Question

I am using a simple table with 6 columns, 3 of which are of XML type, not schema-constrained.
When the table reaches a size around 120,000 or 150,000 rows, I see a dramatic performance cost in doing any query in the table. For comparison, I have another table, which grows in size at about the same rate, but only contain scalar types (int, datetime, a few float columns). That table performs perfectly fine even after 200,000 rows.
And by the way, I am not using XQuery on the xml columns, i am only using regular SQL query statements.

Some specifics: both tables contain a DateTime field called SampleTime.
a statement like (it’s in a stored procedure but I show you the actual statement)

SELECT MAX(sampleTime) SampleTime
FROM dbo.MyRecords
WHERE PlacementID=@somenumber

takes 0 seconds on the table without xml columns, and anything from 13 to 20 seconds on the table with XML columns. That depends on which drive I set my database on. At the moment it sits on a different spindle (not C:) and it takes 13 seconds.

Has anyone seen this behavior before, or have any hint at what I am doing wrong?
I tried this with SQL 2008 EXPRESS and the full-blown SQL Server 2008, that made no difference.
Oh, one last detail: I am doing this from a C# application, .NET 3.5, using SqlConnection, SqlReader, etc..

I’d appreciate some insight into that, thanks!

Sam

Answer 1

Do you have an index on PlacementID and sampleTime in that order?

Both the size of the table and columns types are irrelevant if an index can satisfy the query (“covering”)