Re: [bdbxml] indeces are not used in LET clauses of a FLWOR expression

John Snelson <jsnelson@...> writes:

> 
> Indexes are indeed used in let and return statements. You might want to 
> read a series of blog entries I have been writing about indexing in DB XML:
> 
> http://blog.parthenoncomputing.com/dbxml/archives/discussion/index.html
> 
> Also, you can use the "queryplan" comand in the dbxml shell (called 
> "dmxml.exe") to look at which indexes would be used during the execution 
> of your query.
> 
> If you send the query you are using and the output from the "queryplan" 
> command to the list, I can help you understand which indexes are being 
> used, and why.
> 
> John
> 



Hi John,
Thanks for the reply.

In fact I used the query planner command to view the query plan.
I used it again but it didn't used the index then the expression was in the Let
clause.
The database has an edge-attribute-equality-string default index
The query looks like this:

for $p in collection('mydb.db')\\XMI.Content
let $f1 := $p//SSL.Core.ServiceProfile/@name[. = 'TaxiReservation'],
    $r := if (fn:count($f1) > 0) then 1 else 0
where $r > 0 
oder by $r descending
return <group><id> {dbxml:metadata('id' ,$p)} </id><rank>{$r}</rank></group>

I want the index to be used when calculating the $f1 variable. 
 if I put the expression in the where clase the index is used normally:
(<OQPlan>V(edge-attribute-equality-string,
SSL.Core.ServiceProfile.@name,=,'TaxiReservation')</OQPlan>)

but when I ask for the above query the following query plan is produced:

<XQuery>
  <FLWOR>
    <ForBinding name="p">
      <Navigation>
        <QueryPlanFunction result="collection">
          <OQPlan>U</OQPlan>
        </QueryPlanFunction>
        <Step axis="descendant-or-self" uri="*" name="*" nodeType="*"/>
        <Step axis="child" name="XMI.content" nodeType="element"/>
      </Navigation>
    </ForBinding>
    <LetBinding name="f1">
      <Navigation>
        <Variable name="p"/>
        <Step axis="descendant-or-self" uri="*" name="*" nodeType="*"/>
        <Step axis="child" name="SSL.Core.ServiceProfile" nodeType="element"/>
        <Step axis="attribute" name="name" nodeType="attribute">
          <Predicates>
            <Operator name="equal">
              <ContextItem/>
              <Sequence>
                <AnyAtomicTypeConstructor value="TaxiReservation"
typeuri="http://www.w3.org/2001/XMLSchema" typename="string"/>
              </Sequence>
            </Operator>
          </Predicates>
        </Step>
      </Navigation>
    </LetBinding>
    <LetBinding name="r">
      <If>
        <Test>
          <Operator name="greater_than">
            <Function name="{http://www.w3.org/2004/07/xpath-functions}:count">
              <Variable name="f1"/>
            </Function>
            <Sequence>
              <AnyAtomicTypeConstructor value="0"
typeuri="http://www.w3.org/2001/XMLSchema" typename="integer"/>
            </Sequence>
          </Operator>
        </Test>
        <Then>
          <Sequence>
            <AnyAtomicTypeConstructor value="1"
typeuri="http://www.w3.org/2001/XMLSchema" typename="integer"/>
          </Sequence>
        </Then>
        <Else>
          <Sequence>
            <AnyAtomicTypeConstructor value="0"
typeuri="http://www.w3.org/2001/XMLSchema" typename="integer"/>
          </Sequence>
        </Else>
      </If>
      <Where>
        <Operator name="greater_than">
          <Variable name="r"/>
          <Sequence>
            <AnyAtomicTypeConstructor value="0"
typeuri="http://www.w3.org/2001/XMLSchema" typename="integer"/>
          </Sequence>
        </Operator>
      </Where>
    </LetBinding>
    <Sort>
      <Specification modifier="descending|empty_least">
        <Variable name="r"/>
      </Specification>
    </Sort>
    <DOMConstructor type="element">
      <Name>
        <Sequence>
          <AnyAtomicTypeConstructor value="group"
typeuri="http://www.w3.org/2001/XMLSchema" typename="string"/>
        </Sequence>
      </Name>
      <Children>
        <DOMConstructor type="element">
          <Name>
            <Sequence>
              <AnyAtomicTypeConstructor value="docid"
typeuri="http://www.w3.org/2001/XMLSchema" typename="string"/>
            </Sequence>
          </Name>
          <Children>
            <Variable name="p"/>
          </Children>
        </DOMConstructor>
        <DOMConstructor type="element">
          <Name>
            <Sequence>
              <AnyAtomicTypeConstructor value="rank"
typeuri="http://www.w3.org/2001/XMLSchema" typename="string"/>
            </Sequence>
          </Name>
          <Children>
            <Variable name="r"/>
          </Children>
        </DOMConstructor>
      </Children>
    </DOMConstructor>
  </FLWOR>
</XQuery>

So, I guess that no index is used...
If you have any other ideas or recomedations please post.

Thanx in advance 
George



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