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https://issues.apache.org/jira/browse/LUCENE-8878?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16872734#comment-16872734
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Tony Xu commented on LUCENE-8878:
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> As long as we can keep comparing strings using their ordinals instead of
>their actual values, it should be good.
Is it the case today? I wonder whether the ordinals are comparable across
segments (likely not...); To support this I think the the {{ValueAccessor}} for
{{SortField.Type.String}} needs to return a 3-tuple (segmentId, ord, byteRef)
so the compare logic has enough context to compare ord if possible.
> I was hoping we could soon replace FunctionValues with the new
>oal.search.LongValues/DoubleValues.
+1. I'm still exploring the whole code base but I'm already overwhelmed by the
number of classes for valueSource and values representations which are
descendants of org.apache.lucene.queries.function.ValueSource... Any suggestion
on which class/interface to extend/implement for non-numeric {{ValueAccessor}}?
> Provide alternative sorting utility from SortField other than FieldComparator
> -----------------------------------------------------------------------------
>
> Key: LUCENE-8878
> URL: https://issues.apache.org/jira/browse/LUCENE-8878
> Project: Lucene - Core
> Issue Type: Improvement
> Components: core/search
> Affects Versions: 8.1.1
> Reporter: Tony Xu
> Priority: Major
>
> The `FieldComparator` has many responsibilities and users get all of them at
> once. At high level the main functionalities of `FieldComparator` are
> * Provide LeafFieldComparator
> * Allocate storage for requested number of hits
> * Read the values from DocValues/Custom source etc.
> * Compare two values
> There are two major areas for improvement
> # The logic of reading values and storing them are coupled.
> # User need to specify the size in order to create a `FieldComparator` but
> sometimes the size is unknown upfront.
> # From `FieldComparator`'s API, one can't reason about thread-safety so it
> is not suitable for concurrent search.
> E.g. Can two concurrent thread use the same `FieldComparator` to call
> `getLeafComparator` for two different segments they are working on? In fact,
> almost all existing implementations of `FieldComparator` are not thread-safe.
> The proposal is to enhance `SortField` with two APIs
> # {color:#14892c}int compare(Object v1, Object v2){color} – this is to
> compare two values from different docs for this field
> # {color:#14892c}ValueAccessor newValueAccessor(LeafReaderContext
> leaf){color} – This encapsulate the logic for obtaining the right
> implementation in order to read the field values.
> `ValueAccessor` should be accessed in a similar way as `DocValues` to
> provide the sort value for a document in an advance & read fashion.
> With this API, hopefully we can reduce the memory usage when using
> `FieldComparator` because the users either store the sort values or at least
> the slot number besides the storage allocated by `FieldComparator` itself.
> Ideally, only once copy of the values should be stored.
> The proposed API is also more friendly to concurrent search since it provides
> the `ValueAccessor` per leaf. Although same `ValueAccessor` can't be shared
> if there are more than one thread working on the same leaf, at least they can
> initialize their own `ValueAccessor`.
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