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https://issues.apache.org/jira/browse/LUCENE-831?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12654057#action_12654057
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Michael McCandless commented on LUCENE-831:
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One more thing here... while random-access lookup of a field's value
via MultiReader dispatch requires the binary search to find the right
sub-reader, I think in most internal uses, the access could be
switched to an iterator instead, in which case the lookup should be
far faster.

EG when sorting by field, we could pull say an IntData iterator from
the reader, and then access the int values in docID order as we visit
the docs.

For norms, which we should eventually switch to FieldCache +
column-stride fields, it would be the same story.

Accessing via iterator should go a long ways to reducing the overhead
of "using a method" instead of accessing the full array directly.


> Complete overhaul of FieldCache API/Implementation
> --------------------------------------------------
>
>                 Key: LUCENE-831
>                 URL: https://issues.apache.org/jira/browse/LUCENE-831
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Search
>            Reporter: Hoss Man
>             Fix For: 3.0
>
>         Attachments: fieldcache-overhaul.032208.diff, 
> fieldcache-overhaul.diff, fieldcache-overhaul.diff, 
> LUCENE-831.03.28.2008.diff, LUCENE-831.03.30.2008.diff, 
> LUCENE-831.03.31.2008.diff, LUCENE-831.patch, LUCENE-831.patch, 
> LUCENE-831.patch, LUCENE-831.patch, LUCENE-831.patch
>
>
> Motivation:
> 1) Complete overhaul the API/implementation of "FieldCache" type things...
>     a) eliminate global static map keyed on IndexReader (thus
>         eliminating synch block between completley independent IndexReaders)
>     b) allow more customization of cache management (ie: use 
>         expiration/replacement strategies, disk backed caches, etc)
>     c) allow people to define custom cache data logic (ie: custom
>         parsers, complex datatypes, etc... anything tied to a reader)
>     d) allow people to inspect what's in a cache (list of CacheKeys) for
>         an IndexReader so a new IndexReader can be likewise warmed. 
>     e) Lend support for smarter cache management if/when
>         IndexReader.reopen is added (merging of cached data from subReaders).
> 2) Provide backwards compatibility to support existing FieldCache API with
>     the new implementation, so there is no redundent caching as client code
>     migrades to new API.

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