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https://issues.apache.org/jira/browse/SOLR-9437?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15437150#comment-15437150
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Ronald S. Wood commented on SOLR-9437:
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I realize this request may be too specialized, and appears out of place
compared to other schema field properties. Perhaps there are other, cleaner
ways to hint at this.
[~alessandro.benedetti] argues on solr-user that this opens a can of worms with
regard to other kinds of schema migrations. If a method for docValues migration
is allowed then maybe a generalized mechanism should be implemented that
accounts for similar migrations like turning term vectors, positions, offsets
on/off. E.g. hint="ignoreDocValues,ignoreTermVectors", I suppose.
I would just argue that a safe in-place migration path to docValues might be a
special, important case given the growing importance of docValues for most new
features.
Especially if it should come to pass one day that docValues becomes mandatory
(now that docValues are on by default), it may be desirable to allow users to
migrate safely and not be stuck in obsolete versions.
> Add schema property useDocValues to control use of docValues at query time
> --------------------------------------------------------------------------
>
> Key: SOLR-9437
> URL: https://issues.apache.org/jira/browse/SOLR-9437
> Project: Solr
> Issue Type: Improvement
> Security Level: Public(Default Security Level. Issues are Public)
> Affects Versions: 5.5.2
> Reporter: Ronald S. Wood
>
> Currently migration to docValues appears to be an all or nothing proposition,
> requiring a full reindexing when an existing field with indexed records gets
> docValues set to true. This can be a painful process for users with very
> large indexes.
> If a new schema property called useDocValues is created (with a default of
> true), this can be used to allow incremental re-indexing of records.
> The idea is that while docValues=true indexing docValues will happen as
> before, but as long as useDocValues=false, the partially indexed docValues
> will not be used so that searches, faceting, etc. will behave as if
> docValues=false.
> Once the user is satisfied that all records have docValues, they can set
> useDocValues=true or remove the property altogether.
> From what I can see in the Solr core code, where field.hasDocValues is now
> used to determine whether to use docValues at query time, the code can be
> changed to (field.hasDocValues && field.useDocValues).
> I am uncertain whether any changes would be needed at the Lucene level.
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