garydevstore]

Yea ive tried a test on both and a query using the indexes take about
20 seconds less to run.

I tried the CTRL + L but its not making much sense to me.

[Erland Sommarskog]

garydevstore (Ga***********@gmail.com) writes:

Which contains 5.6 Million rows and has non clustered indexes on Date,
ReasonID, maj_Reason, Connection. Compared to other tables on the same
server this one is extremely slow. A simple query such as :
Maybe some terminology is in order here. A road can be fast, but that
does not help you, if you car has a steering wheel out of order causing
you to zig-zag over the road. A car can be fast, but that does not help
if the road is in poor condition, so you cannot driver faster than 30 km/h
anyway.

In this case, the table is the road, and the query plan is the car. A
table itself does not move, but it can be badly fragmented in which case
it can be slow to drive through.

More likely, the query plan is not the best for the query. This is your
query:
SELECT
SD.reason_desc,
SD.Duration,
SD.maj_reason_desc,
SD.[Connection],
SD.aolEnteredBy
FROM dbo.[Sherlock Data] SD
Where SD.[Date] > Dateadd(Month,-2,Getdate())

There is a non-clustered index on Date. Assuming that rows are added
to this table regularly, there are presumably quite a few rows that
fits this condition. There are two ways for the optimizer to evaluate
this query: using the index, or scanning the table. The index is good
if only few rows are hit, but if many rows are hit the table scan is
faster. This is because, with the index you will need to read the same
page more than once.

The optimizer makes it choice of plan from the statistics SQL Server
has sampled about the table. The statistics may be out of date (even
if by default SQL Server auto-updates statistics). Try an UPDATE
STATISTICS WITH FULLSCAN, to see if this makes any difference.

But the road can also be in poor condition, that is the table can be
badly fragmented. This can be analysed with DBCC SHOWCONTIG and
remedied with DBCC DBREINDEX.

As suggested in other posts, you should look at the query plan, and see
if it says Clustered Index Scan or Index Seek + Bookmark Lookup.


--
Erland Sommarskog, SQL Server MVP, es****@sommarskog.se

Books Online for SQL Server SP3 at
http://www.microsoft.com/sql/techinf...2000/books.asp

[Robert Klemme]

Simon Hayes wrote:

One possiblility is to change the PK to nonclustered and the index on
[Date] to clustered. If you often do range/grouping queries based on
[Date] then that should be useful, but it might also impact queries
using TroubleshootId, so you need to test any change with a number of
representative queries.
A covering index might be an option, too, especially if there are several
queries with multiple criteria.

One question to the OP: why do you have Date and Time both as timestamp
columns? Other remarkable things: all char columns seem to be unicode
(nvarchar) and have length (255). You might save space by changing to
varchar (if possible) and / or reducing the length. Also, this doesn't
really look like a normalized schema. I would at least expect having ids
for EnteredBy and probably some others.
Other general advice would be to review the query plan in QA (Ctrl+K),
run UPDATE STATISTICS on the table, and also try tracing a typical
workload and running it through the Index Tuning Wizard to see what it
can recommend.

If you need more specific comments, you should post the query plan
(using SET SHOWPLAN_TEXT), and it might also be useful to know how
many rows are returned by the query.

Plus complete index DDL.

robert