[Xapian-devel] Re: [Xapian-commits] 9476: trunk/xapian-core/ trunk/xapian-core/include/xapian/ trunk/xapian-core/queryparser/ trunk/xapian-core/tests/

Richard Boulton richard at lemurconsulting.com
Fri Oct 19 11:20:49 BST 2007

olly wrote:
> SVN root:       svn://svn.xapian.org/xapian
> Changes by:     olly
> Revision:       9476
> Date:           2007-10-19 03:47:11 +0100 (Fri, 19 Oct 2007)
> Log message (14 lines):
> include/xapian/queryparser.h,queryparser/queryparser.cc,
> queryparser/queryparser.lemony,queryparser/queryparser_internal.h,
> tests/queryparsertest.cc: Since calling QueryParser::add_prefix()
> or QueryParser::add_boolean_prefix() a second time with the same
> field name was ignored before (rather than overriding as we had
> thought) it seems reasonable to change this behaviour.  This
> also avoids the need to deprecate these methods which will force all
> users to update their code.

I think leaving the old methods in place and undeprecated with these 
semantics is fine, but I'd argue the the new, 3 argument, form of 
add_prefix, and the prefix_type enum are good additions because they 
lead to future improvements (although, they're possibly not needed yet).

I thought that the consensus on the query parser was that it should 
become more configurable, and one of the ways in which it should be more 
configurable is being able to generate terms according to the prefixes 
used in a more flexible way.  In particular, it should be possible to 
decouple the status of a prefix as a "filter" prefix from the way in 
which terms are generated from the text.

Currently, we generate terms in two ways:

1. By taking the text after the field specifier, and processing it as 
free text (stemming, lowercase conversion, end at punctuation or space, 
may generate phrases).

2. By taking the text after the field specifier, and processing it as a 
single unit (currently, lowercase conversion only, end at space, no 

We also use these terms in two ways:

A. Involving the term in the query (so, joining it with surrounding 
terms according to boolean operators, and using the default operator if 
there aren't any such operators).

B. Adding the term to a list of filter terms, which are then combined 
with the rest of the query to restrict the set of matching documents, 
but not used to affect rankings.  Surrounding boolean operators are 
ignored or invalid (can't remember which).

Currently, add_prefix(field, prefix) allows the combination of "1" and 
"A" above to be used, and add_boolean_prefix(field, prefix) allows the 
combination of "2" and "B" above to be used.

There is really no reason that options 1B and 2A shouldn't be available: 
for example, option 2A on field "code" would allow something like

  "code:xkcd.2354/sad OR richard"

to generate the query "XCODE:xkcd.2354/sad OR richard"; ie, be used as a 
normal weighted term, and included in boolean operators.

My idea when introducing the prefix_type enum, and when naming its 
values as PREFIX_INLINE and PREFIX_FILTER, was that the prefix_type 
would determine how the term was "mixed" into the query (so, 
PREFIX_INLINE correlated to option "A" above, and PREFIX_FILTER 
correlated to option "B" above).  My patch also hard-coded the 
combination of option A with "1" and option B with "2", but my thinking 
was that we could later add further parameters to add_prefix() 
specifying how the term generation should be done.  Probably, this would 
simply involve adding a single extra parameter holding a "TermProcessor" 
class, which would generate a term, (or possibly a query), from a piece 
of text.  There would be default TermProcessor classes which followed 
option "1" or option "2" above, and the user would be free to define 
further such classes.

Now, it's reasonable to argue, since you're happy with changing the 
semantics of add_prefix and add_boolean_prefix, that we don't need the 
new form of add_prefix() just yet, since it doesn't add any new 
functionality; there's only any point in adding the new form of 
add_prefix() once we have an implementation which takes 4 arguments, 
allowing "TermProcessors" to be added.  I'm not too bothered about this 
right now, but it does seem that add_boolean_prefix() will then become a 
limited form of add_prefix() and would eventually be deprecated to clean 
up the API, so adding a 3 argument form of add_prefix() now to allow 
users to start using it instead of add_boolean_prefix() doesn't seem 
unreasonable either.

The main concern I have is to check that we're still heading in the same 
direction: making the query parser more configurable, and decoupling the 
status of a prefix as an inline or a filter prefix from the way in which 
term processing is done.

However, I would say that changing from using an enum internally to 
using a boolean value for the prefix type seems like a backwards step - 
use of an enum makes the code more readable, in my opinion.  I'd be 
happier if the enum was still used internally (though not exposing it in 
the API until we have a 3 or 4 argument form of add_prefix() is 
perfectly sensible).


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