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https://issues.apache.org/jira/browse/LUCENE-4764?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13576660#comment-13576660
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Robert Muir commented on LUCENE-4764:
-------------------------------------
{quote}
I'm still not sure what's the best (or common) way to work w/ Codecs. When do
you create your own FilterCodec and override the relevant method, and when are
you expected to extend another Codec?
{quote}
The rule is that an index needs to be self-describing: basically I should be
able to open any index if i have the right stuff in my classpath.
This would be somewhat of a burden for users who just want to change their "id"
field to use a different postings format (they would have to make a whole codec
with their own unique name), and so on.
So Lucene42Codec uses PerField[Postings/DocValuesFormat] (note this is final!),
which separately record the name of the format used on a per-field basis.
Because of this, its ok that it not final and exposes these hooks to custom
postings/docvalues per-field, because it writes the name of those formats into
the index for the field.
> Faster but more RAM/Disk consuming DocValuesFormat for facets
> -------------------------------------------------------------
>
> Key: LUCENE-4764
> URL: https://issues.apache.org/jira/browse/LUCENE-4764
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: Michael McCandless
> Assignee: Michael McCandless
> Fix For: 4.2, 5.0
>
> Attachments: LUCENE-4764.patch, LUCENE-4764.patch, LUCENE-4764.patch
>
>
> The new default DV format for binary fields has much more
> RAM-efficient encoding of the address for each document ... but it's
> also a bit slower at decode time, which affects facets because we
> decode for every collected docID.
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