Why insert-select to variable table from XML variable so slow?

Question

I'm trying to insert some data from a XML document into a variable table. What blows my mind is that the same select-into (bulk) runs in no time while insert-select takes ages and holds SQL server process accountable for 100% CPU usage while the query executes.

I took a look at the execution plan and INDEED there's a difference. The insert-select adds an extra "Table spool" node even though it doesn't assign cost. The "Table Valued Function [XML Reader]" then gets 92%. With select-into, the two "Table Valued Function [XML Reader]" get 49% each.

Please explain "WHY is this happening" and "HOW to resolve this (elegantly)" as I can indeed bulk insert into a temporary table and then in turn insert into variable table, but that's just creepy.

I tried this on SQL 10.50.1600, 10.00.2531 with the same results

Here's a test case:

declare @xColumns xml
declare @columns table(name nvarchar(300))

if OBJECT_ID('tempdb.dbo.#columns') is not null drop table #columns

insert @columns select name from sys.all_columns

set @xColumns = (select name from @columns for xml path('columns'))

delete @columns

print 'XML data size: ' + cast(datalength(@xColumns) as varchar(30))

--raiserror('selecting', 10, 1) with nowait

--select ColumnNames.value('.', 'nvarchar(300)') name
--from @xColumns.nodes('/columns/name') T1(ColumnNames)

raiserror('selecting into #columns', 10, 1) with nowait

select ColumnNames.value('.', 'nvarchar(300)') name
into #columns
from @xColumns.nodes('/columns/name') T1(ColumnNames)

raiserror('inserting @columns', 10, 1) with nowait

insert @columns
select ColumnNames.value('.', 'nvarchar(300)') name
from @xColumns.nodes('/columns/name') T1(ColumnNames)

Thanks a bunch!!

Answer 1

This is a bug in SQL Server 2008. Use

insert @columns 
select ColumnNames.value('.', 'nvarchar(300)') name
from @xColumns.nodes('/columns/name') T1(ColumnNames)
OPTION (OPTIMIZE FOR ( @xColumns = NULL ))

This workaround is from an item on the Microsoft Connect Site which also mentions a hotfix for this Eager Spool / XML Reader issue is available (under traceflag 4130).

The reason for the performance regression is explained in a different connect item

The spool was introduced due to a general halloween protection logic (that is not needed for the XQuery expressions).