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https://issues.apache.org/jira/browse/SOLR-236?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12792190#action_12792190
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Martijn van Groningen commented on SOLR-236:
--------------------------------------------
I have updated the response examples on the wiki.
Some time ago I tried to come up with an *accurate* distributed solution, but I
ran a problem as I have described in a previous comment:
{quote}
....
Field collapsing keeps track of the number of document collapsed per unique
field value and the total count documents encountered per unique field. If the
total count is greater than the specified collapse
threshold then the number of documents collapsed is the difference between the
total count and threshold. Lets say we have two shards each shard has one
document with the same field value. The collapse threshold is one, meaning that
if we run the collapsing algorithm on the shard individually both documents
will never be collapsed. But when the algorithm applies to both shards, one of
the documents must be collapsed however neither shared knows that its document
is the one to collapse.
There are more situations described as above, but it all boils down to the fact
that each shard does not have meta information about the other shards in the
cluster. Sharing the intermediate collapse results between the shards is in my
opinion not an option. This is because if you do that then you also need to
share information about documents / fields that have a collapse count of zero.
This is totally impractical for large indexes.
....
{quote}
I'm really curious how others have addressed this issue. I have not stumbled on
any literature on this particular issue, maybe someone else has.
> Field collapsing
> ----------------
>
> Key: SOLR-236
> URL: https://issues.apache.org/jira/browse/SOLR-236
> Project: Solr
> Issue Type: New Feature
> Components: search
> Affects Versions: 1.3
> Reporter: Emmanuel Keller
> Assignee: Shalin Shekhar Mangar
> Fix For: 1.5
>
> Attachments: collapsing-patch-to-1.3.0-dieter.patch,
> collapsing-patch-to-1.3.0-ivan.patch, collapsing-patch-to-1.3.0-ivan_2.patch,
> collapsing-patch-to-1.3.0-ivan_3.patch, field-collapse-3.patch,
> field-collapse-4-with-solrj.patch, field-collapse-5.patch,
> field-collapse-5.patch, field-collapse-5.patch, field-collapse-5.patch,
> field-collapse-5.patch, field-collapse-5.patch, field-collapse-5.patch,
> field-collapse-5.patch, field-collapse-5.patch, field-collapse-5.patch,
> field-collapse-5.patch, field-collapse-5.patch, field-collapse-5.patch,
> field-collapse-5.patch, field-collapse-5.patch,
> field-collapse-solr-236-2.patch, field-collapse-solr-236.patch,
> field-collapsing-extended-592129.patch, field_collapsing_1.1.0.patch,
> field_collapsing_1.3.patch, field_collapsing_dsteigerwald.diff,
> field_collapsing_dsteigerwald.diff, field_collapsing_dsteigerwald.diff,
> quasidistributed.additional.patch, SOLR-236-FieldCollapsing.patch,
> SOLR-236-FieldCollapsing.patch, SOLR-236-FieldCollapsing.patch,
> SOLR-236.patch, solr-236.patch, SOLR-236_collapsing.patch,
> SOLR-236_collapsing.patch
>
>
> This patch include a new feature called "Field collapsing".
> "Used in order to collapse a group of results with similar value for a given
> field to a single entry in the result set. Site collapsing is a special case
> of this, where all results for a given web site is collapsed into one or two
> entries in the result set, typically with an associated "more documents from
> this site" link. See also Duplicate detection."
> http://www.fastsearch.com/glossary.aspx?m=48&amid=299
> The implementation add 3 new query parameters (SolrParams):
> "collapse.field" to choose the field used to group results
> "collapse.type" normal (default value) or adjacent
> "collapse.max" to select how many continuous results are allowed before
> collapsing
> TODO (in progress):
> - More documentation (on source code)
> - Test cases
> Two patches:
> - "field_collapsing.patch" for current development version
> - "field_collapsing_1.1.0.patch" for Solr-1.1.0
> P.S.: Feedback and misspelling correction are welcome ;-)
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