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https://issues.apache.org/jira/browse/LUCENE-8264?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16449702#comment-16449702
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Uwe Schindler commented on LUCENE-8264:
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Simon: This is exactly the case: People have simple analysis chains or they are
using ClassicAnalyzer that is still supported. In my case, one of the customers
just had his own version of the old analyzer from 2.x days and ported it, jut
to make the indexes still working. The only issue was missing doc values (no
fieldcache anymore), so there was the requirement to add doc values. Based on
that, the upgrade path was easy: Take the tokens as is (offsets were not an
issue, because no highlighting - only "old-style" highlighting with reanalyzing
text). The Analyzers were ported. The IndexUpgarde we wrote was just like the
above code (using UninvertingReader to add doc values) and additionally adding
all (filter-wrapped) segments of the original index one by one so keeping the
old segment structure.
So yes, we should think of adding some infrastructure to do manual upgrades
(configurable), so you don't have to hack crazy filterreaders. Maybe add some
options like "add docvalues for field x, convert Numeric/Trie field to Points -
even that is possible with some limitations by using UninvertingReader!!!),
keep tokens alive, drop offsets completely (e.g., if broken).
But as said before, by default: Don't support that without manual intervention.
But we should not let the people fail completely when upgrading.
> Allow an option to rewrite all segments
> ---------------------------------------
>
> Key: LUCENE-8264
> URL: https://issues.apache.org/jira/browse/LUCENE-8264
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: Erick Erickson
> Assignee: Erick Erickson
> Priority: Major
>
> For the background, see SOLR-12259.
> There are several use-cases that would be much easier, especially during
> upgrades, if we could specify that all segments get rewritten.
> One example: Upgrading 5x->6x->7x. When segments are merged, they're
> rewritten into the current format. However, there's no guarantee that a
> particular segment _ever_ gets merged so the 6x-7x upgrade won't necessarily
> be successful.
> How many merge policies support this is an open question. I propose to start
> with TMP and raise other JIRAs as necessary for other merge policies.
> So far the usual response has been "re-index from scratch", but that's
> increasingly difficult as systems get larger.
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