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https://issues.apache.org/jira/browse/SOLR-2338?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12992728#comment-12992728
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Hoss Man commented on SOLR-2338:
--------------------------------
Most existing situations where plugins are dereferenced by name are so we can
reuse the exact same object instance (ie: for recording stats, or because they
are heavyweight to construct on the fly)
in the case of similarity, the main advantage i can think of would be if we
wanted true per-field similiarity declaration, not just per field type ie...
{code}
<similarity name="S_XX" class=...></similarity>
<similarity name="S_YY" class=...></similarity>
...
<fieldType name="FT_AA">
<analyzer>...</analyzer>
<similarity name="S_XX"/>
</fieldType>
...
<field name="F_111" type="FT_AA" /><!-- implied S_XX -->
<field name="F_222" type="FT_AA" similarity="S_YY" />
{code}
...but even if we don't do that, i suppose it's also conceivable that someone
might have their own Similarity implementation that is expensive to instantiate
(ie: maintains some big in memory data structures?) and might want to be able
to declare one instance and then refer to it by name in many different
fieldType declarations.
I think for now just supporting the first example yonik cited...
{code}
<fieldType>
<analyzer>...</analyzer>
<similarity class=...></similarity>
</fieldType>
{code}
would be a huge win, and we can always enhance to add name derefrencing later.
> improved per-field similarity integration into schema.xml
> ---------------------------------------------------------
>
> Key: SOLR-2338
> URL: https://issues.apache.org/jira/browse/SOLR-2338
> Project: Solr
> Issue Type: Improvement
> Components: Schema and Analysis
> Affects Versions: 4.0
> Reporter: Robert Muir
>
> Currently since LUCENE-2236, we can enable Similarity per-field, but in
> schema.xml there is only a 'global' factory
> for the SimilarityProvider.
> In my opinion this is too low-level because to customize Similarity on a
> per-field basis, you have to set your own
> CustomSimilarityProvider with <similarity class=.../> and manage the
> per-field mapping yourself in java code.
> Instead I think it would be better if you just specify the Similarity in the
> FieldType, like after <analyzer>.
> As far as the example, one idea from LUCENE-1360 was to make a "short_text"
> or "metadata_text" used by the
> various metadata fields in the example that has better norm quantization for
> its shortness...
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