[MarkLogic Dev General] query execution plan question

Mon Jul 16 11:46:44 PDT 2007

No it's not.  doc is, but not body.

Jason Hunter wrote:
> Hi Mike,
>
> You don't mention it, but <body> is a fragment root, right?
>
> -jh-
>
> Mike Sokolov wrote:
>> I stumbled on a surprising behavior and wanted to know if anybody has 
>> a good heuristic for identifying this odd little performance 
>> bottleneck in MarkLogic so as to avoid it (there is an easy 
>> alternative expression that works better).
>>
>> If I run the query:
>>
>> cts:search(//doc, cts:element-attribute-value-query 
>> (xs:QName('doc'),  xs:QName('e:id'),  '/opr/t125/e1646'))/body,
>> I get results quickly (.002 sec)
>>
>> However the very similar query
>>
>> //doc[@id='/opr/t125/e1646']/body
>>
>> takes 1.37 sec, almost 1000x slower!
>>
>> query-meters reveals that it involves 85,000 tree-cache-hits and 
>> touches various documents, when in fact there is only a single 
>> fragment in the system matching the given attribute value.  Similarly 
>> query-trace says "Selected 7212 fragments to filter," although I'm 
>> not sure why.
>>
>> Somehow adding the /body element seems to be the culprit since:
>>
>> //doc[@id='/opr/t125/e1646']
>>
>> is quite fast, as is
>>
>> let $doc := //doc[@id='/opr/t125/e1646']
>>  return $doc/body
>>
>>
>> Does this seem surprising to anyone else?  I have been trying to use 
>> xpath whenever I can due to its elegant simplicity in comparison with 
>> the cts: functional operators, and I thought I had a good internal 
>> sense of when an expression could get run quickly using indexes, but 
>> now I am beginning to worry about other possible performance 
>> bottlenecks I may have overlooked.
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