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https://issues.apache.org/jira/browse/LUCENE-3866?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13395968#comment-13395968
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Michael McCandless commented on LUCENE-3866:
--------------------------------------------
I ran quick perf test:
{noformat}
Task QPS base StdDev base QPS patchStdDev patch Pct
diff
Respell 92.15 1.40 90.30 2.70 -6% -
2%
PKLookup 130.68 4.16 128.24 2.28 -6% -
3%
Fuzzy2 41.79 0.40 41.14 1.11 -5% -
2%
Fuzzy1 108.97 2.40 107.33 2.87 -6% -
3%
AndHighHigh 16.19 0.48 15.97 0.31 -6% -
3%
Phrase 12.90 0.32 12.74 0.36 -6% -
4%
AndHighMed 64.18 1.81 63.46 1.83 -6% -
4%
SloppyPhrase 8.37 0.29 8.29 0.11 -5% -
3%
SpanNear 5.51 0.12 5.46 0.18 -6% -
4%
TermGroup1M 36.19 0.60 35.89 0.74 -4% -
2%
TermBGroup1M 70.64 0.49 70.74 0.57 -1% -
1%
Prefix3 61.07 3.58 61.25 1.35 -7% -
8%
Wildcard 40.84 2.20 41.00 0.98 -7% -
8%
Term 147.32 3.85 149.65 4.87 -4% -
7%
TermBGroup1M1P 50.32 1.58 51.29 0.88 -2% -
7%
IntNRQ 9.96 1.40 10.18 0.56 -15% -
25%
OrHighHigh 10.31 0.74 10.56 0.57 -9% -
16%
OrHighMed 12.95 1.01 13.26 0.79 -10% -
17%
{noformat}
Basically no real change ... good!
> Make CompositeReader.getSequentialSubReaders() and the corresponding
> IndexReaderContext methods return unmodifiable List<R extends IndexReader>
> -----------------------------------------------------------------------------------------------------------------------------------------------
>
> Key: LUCENE-3866
> URL: https://issues.apache.org/jira/browse/LUCENE-3866
> Project: Lucene - Java
> Issue Type: Improvement
> Reporter: Uwe Schindler
> Assignee: Uwe Schindler
> Fix For: 4.0
>
> Attachments: LUCENE-3866-step1.patch, LUCENE-3866-step2.patch,
> LUCENE-3866-step2.patch
>
>
> Since Lucene 2.9 we have the method getSequentialSubReader() returning
> IndexReader[]. Based on hardly-to-debug errors in user's code, Robert and me
> realized that returning an array from a public API is an anti-pattern. If the
> array is intended to be modifiable (like byte[] in terms,...), it is fine to
> use arrays in public APIs, but not, if the array must be protected from
> modification. As IndexReaders are 100% unmodifiable in trunk code (no
> deletions,...), the only possibility to corrumpt the reader is by modifying
> the array returned by getSequentialSubReaders(). We should prevent this.
> The same theoretically applies to FieldCache, too - but the party that is
> afraid of performance problems is too big to fight against that :(
> For getSequentialSubReaders there is no performance problem at all. The
> binary search of reader-ids inside BaseCompositeReader can still be done with
> the internal protected array, but public APIs should expose only a
> unmodifiable List. The same applies to leaves() and children() in
> IndexReaderContext. This change to list would also allow to make
> CompositeReader and CompositeReaderContext Iterable<R extends IndexReader>,
> so some loops would look nice.
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