MS SQL XML - Re: FOR XML performance question

Author

Re: FOR XML performance question

Hilary Cotter

2006-01-06, 9:24 am

I'm cross posting this to microsoft.public.sqlserver.xml, a more appropriate
forum for this question.
--
Hilary Cotter
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"Lee" <Lee@discussions.microsoft.com> wrote in message
news:F314E284-6E34-403C-94C1- B307682B1418@microso
ft.com...
>I am currently rewriting a data access component to make use of the FOR XML
> SQL statement to return XML data as an ADO stream from a specified
> source.
> The older current component requests this data using an ADO recordset and
> then manually converts this to XML.
>
> I have run several performance tests comparing the 2 and on narrow and
> medium width tables I have found that the performance gain is massive
> (approx
> 80% gain). However, when I run the 2 on very wide tables, ones which
> contain
> text/ntext columns, FOR XML only performs about 10% better pulling back 1
> row
> but pulling back 20 rows it becomes over twice as slow as the older
> component.
>
> Can anyone suggest why? Or even better, any methods/tips that could
> improve
> performance in this instance?
>
> Thanks in advance.

Lee

2006-01-06, 11:24 am

I am currently rewriting a data access component to make use of the FOR XML
SQL statement to return XML data as an ADO stream from a specified source.
The older current component requests this data using an ADO recordset and
then manually converts this to XML.

I have run several performance tests comparing the 2 and on narrow and
medium width tables I have found that the performance gain is massive (approx
80% gain). However, when I run the 2 on very wide tables, ones which contain
text/ntext columns, FOR XML only performs about 10% better pulling back 1 row
but pulling back 20 rows it becomes over twice as slow as the older component.

Can anyone suggest why? Or even better, any methods/tips that could improve
performance in this instance?

Thanks in advance.

-----------------------------------------------------------------------

In addition to the above I have done some further investigation. On a query
which returns the top row from a table the FOR XML method performed 83.5%
faster than the recordset version. However, when I run a where query which I
know returns a single row the FOR XML method performance plunges and is
actually 6% slower than the recordset version.

Is SQLXML just one of those things which seems like a great idea but has no
real practical use in an enterprise environment? I find it very frustrating
that its performance is superb in some situations but is so awful in others.
Is it a work in progress?

That said, are there any resources which discuss various ways to pull data
from SQL server 2000 as(and convert to) XML format? Surely there is a better
way than using a ADO recordset as described above?

Thanks.

Michael Rys [MSFT]

2006-01-12, 8:24 pm

Hi Lee

This is hard to answer without having more specifics.

How does your FOR XML query look like? How does it compare to the previous
query, what indices do yo have on it?

Etc.

Best regards
Michael

"Lee" <Lee@discussions.microsoft.com> wrote in message
news:95F97E2B-CCE3-415F-AEBE- 25E7B499825E@microso
ft.com...
>I am currently rewriting a data access component to make use of the FOR XML
> SQL statement to return XML data as an ADO stream from a specified
> source.
> The older current component requests this data using an ADO recordset and
> then manually converts this to XML.
>
> I have run several performance tests comparing the 2 and on narrow and
> medium width tables I have found that the performance gain is massive
> (approx
> 80% gain). However, when I run the 2 on very wide tables, ones which
> contain
> text/ntext columns, FOR XML only performs about 10% better pulling back 1
> row
> but pulling back 20 rows it becomes over twice as slow as the older
> component.
>
> Can anyone suggest why? Or even better, any methods/tips that could
> improve
> performance in this instance?
>
> Thanks in advance.
>
> -----------------------------------------------------------------------
>
> In addition to the above I have done some further investigation. On a
> query
> which returns the top row from a table the FOR XML method performed 83.5%
> faster than the recordset version. However, when I run a where query
> which I
> know returns a single row the FOR XML method performance plunges and is
> actually 6% slower than the recordset version.
>
> Is SQLXML just one of those things which seems like a great idea but has
> no
> real practical use in an enterprise environment? I find it very
> frustrating
> that its performance is superb in some situations but is so awful in
> others.
> Is it a work in progress?
>
> That said, are there any resources which discuss various ways to pull data
> from SQL server 2000 as(and convert to) XML format? Surely there is a
> better
> way than using a ADO recordset as described above?
>
> Thanks.

Lee

2006-01-13, 7:23 am

The query is a simple :-

SELECT stuff
FROM table
WHERE condition (optional)
FOR XML RAW

There is a single index on the primary key of the table and that was the
field I did my where clause on as described in my above posts.

"Michael Rys [MSFT]" wrote:

> Hi Lee
>
> This is hard to answer without having more specifics.
>
> How does your FOR XML query look like? How does it compare to the previous
> query, what indices do yo have on it?
>
> Etc.
>
> Best regards
> Michael
>
> "Lee" <Lee@discussions.microsoft.com> wrote in message
> news:95F97E2B-CCE3-415F-AEBE- 25E7B499825E@microso
ft.com...
>
>
>

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