Florin Iordache created OAK-2725:
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Summary: Wrong indexed query estimates exceed more than double the
actual index entries
Key: OAK-2725
URL: https://issues.apache.org/jira/browse/OAK-2725
Project: Jackrabbit Oak
Issue Type: Bug
Components: query
Affects Versions: 1.1.8
Reporter: Florin Iordache
Priority: Critical
Fix For: 1.2
The {{ApproximateCounter.adjustCountSync}} public method that is used by the
indexing engine will sometimes produce very unrealistic cost estimates.
The problem is that it can produce an estimated cost that exceeds the estimated
cost of the full traversal query, thus causing the index to be bypassed
altogether, resulting in a full traversal rather than the use of the existing
index.
Problem resides in the way the property counts are updated:
* The count property update goes through if two randoms are not zero:
random(100) and random({1, 2, 4, 8, 16, ...}).
* Same static pseudo random generator for all invocations.
Even if #1 might seem improbable, it is statistically possible to reach a very
high count with only a handful of invocations.
In practice I've found that running 100 tests with 1000 invocations if the
adjustCountSync method will yield costs exceeding value 2000 in 4-10% of the
tests. Attaching a patch for {{ApproximateCounterTest}} with this test case.
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