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https://issues.apache.org/jira/browse/SOLR-5670?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15190785#comment-15190785
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Per Steffensen commented on SOLR-5670:
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Well, no, I have not.
Our reason for doing this is not necessarily to make things faster. It
basically boils down to avoiding OOMs. When you want to get a value for a
particular field on a particular document in Solr/Lucene there are several
places to find the information. You can find it in the store (if stored=true
for that field), which is fairly slow. You can also find it in the FieldCache,
which is an in-memory doc-id to field-value map. Problem with FieldCache is
that if you load it, you need to load values for ALL the documents in the index
(or at least the segment). If you have enormous amounts of documents in your
index this can cause OOM, because you simple cannot have all those values in
memory. Then doc-value is for the rescue. Doc-value basically holds the same
doc-id to field-value data-structure as the FieldCache, but doc-values are
maintained continuously in additional files in your index-folder, so you can
just go read the particular value you need, hence avoiding OOM. FieldCache is
something you "calculate" into memory, based on data form store or index.
> _version_ either indexed OR docvalue
> ------------------------------------
>
> Key: SOLR-5670
> URL: https://issues.apache.org/jira/browse/SOLR-5670
> Project: Solr
> Issue Type: Improvement
> Components: SolrCloud
> Affects Versions: 4.7
> Reporter: Per Steffensen
> Assignee: Per Steffensen
> Labels: solr, solrcloud, version
> Fix For: 4.7, master
>
> Attachments: SOLR-5670.patch, SOLR-5670.patch
>
>
> As far as I can see there is no good reason to require that "_version_" field
> has to be indexed if it is docvalued. So I guess it will be ok with a rule
> saying "_version_ has to be either indexed or docvalue (allowed to be both)".
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