Re: [bdbxml] Varying XQuery performance with change in 'return' clause

Deepak Jois wrote:
> =====
> for $x in collection("genbank_bct.dbxml")/INSDSet/INSDSeq[INSDSeq_locus="AE017283"]
> return
> $x
> =====
> Returns instantly.

This could just as well be written:

collection("genbank_bct.dbxml")/INSDSet/INSDSeq[INSDSeq_locus="AE017283"]

> whereas,
> =====
> for $x in collection("genbank_bct.dbxml")/INSDSet/INSDSeq
> let $y := $x/INSDSeq_locus
> where $y = "AE017283"
> return
> $x
> =====
> 
> Takes extremely long!

It is the let clause, that seems to force the query to execute as a loop 
instead of a pipeline. I'll have to look into how we can optimise these 
situations.

In cases where the comparison operator does not form a "join" (it has 
nodes on both sides of it), the size of the node producing expression 
does not matter. You can just as well write:

for $x in collection("genbank_bct.dbxml")/INSDSet/INSDSeq
where $x/INSDSeq_locus = "AE017283"
return
$x

In fact, if you look at the query plan for this query and compare it to 
your first query, they will be almost identical.

John

-- 
John Snelson, Berkeley DB XML Engineer
Sleepycat Software, Inc
http://www.sleepycat.com

Contracted to Sleepycat through Parthenon Computing Ltd
http://blog.parthcomp.com/dbxml


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