[jira] [Commented] (LUCENE-8878) Provide alternative sorting utility from SortField other than FieldComparator

["Michael McCandless (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/LUCENE-8878?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16879274#comment-16879274 ] 

Michael McCandless commented on LUCENE-8878:
--------------------------------------------

{quote}I believe you are talking about Scorer#setMinCompetitiveScore, ie. changing the FieldComparator API to only track the bottom bucket as opposed to every bucket? If this is the case I agree that it sounds like a good idea to explore.
{quote}
Ahh, yes, that ;)  +1

> 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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