How to optimise the 'XQuery' SQL

问题

I have an XML hierarchy like this in an XML type column of a table with 10,000 records-

<Root>
     <Elem1>
         <Parent1>
              <Separator>
                  <Child1/>
              </Separator>
         </Parent1>
     </Elem1>
</Root>

I have a query like this -

DECLARE @Root VARCHAR(50)
DECLARE @Entity VARCHAR(50)
DECLARE @ParentNode VARCHAR(50)
DECLARE @Separator VARCHAR(50)
DECLARE @ChildNode VARCHAR(50)


SET @Root = 'Root'
SET @Entity = 'Elem1'
SET @ParentNode = 'Parent1'
SET @Separator = 'separator'
SET @ChildNode = 'Child1'

select Parent.P.value('.', 'varchar(max)') as MyValue, 
T.uniqueId, T.XMLCol
from [XMLTable] as T
cross apply 
 (SELECT
         XMLTable.XMLCol.query('(/*[local-name()=sql:variable("@Root")]/*[local-name(.)=sql:variable("@Entity")]/*[local-name(.)=sql:variable("@ParentNode")]/*[local-name(.)=sql:variable("@Separator")]/*[local-name(.)=sql:variable("@ChildNode")])[1]'
 )  as Parent(P)

How can I further Optimize this query. Currently, it is taking 2 secs and if I do further INNER JOINS, it adds up to the time. I tried creating a PRIMARY Index on the XML Column, but it takes more time !

EDIT- If I hardcode the path instead of using variables, then it takes less than a second. But, I want it in a Table-Valued function and cannot hardcode path ?

EDIT - Solution

select x.value('(Parent1/Separator1/Child1)[1]', 'varchar(max)') as Col1,
x.value('(Parent2/Separator2/Child2)[1]', 'varchar(max)') as Col2,
x.value('(Parent3)[1]', 'varchar(max)') as Col3
from [XMLTable] T 
cross apply T.XMLCOL.nodes('/Root/Elem1') a(x)

The above query takes just about one sec. It seems to be the fastest of all. So, instead of going for a Table valued function with params, the above query can be dynamically prepared and executed on the fly using ADO.NET capabilities

Correct me please ... ?

回答1:

If you only want one value out of every row there is no need to use cross apply.

select XMLCol.value('(/*[local-name()=sql:variable("@Root")]
                      /*[local-name(.)=sql:variable("@Entity")]
                      /*[local-name(.)=sql:variable("@ParentNode")]
                      /*[local-name(.)=sql:variable("@Separator")]
                      /*[local-name(.)=sql:variable("@ChildNode")])[1]', 'varchar(max)')
from XMLTable

Another way to get the same is to use FLWOR. In my limited tests this will run a bit faster.

select XMLCol.value('(for $n1 in /*,
                          $n2 in $n1/*,
                          $n3 in $n2/*,
                          $n4 in $n3/*,
                          $n5 in $n4/*
                      where $n1[local-name(.) = sql:variable("@Root")] and
                            $n2[local-name(.) = sql:variable("@Entity")] and
                            $n3[local-name(.) = sql:variable("@ParentNode")] and
                            $n4[local-name(.) = sql:variable("@Separator")] and
                            $n5[local-name(.) = sql:variable("@ChildNode")]
                      return $n5
                     )[1]', 'varchar(max)')
from XMLTable

来源：https://stackoverflow.com/questions/7064207/how-to-optimise-the-xquery-sql

标签

sql-server

sql-server-2008

query-optimization

sql-server-2008-r2

xquery-sql