Xapian discuss - Jan 2018 - How to get the serialise score returned in Xapian::KeyMaker->operator().

HI, all,

I am a user of Xapian, and now I have a problem in using it. 

After using boolean terms to get some candidates of documents (still too much),
we want sorted them by self-defined function which is used in
Xapian::KeyMaker->operator(). But how can I get the serialise score in
Xapian::MSetIterator object.

c++ code likes this:

class SortKeyMaker : public Xapian::KeyMaker {
std::string operator()(const Xapian::Document& doc) const {
    double score = self-defined-function(doc);
    return Xapian::sortable_serialise(score);   // How get this value in
Xapian::MSetIterator
}






--
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On Fri, Dec 15, 2017 at 03:10:48PM +0800, 张少华 wrote:
> After using boolean terms to get some candidates of documents (still
> too much), we want sorted them by self-defined function which is used
> in Xapian::KeyMaker->operator(). But how can I get the serialise score
> in Xapian::MSetIterator object.
> 
> c++ code likes this:
> 
> class SortKeyMaker : public Xapian::KeyMaker {
> std::string operator()(const Xapian::Document& doc) const {
>     double score = self-defined-function(doc);
>     return Xapian::sortable_serialise(score);   // How get this value in
Xapian::MSetIterator
> }
Unfortunately the sort key isn't currently exposed via the public API.
It's available internally and it seems like it ought to be accessible
but there's no accessor method for it - I can add one but that won't
help for existing releases.

A possible workaround (and perhaps a better approach) would be to
set BoolWeight as the weighting scheme, then feed in your score as
a weight using a PostingSource.  Then it's available via get_weight()
on the MSetIterator object:

https://getting-started-with-xapian.readthedocs.io/en/latest/advanced/postingsource.html

You may find that's faster because it'll mean sorting by doubles instead
of strings.

Also you'll get told the lowest weight that's of interest each time
you need to calculate a score - if your scoring function is expensive
you might be able to skip documents if a cheap calculation can show they
can't beat this lowest weight.

Cheers,
    Olly

On Sat, Dec 16, 2017 at 10:11:40PM +0000, Olly Betts
wrote:
> Unfortunately the sort key isn't currently exposed via the public API.
> It's available internally and it seems like it ought to be accessible
> but there's no accessor method for it - I can add one but that
won't
> help for existing releases.
I've added MSetIterator::get_sort_key() to master in
9f807b83ab61a943a355a9ff6733299eab8e6bb1, and backported to the
RELEASE/1.4 branch in 93ea6216fe8141d6223c869c6bccb039414db0fa, so this
should be in 1.4.6 when that's released.

Cheers,
    Olly

HI, We have an index database of products, about 20 million. We had constructed
the title and description of products into posting list, and also stored some
values of properties into slot, such as the price, comment count, production
date, click number of the products.

Now we want select some products which satisties specific condition, such as
contain the term of "shirt" and "white", and "price
<= 500" and "comment count >= 100", "1000 <=
click_number <= 2000".

And we have two methods:
1, use xapian.Query for terms and xapian.Query.OP_VALUE_RANGE to filter the
value.
2, use xapian.Query for terms to get candidates, then use xapian.MatchDecider to
filter the value.

Which method get a better performance?

On Thu, Jan 11, 2018 at 05:11:20PM +0800, 张少华 wrote:
> HI, We have an index database of products, about 20 million. We had
> constructed the title and description of products into posting list,
> and also stored some values of properties into slot, such as the
> price, comment count, production date, click number of the products.
> 
> Now we want select some products which satisties specific condition,
> such as contain the term of "shirt" and "white", and
"price <= 500"
> and "comment count >= 100", "1000 <= click_number
<= 2000".
> 
> And we have two methods:
> 1, use xapian.Query for terms and xapian.Query.OP_VALUE_RANGE to
> filter the value.
> 2, use xapian.Query for terms to get candidates, then use
> xapian.MatchDecider to filter the value.
> 
> Which method get a better performance?
Use OP_VALUE_RANGE for a range check on a value - then the matcher
actually knows what the check being performed is, which means it can
optimise better.

That's probably doubly true if you're using Xapian via one of the
bindings (which I'm guessing you are from "xapian.Query") since a
MatchDecider subclass in that language will require a call between
languages for every candidate document considered, and that's likely to
be significantly slower than staying within C++.  The matcher will try
to call it as little as it can, but in cases where a lot of documents
match without the filter but the filter rejects most of them it may need
to call it millions of times if you have 20 million documents.

Cheers,
    Olly

>A possible workaround (and perhaps a better approach) would be to
>set BoolWeight as the weighting scheme, then feed in your score as
>a weight using a PostingSource.  Then it's available via get_weight()
>on the MSetIterator object:
>
>https://getting-started-with-xapian.readthedocs.io/en/latest/advanced/postingsource.html
>
>You may find that's faster because it'll mean sorting by doubles
instead
>of strings.

We realise our score function using PostingSource instead of using KeyMaker,  we
reference your python example and source code of xapian, the simple demo  is
here.
https://github.com/xiangqianzsh/xapian_leaning/blob/master/postingsource/ExternalWeightPostingSource.h
But we found that using PostingSource is more slower than KeyMaker.  I think the
reason maybe:
We only use one Xapian::Query of PostingSource and the upper bound of our
get_weight() can not work on a single PostingSource.  So some optimizing 
don't work, but waste time  oppositely.
How do you think about this?




Also, We found the BM25 algorithm is fast in xapian, so we think if we can
modify our get_weight() function to adjust the BM25 algorithm. If so, the type
of termfreq of document should be double. I am wondering if it works just
re-typedef Xapian::termcount to double? Does it has a negative impact on other
place of xapian source.


Thanks.