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https://issues.apache.org/jira/browse/LUCENE-4560?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13500275#comment-13500275
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Tim Smith commented on LUCENE-4560:
-----------------------------------
offsets can be used for highlighting
users want to configure highlighting per field
users don't always know what fields they want to highlight and may change this
setting frequently
setting "highlighting=true" on a field should be fully possible without full
reindex required (old documents of course will not be highlighted, or may
default to a slower highlighting method that does not require offsets) (slowly
refeeding old documents will allow users to get full functionality for old docs
as well, however refeeding may take weeks and should not impact indexing of new
content)
i can't proactively always enable offsets on the off chance they will enable
highlighting in the future as this implies additional disk requirements
this is the primary use case that spawned this ticket
right now, due to the merging behavior, i cannot use indexed offsets for
highlighting as a setting change will result in merges destroying offsets.
this filtering merge reader approach also fulfills other requirements i have
for migrating old indexed content to use new features so it would be a win-win
for me to use this filtered merge reader approach to ensure consistency and
conformance with my schema.
> Support Filtering Segments During Merge
> ---------------------------------------
>
> Key: LUCENE-4560
> URL: https://issues.apache.org/jira/browse/LUCENE-4560
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: Tim Smith
> Attachments: LUCENE-4560.patch
>
>
> Spun off from LUCENE-4557
> It is desirable to be able to filter segments during merge.
> Most often, full reindex of content is not possible.
> Merging segments can sometimes have negative consequences when fields are
> have different options (most restrictive option is forced during merge)
> Being able to filter segments during merges will allow gradually migrating
> indexed data to new index settings, support pruning/enhancing existing data
> gradually
> Use Cases:
> * Migrate IndexOptions for fields (See LUCENE-4557)
> * Gradually Remove index fields no longer used
> * Migrate indexed sort fields to DocValues
> * Support converting data types for indexed data
> * and so on
> patch will be forthcoming
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