Jack Krupansky created SOLR-3723:
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Summary: Improve OOTB behavior: English word-splitting should
default to autoGeneratePhraseQueries=true
Key: SOLR-3723
URL: https://issues.apache.org/jira/browse/SOLR-3723
Project: Solr
Issue Type: Improvement
Components: Schema and Analysis
Affects Versions: 3.6.1, 4.0-ALPHA, 3.6, 3.5, 3.4
Reporter: Jack Krupansky
Digging through the Jira and revision history, I discovered that back at the
end of May 2011, a change was made to Solr that fairly significantly
degrades the OOTB behavior for English Solr queries, namely for word-splitting
of terms with embedded punctuation, so that they end up, by default, doing the
OR of the sub-terms, rather than doing the obvious phrase query of the
sub-terms.
Just a couple of examples:
1. CD-ROM => CD OR ROM rather than “CD ROM”
2. 1,000 => 1 OR 000 rather than “1 000” (when using the WordDelimiterFilter
innocently added to text_general or text_en)
3. out-of-the-box => out OR of OR the OR box rather than “out of the box”
4. 3.6 => 3 OR 6 rather than "3 6" (when using WordDelimiterFilter innocently
added to text_general or text_en)
5. docid-001 => docid OR 001 rather than "DOCID 001"
All of those queries will give surprising and unexpected results.
Note: The hyphen issue is present in StandardTokenizer, even if WDF is not
used. Side note: The full behavior of StandardTokenizer should be more fully
documented on the Analyzers wiki.
Back to the history of the change, there was a lot of lively discussion on
SOLR-2015 - add a config hook for autoGeneratePhraseQueries.
And the actual change to default to the behavior described above was SOLR-2519
- improve defaults for text_* field types.
(Consider the entire discussion in those two issues incorporated here for
reference. Anyone wishing to participate in discussion on this issue would be
well-advised to study those two issues first.)
I gather that the original motivation was for non-European languages, and
that even some European languages might search better without auto-phrase
generation, but the decision to default English terms to NOT automatically
generate phrase queries and to generate OR queries instead is rather surprising
and unexpected and outright undesirable, as my examples above show.
I had been aware of the behavior for quite some time, but I had thought it was
simply a lingering bug so I paid little attention to it, until I
stumbled across this autoGeneratePhraseQueries "feature" while looking at
the query parser code. I can understand the need to disable automatic phrase
queries for SOME languages, but to disable it by default for English seems
rather bizarre, as my simple use cases above show.
Even if no action is taken on this Jira, I feel that it is important that there
be a wider awareness of the significant and unexpected impact from SOLR-2519,
and that what had seemed like buggy behavior was done intentionally.
Unless there has been a change of heart since SOLR-2015/2519, I guess we are
stuck with the default TextField behavior, but at least we could improve the
example schema in several ways:
1. The English text field types should have autoGeneratePhraseQueries=true. If
a user innocently adds a word delimiter to text_en, for example, they need to
know that autoGeneratePhraseQueries=true is needed. Better to preempt that
confusion and put the attribute in now. In fact, hyphenated terms fail as I
have noted above, so the addition is needed even if a WDF is not added.
2. Add commentary about the impact of autoGeneratePhraseQueries=true/false - in
terms of use case examples, as above. Specifically note the ones that will
break with if the feature is disabled.
Another, more controversial change will be:
3. Change text_general to autoGeneratePhraseQueries=true so that English
will be treated reasonably by default. I suspect that most European
languages will be at least "okay". A comment will note that this field
attribute should be removed or set to false for non-whitespace languages, or
that an alternative field type should be used. I suspect that the first thing
any non-whitespace language application will want to do is pick the text field
type that has analysis that makes the most sense for them, so I see no need to
mess up English for no good reason.
Make no mistake, #3 is the primary and only real goal of this OOTB
improvement. Maybe "text_general" could be kept as is for reference as the
purported "general" text field type (except that it doesn't work well for
English. as shown above), and maybe there should be a "text_default" that I
would propose should be text_en with commentary to direct users to the other
choices for language.
I would note that text_ja already has autoGeneratePhraseQueries=false, so
I'm not sure why the default in the TextField code had to be changed to false.
Any languages for which automatic phrase query generation is problematic should
be attributed similarly. But, now that it is wired into the schema defaults, we
may be stuck with it.
I was rather surprised that SOLR-2519 actually changed the default in
TextField rather than simply set the attribute as appropriate for the
various text field types.
There are probably also a couple of places in the wikis where the surprising
behavior should be noted. There is literally no wiki documentation for this
important feature. There are only two references to autoGeneratePhraseQueries,
with no discussion of exactly what this feature does or what the downside is if
it is disabled.
In the past, there was no need to document the treatment of embedded word
delimiters (well, okay, the poor handling for non-whitespace languages SHOULD
have been documented), but now there is no documentation of the degradation of
what was a default and implicit feature that a lot of people assume should be
automatic.
And, I would propose that the 4.0 CHANGES.TXT very clearly highlight the
kinds of use cases that unsuspecting users may not realize were BROKEN by
the commit of SOLR-2519 that is masked under the innocent phrasing of
"improve defaults for text_* field types". How many users seriously
understood that a query with embedded dashes and commas behave differently as a
result of that change?
I am contemplating whether to suggest that the WordDelimiterFilter should
also be part of the default text field type. Right now, it is hidden off in
text_en_splitting.
I think stemming should also be part of the default English field type. The
whole point of the "example" schema is to show-off the best of Lucene/Solr.
I'm not quite ready to propose that English be the default language supported
by the example schema, but I am 99.999% certain that we should focus it on
European, Roman, Latin languages. Non-European languages are indeed important,
and should probably have their own schema. text_general was a good idea, but in
hindsight it appears to have not been such a great idea in light of the
word-splitting problems I have highlighted above.
Maybe I would propose that text_general be left as is, but that we add
text_default which is a copy of text_en (which would have WDF and stemming
added) and fields use text_default as their type. That way, it would be clear
what is going on and users could sensibly see what needs to happen if they wish
to switch default languages.
After discussion settles, a revised final proposal will be composed. And some
specific and non-controversial issues may be split into separate Jira issues.
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