[LUCENE-1372] Proposal: introduce more sensible sorting when a doc has multiple values for a term

• Type: Improvement
• Status: Resolved
• Priority: Minor
• Resolution: Won't Fix
• Affects Version/s: 2.3.2
• Component/s: core/search
• Lucene Fields: New, Patch Available
• Assignee: Unassigned
• Reporter: Paul Cowan
• Created: 01/Sep/08 02:26
• Updated: 28/Aug/22 11:52
• Resolved: 14/Apr/13 12:54

Description

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.

Issue Links

• is related to: LUCENE-1470 Add TrieRangeFilter to contrib
• is related to: LUCENE-831 Complete overhaul of FieldCache API/Implementation