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From Steven Rowe <sar...@syr.edu>
Subject Re: Multi-field distinct query
Date Wed, 16 May 2007 13:31:14 GMT
Hi Terry,

Why not have another index in which a document has one field for the
parent and another field containing all of its children.  An OR query
over the "children" field would return you exactly what you want - one
document for each distinct parent.

Steve

dontspamterry wrote:
> Hi all,
> 
> I know this whole distinct query has been discussed a bunch of times for
> various scenarios because I've been scouring the forums trying to find a
> clue as to how I could solve my problem. I'm indexing a large set of
> parent-child term relations (~1 million). The number of unique terms is
> about ~570,000. Each relation is a document. Each term in a relation
> contains all of the term's attributes. Effectively, a term's attributes will
> be duplicated "x" number of times for the "x" number of relations it
> participates in. For example, say I have the following term tree:
> 
> A
> |--B
>     |--E
>         |--H
>     |--F
> |--C
>     |--G
> |--D
> 
> I would then have documents for:
> A->B, A->C, A->D, B->E, (and so forth...)
> 
> For all relations involving A, A's attributes will be duplicated in 3
> separate documents.
> For all relations involving B, B's attributes will be duplicated in 3
> separate documents.
> (you get the picture...)
> 
> This index structure works great for queries which traverse up and down the
> tree. However, I have a requirement where I would also like to do a distinct
> query which returns the data for each unique term satisfying the query. For
> example, say I have a query which returns all relations where A or B is the
> parent (that would be 5 documents in total),
> but do a distinct on the parent such that I get 2 documents back, one for A
> as the parent (any 1 of the 3 matching docs)  and the other where B is the
> parent (any 1 of the 2 matching docs). For this query, I don't care about
> the child information since I'm only interested in retrieving the distinct
> parent terms. This query is analogous to a 'select distinct <set of parent
> term attributes>' . I played around with caching BitSets for the fields
> which I'd like to do a distinct on, but given the amount of data, I run out
> of memory. I also took the approach where I retrieve the bitset using a
> queryfilter and then process each document id, hashing the field values on
> which I'm doing a distinct to construct my distinct set. Problem with this
> is that I have tree structures where a parent has over 100K children.
> Retrieving each doc for this size is too time- and memory- consuming. Since
> I don't really want to return that much data, I thought that I could use
> paging. The problem I faced is that I do not know if a distinct value in the
> current query was actually returned in some previous query for a previous
> page.
> 
> Sorry for the long description, but wanted to make sure I explained it as
> clearly as I could.
> 
> -Terry


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