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Posted to issues@lucene.apache.org by "Uwe Schindler (Jira)" <ji...@apache.org> on 2019/10/10 13:32:00 UTC
[jira] [Commented] (LUCENE-9003) Should FilterDirectoryReader
compute numDocs lazily?
[ https://issues.apache.org/jira/browse/LUCENE-9003?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16948579#comment-16948579 ]
Uwe Schindler commented on LUCENE-9003:
---------------------------------------
+1 I had that issue also several times. Especially if you hide documents based on a bitset or a query producing a bitset (e.g. filtered aliases, security stuff using a BitsetProducer). Bitsets still require linear time to get their cardinality.
The implementation and the lengthly comment about thread safety are perfectly fine. But as numDocs is a seldomly used statistics (as you say), why not use AtomicInteger with getOpaque()?
> Should FilterDirectoryReader compute numDocs lazily?
> ----------------------------------------------------
>
> Key: LUCENE-9003
> URL: https://issues.apache.org/jira/browse/LUCENE-9003
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: Adrien Grand
> Priority: Minor
> Time Spent: 10m
> Remaining Estimate: 0h
>
> FilterDirectoryReader extends BaseCompositeReader, which computes both maxDoc and numDocs eagerly in its constructor by summing up these values across all sub leaves.
> This is problematic for readers that hide additional documents. Computing numDocs on such leaf readers usually requires iterating over all live documents to count them. This makes creating a FilterDirectoryReader on top run in linear time, which has caused several performance bugs to us over time. This is especially frustrating given that numDocs is a rarely used index statistic.
> I think computing numDocs lazily would be less surprising?
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