Paul Cowan (JIRA | 4 Mar 07:10 2009
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[jira] Updated: (LUCENE-1372) Proposal: introduce more sensible sorting when a doc has multiple values for a term


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https://issues.apache.org/jira/browse/LUCENE-1372?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

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 (termDocs.next()) {
   retArray[termDocs.doc()] = termval;
}{code}
with ones that look like this:
{code}while (termDocs.next()) {
   if (retArray[termDocs.doc() == null) {
     retArray[termDocs.doc()] = termval;
   }
}{code}
(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 performant
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: https://issues.apache.org/jira/browse/LUCENE-1372
>             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 (termDocs.next()) {
>             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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Gmane