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From "Paul Cowan (JIRA)" <>
Subject [jira] Updated: (LUCENE-1372) Proposal: introduce more sensible sorting when a doc has multiple values for a term
Date Wed, 04 Mar 2009 06:10:01 GMT


Paul Cowan updated LUCENE-1372:

    Attachment: LUCENE-1372-MultiValueSorters.patch

I think we're after somewhat different things here Uwe, but still pulling generally in the
same direction.

For your case, I personally am in favour of:
1) replacing (as I did in my original patch) the loops in FieldCacheImpl that look like this:
{code}while ( {
   retArray[termDocs.doc()] = termval;
with ones that look like this:
{code}while ( {
   if (retArray[termDocs.doc() == null) {
     retArray[termDocs.doc()] = termval;
(or == 0 for the int case, == 0.0 for the float case, whatever). This, I think, meets your
sorting goal (order by lexicographically first term using simple binary ordering of the term
text). You then just need either:
a) a code path that uses FieldCacheImpl.getStrings() rather than than .getStringIndex() (the
former doesn't care about the more-terms-than-documents case), but this is obviously not optimally
b) a change to .getStringIndex which doesn't assume that there are fewer terms than documents.
Not sure if this is harder or not.... don't know if there is an easy way to find the number
of terms for a field in advance to size the array?
I think 'multi-value fields order by the first term in binary string order' is a valid behaviour,
doesn't 'dirty' the codebase, is easy to document + explain, and suits cases like Uwe's where
the fact that it's stored as a String is kind of irrelevant (for TrieRange, you'd be just
as happy with a byte[] as a String, right?) So that, I think, would suit you fine.

2) For OUR case, we might have docs indicated above:
doc 1: {"apple"}
doc 2: {"banana"}
doc 3: {"apple", "banana"}
doc 4: {"apple", "zebra"}
and we'd like them sorted lexicographically in what most english speakers would call the 'expected'
order (1, 3, 4, 2) this won't really help (the case above was really just a half-hearted compromise).
You might ask why we don't just index a single term for each ("apple", "banana", "apple/banana",
"apple/zebra" and sort by that, but as well as being flaky if the separator character is used
in an actual term, this breaks for multi-language sorting; 'banana' might sort before 'apple'
in another locale. Imagine if these were people's surnames, we need to follow expected order.
If you have 1000 values in random combinations of 10 this also makes the index terms eat up
serious memory)

For this case, I have attached a patch which may or may not be a useful basis for doing this
behaviour 'correctly'. It's implemented as a static factory class for producing SortComparatorSources
which have the correct behaviour. There's little javadoc for now, but a test case which should
explain relatively easily how it works. This is very low-impact on Lucene; if people want
it in Lucene-core, great; if it can go in contrib, great; if not, we can keep it separate,
though for it to work there is a minor change required to [Extended]FieldCacheImpl to expose
the default parsers. We could remove that, but it would be good to up those default parsers
to default access.

Please let me know what you think of this patch; it's not overly performance (rather than
a String[] or a float[] for the terms, it uses a ArrayList<String>[] or ArrayList<Float>[],
which is more overhead (especially in the latter case; 4 bytes per doc for a primitive float
explodes a bit for an ArrayList of Float objects) but I suspect this will be acceptable for
certain cases and can be appropriately documented.

> Proposal: introduce more sensible sorting when a doc has multiple values for a term
> -----------------------------------------------------------------------------------
>                 Key: LUCENE-1372
>                 URL:
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Search
>    Affects Versions: 2.3.2
>            Reporter: Paul Cowan
>            Priority: Minor
>         Attachments: LUCENE-1372-MultiValueSorters.patch, lucene-multisort.patch
> At the moment, FieldCacheImpl has somewhat disconcerting values when sorting on a field
for which multiple values exist for one document. For example, imagine a field "fruit" which
is added to a document multiple times, with the values as follows:
> doc 1: {"apple"}
> doc 2: {"banana"}
> doc 3: {"apple", "banana"}
> doc 4: {"apple", "zebra"}
> if one sorts on the field "fruit", the loop in FieldCacheImpl.stringsIndexCache.createValue()
(and similarly for the other methods in the various FieldCacheImpl caches) does the following:
>           while ( {
>             retArray[termDocs.doc()] = t;
>           }
> which means that we look over the terms in their natural order and, on each one, overwrite
retArray[doc] with the value for each document with that term. Effectively, this overwriting
means that a string sort in this circumstance will sort by the LAST term lexicographically,
so the docs above will effecitvely be sorted as if they had the single values ("apple", "banana",
"banana", "zebra") which is nonintuitive. To change this to sort on the first time in the
TermEnum seems relatively trivial and low-overhead; while it's not perfect (it's not local-aware,
for example) the behaviour seems much more sensible to me. Interested to see what people think.
> Patch to follow.

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