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https://issues.apache.org/jira/browse/SOLR-14365?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17069285#comment-17069285
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Shalin Shekhar Mangar commented on SOLR-14365:
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I think we should add another method and make it configurable.
> CollapsingQParser - Avoiding always allocate int[] and float[] with size
> equals to number of unique values
> ----------------------------------------------------------------------------------------------------------
>
> Key: SOLR-14365
> URL: https://issues.apache.org/jira/browse/SOLR-14365
> Project: Solr
> Issue Type: Improvement
> Security Level: Public(Default Security Level. Issues are Public)
> Affects Versions: 8.4.1
> Reporter: Cao Manh Dat
> Assignee: Cao Manh Dat
> Priority: Major
>
> Since Collapsing is a PostFilter, documents reach Collapsing must match with
> all filters and queries, so the number of documents Collapsing need to
> collect/compute score is a small fraction of the total number documents in
> the index. So why do we need to always consume the memory (for int[] and
> float[] array) for all unique values of the collapsed field? If the number of
> unique values of the collapsed field found in the documents that match
> queries and filters is 300 then we only need int[] and float[] array with
> size of 300 and not 1.2 million in size. However, we don't know which value
> of the collapsed field will show up in the results so we cannot use a smaller
> array.
> The easy fix for this problem is using as much as we need by using IntIntMap
> and IntFloatMap that hold primitives and are much more space efficient than
> the Java HashMap. These maps can be slower (10x or 20x) than plain int[] and
> float[] if matched documents is large (almost all documents matched queries
> and other filters). But our belief is that does not happen that frequently
> (how frequently do we run collapsing on the entire index?).
> For this issue I propose adding 2 methods for collapsing which is
> * array : which is current implementation
> * hash : which is new approach and will be default method
> later we can add another method {{smart}} which is automatically pick method
> based on comparision between {{number of docs matched queries and filters}}
> and {{number of unique values of the field}}
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