If an xpath expression has a predicate with value, then element range index
will be used, if available.
For example, in the following two cases a range index on xs:QName("title") will
be used:
a. /bookstore/book[title="XQuery Programming"]
b. /bookstore/book[title="XQuery Programming"]/author
However, if there is no value in the predicate then the range index will not be
used:
c. . /bookstore/book[title]
The same logic applies to where clause. If there is a value in the where clause
and a corresponding element range index, it will be used.
It is different in order by, where an index gets used even though there is no
comparison predicate and value.
In 6.0.1-1, XPath based index can be defined. There are many situations when
MarkLogic server attempts to find matching path range index. Again in order to
use range index, there must be a comparison predicate in the query.
Thanks,
Gajanan
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Michael Blakeley
Sent: Monday, October 22, 2012 9:47 AM
To: MarkLogic Developer Discussion
Subject: Re: [MarkLogic Dev General] Range Index in Where Clause
I would rephrase this as "does the evaluator ever use indexes for where
clauses?" If indexes are used at all, I would expect ranges to work too. And
with 6.0-1.1 the answer seems to be "yes" - but I would still recommend doing
as much as possible in XPath predicates.
Here's a contrived test. In real life it would be very odd to have a
dls:created element in the main document, but since the range index comes
preconfigured it's handy for testing.
declare namespace dls="http://marklogic.com/xdmp/dls";;
xdmp:document-insert(
'test/1',
element a {
element b { 'c' },
element dls:created { xs:dateTime(xs:date('2012-12-01')) } }),
xdmp:document-insert(
'test/2',
element a {
element b { 'c' },
element dls:created { xs:dateTime(xs:date('2012-12-12')) } }) => ()
Now we have two documents that match the /a[b eq 'c'] portion of the
expression, but with different dsl:created values. Let's write a test query
that matches one, but not the other:
declare namespace dls="http://marklogic.com/xdmp/dls";;
xdmp:query-trace(true()),
for $n in /a[ b eq 'c' ]
where $n/dls:created < xs:dateTime(xs:date('2012-12-12'))
return $n
One document should match the dls:created constraint, and the other shouldn't.
The actual results match expectations, but that could be due to filtering.
Let's check the trace:
2012-10-22 09:06:18.661 Info: 8003-cq: at 4:11: Analyzing path for $n:
fn:collection()/a[b eq "c"]
2012-10-22 09:06:18.661 Info: 8003-cq: at 4:11: Step 1 is searchable:
fn:collection()
2012-10-22 09:06:18.661 Info: 8003-cq: at 4:11: Step 2 is searchable: a[b eq
"c"]
2012-10-22 09:06:18.661 Info: 8003-cq: at 4:11: Path is fully searchable.
2012-10-22 09:06:18.661 Info: 8003-cq: at 4:11: Gathering constraints.
2012-10-22 09:06:18.661 Info: 8003-cq: at 4:14: Comparison contributed hash
value constraint: b = "c"
2012-10-22 09:06:18.661 Info: 8003-cq: at 4:11: Step 2 predicate 1 contributed
1 constraint: b eq "c"
2012-10-22 09:06:18.661 Info: 8003-cq: at 4:14: Comparison contributed hash
value constraint: b = "c"
2012-10-22 09:06:18.661 Info: 8003-cq: at 4:11: Step 2 predicate 1 contributed
1 constraint: b eq "c"
2012-10-22 09:06:18.661 Info: 8003-cq: at 4:11: Step 2 contributed 2
constraints: a[b eq "c"]
2012-10-22 09:06:18.661 Info: 8003-cq: at 5:9: Comparison contributed dateTime
range value constraint: dls:created < xs:dateTime("2012-12-12T00:00:00")
2012-10-22 09:06:18.661 Info: 8003-cq: at 4:11: Where clause contributed 1
constraint for $n: $n/dls:created < xs:dateTime("2012-12-12T00:00:00")
2012-10-22 09:06:18.661 Info: 8003-cq: at 4:11: Executing search.
2012-10-22 09:06:18.667 Info: 8003-cq: at 4:11: Selected 1 fragment to filter
That looks exactly as expected (aside from the duplication of b=c, anyway). If
we increment the date to the 13th or the 31st, both fragments match. If we try
to use a QName that doesn't have a range index, we lose the "Comparison
contributed dateTime range value constraint" line. So it seems clear that the
range index is used for the where-clause.
Note that the same expression will be unsearchable according to xdmp:plan,
though. Presumably that's because the FLWOR expression taken as a whole is
unsearchable. That's understandable, but a little frustrating.
Even knowing this, I would still recommend doing as much as possible in XPath
predicates. For one thing, you can use xdmp:plan instead of xdmp:query-trace.
-- Mike
On 22 Oct 2012, at 08:02 , David Sargeant <[email protected]> wrote:
> I'm wondering if ML uses a range index in the where portion of a FLOWR
> expression where applicable. I could only find examples of 'order by'.
> Thanks.
>
>
>
> David
>
> _______________________________________________
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