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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