Wrong index usage in execution plan

Question

I have one large document in container created with DBXML_INDEX_NODES flag.
There is an index by "high" attribute node
self.container.addIndex ( "", "high", "edge-attribute-equality-double", self.uc )
About 50% of elements in document have structure:
<record><data high="XX.XX" /></record>
Which are inside "session" element which is itself inside "log" element. Only one "log" and one "session" element exist in document
Index by “high” attribute node is very selective. Below is execution plan (changing index to "node" doesn't change things). Index is used on "presence" of "high" attribute and not "value" of it. I think using it for value would improve the speed of the query. Here is the query produced the execution plan below "collection ( 'test.dbxml' )/log/session/record[data/@high=79.37]"
<XQuery>
<Navigation>
<QueryPlanFunction result="collection" container="seasonal.dbxml">
<OQPlan>P(edge-attribute-equality-double,prefix,data.@high)</OQPlan>
</QueryPlanFunction>
<Step axis="child" name="log" nodeType="element"/>
<Step axis="child" name="session" nodeType="element"/>
<Step axis="child" name="record" nodeType="element"/>
<DbXmlFilter>
<Navigation>
<Step axis="child" name="data" nodeType="element">
<OQPlan>P(edge-attribute-equality-double,prefix,data.@high)</OQPlan>
</Step>
<Step axis="attribute" name="high" nodeType="attribute"/>
<DbXmlCompare name="equal">
<OQPlan>P(edge-attribute-equality-double,prefix,@high)</OQPlan>
<Sequence>
<AnyAtomicTypeConstructor value="79.37" typeuri="http://www.w3.org/2001/XMLSchema" typename="decimal"/>
</Sequence>
</DbXmlCompare>
</Navigation>
</DbXmlFilter>
</Navigation>
</XQuery>
Also, why is the “typename” attribute of “AnyAtomicTypeConstructor” element is “decimal”?
Removing flag DBXML_INDEX_NODES on the container increases speed significantly and produces this execution plan, speed is not acceptable though.
<XQuery>
<Navigation>
<QueryPlanFunction result="collection" container="seasonal.dbxml">
<OQPlan>P(edge-attribute-equality-double,prefix,data.@high)</OQPlan>
</QueryPlanFunction>
<Step axis="child" name="log" nodeType="element"/>
<Step axis="child" name="session" nodeType="element"/>
<Step axis="child" name="record" nodeType="element"/>
<DbXmlFilter>
<Navigation>
<Step axis="child" name="data" nodeType="element"/>
<Step axis="attribute" name="high" nodeType="attribute"/>
<DbXmlCompare name="equal">
<Sequence>
<AnyAtomicTypeConstructor value="79.37" typeuri="http://www.w3.org/2001/XMLSchema" typename="decimal"/>
</Sequence>
</DbXmlCompare>
</Navigation>
</DbXmlFilter>
</Navigation>
</XQuery>
Using index lookup allows achieving speed I need. I'd like to know how to properly write the query and what index to use in my case, though.

Answer 1

Your query is not using the index because the numeric literal you are using is an xs:decimal, not an xs:double. Wrapping your literal in a cast should make the query use your index:
collection ( 'test.dbxml' )/log/session/record[data/@high=xs:double(79.37)]
Having said that, this is only the case for DB XML 2.2.13 and before. It turns out that DB XML has been choosing the wrong type to perform comparisons under - and this has been remedied in the forthcoming version. In your case, the new behaviour will mean that the comparison will always be performed as an xs:double.
John