[MarkLogic Dev General] query execution plan question

[Mike Sokolov]

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.