Unique terms not available in IndexReaderUtils

[djoerd]

I want to know the number of unique terms in my index and got: -1

Steps:
IndexCollection -collection TrecCollection -input /home/hiemstra/Data/robust04/ -index lucene-index.robust04.pos+docvectors -threads 16 -storePositions -storeDocvectors
IndexReaderUtils -stats -index lucene-index.robust04.pos+docvectors/

Results:
Index statistics
----------------
documents: 528030
documents (non-empty): 528030
unique terms: -1
total terms: 174540872

Turns out that: "Terms.size(): (...) may be unavailable (returns -1) for some Terms implementations such as MultiTerms where it cannot be efficiently computed.

I already solved this myself: I will add a pull request.

[lintool]

To get an accurate count of the vocab size, you have to use the -optimize flag, which merges all the index segments down into a single one.

[djoerd]

Thanks a lot (also for doing this on SIGIR deadline day!): That solves my problem. The optimize flag is costly for a large index though, so the PR may still be helpful. I checked and it gives the exact same number of unique terms for my (non-optimized) Robust04 index: 923436 unique terms, i.e., the Lucene term iterator seems to work correctly on multiple segments.

BTW, I forgot to remove the "terms" declaration from the original getIndexStats() method (should have installed Eclipse directly)