Thanks Erick!!
Index sorting and early termination is what I am looking for.
On 3/6/18, 11:33 PM, "Erick Erickson" <erickerick...@gmail.com> wrote:
OK, you're asking a different question I think.
See SOLR-5730 and SOLR-8621, particularly SOLR-5730. This will work
only a single field which you decide at index time. You can still sort
by any field at the same expense as now, but since your docs are
ordered by one field the early termination part won't be applicable to
other fields.
Best,
Erick
On Mon, Mar 5, 2018 at 6:28 PM, Tony Ma <t...@opentext.com> wrote:
> Hi Erick,
>
> I raise this question is about the sorting scenario as you mentioned in
#2.
>
> If the hit docs are about 100, and my query just want top 2. If the
values are not sorted, it has to iterate all 100 docs and find top2 in a
priority queue. If the values are already sorted, it just need to iterate first
2. If the query is unselective, the hit doc might be huge, pre-sort or not will
have big differences.
>
> I understand your thinking that if the doc values are not persisted with
doc id sequence, it is unable to retrieve field value by doc id.
>
> Actually, I am just wondering how lucene handle the sorting scenario, is
iterating all values of all docs unavoidable?
>
>
> On 3/6/18, 6:50 AM, "Erick Erickson" <erickerick...@gmail.com> wrote:
>
> I think there are two issues here that are being conflated
> 1> _within_ a document, i.e. for a multi-valued field the values are
> stored as Dominik says as a SORTED_SET. Not only will they be returned
> (if you return from docValues rather than stored) in lexical order,
> but identical values will be collapsed
>
> 2> across multiple documents, the question about "...persisted with
> order of values, not document id..." really makes no sense. The point
> of DocValues is to answer the question "for document X what is the
> value of field Y". X here is the _internal_ document ID. Now consider
> a search. There are two documents that are hits, doc 35 and doc 198
> (internal lucene doc ID). To sort them by field Y you have to know
> what the value in that field is for those two docs is. How would
> "pre-ordering" the values help here? If I have the _values_ in order,
> I have no clue what docs are associated with them. That question is
> what the "inverted index" is there to answer.
>
> So I have doc 35 and 198. Think of DocValues as a large array indexed
> by internal doc id. To know how these two docs sort all I have to do
> is index into the array. It's slightly more complicated than that, but
> conceptually that's what happens.
>
> Best,
> Erick
>
> On Mon, Mar 5, 2018 at 11:29 AM, Dominik Safaric
> <dominiksafa...@gmail.com> wrote:
> >> So, can doc values be persisted with order of values, not document
id? This should be fast in sort scenario that the values are pre-ordered
instead of scan/sort at runtime.
> >
> >
> > No, unfortunately doc values cannot be persisted in order. Lucene
stores this values internally as a DocValuesType.SORTED_SET, where the values
are being stored using for example Long.compareTo().
> >
> > If you'd like to retrieve the values in insertion order, use stored
instead of doc values instead of. Then you might access the values in order
using the LeafReader's document function. However, beware that may induce
performance issues because it requires loading the document from disk.
> >
> > If you require to store and retrieve multiple numeric values per
document in order, you might consider using PointValues. PointValues are
internally indexed with KD-trees. But, beware that PointValues have a limited
dimensionality, in terms that you can for example store values in 8 dimensions,
each of max 16 bytes.
> >
> >> On 5 Mar 2018, at 15:33, Tony Ma <t...@opentext.com> wrote:
> >>
> >> Per my understanding, doc values (binary doc values / numeric doc
values) are stored with sequence of document id. Sorted numeric doc values just
means if a document has multiple values, the values will be sorted for same
document, but for different documents, the value is still ordered by document
id. Is that true?
> >> So, can doc values be persisted with order of values, not document
id? This should be fast in sort scenario that the values are pre-ordered
instead of scan/sort at runtime.
> >
> >
> >
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