clintropolis opened a new pull request, #13561:
URL: https://github.com/apache/druid/pull/13561
### Description
This PR fixes a bug with nested column "value set" indexes caused by not
properly validating that the globalId looked up for value is present in the
global dictionary prior to looking it up in the local dictionary, which when
"adjusting" the global ids for value type can cause incorrect selection of
value indexes.
To use an example of a variant typed nested column with 3 values `["1",
null, -2]` (the test case I added). The string dictionary is `[null, "1"]` and
the long dictionary is `[-2]` and our local dictionary is `[0, 1, 2]`.
The code for variant typed indexes checks if the value is present in all
global dictionaries and returns indexes for all matches. So in this case, we
first lookup "1" in the string dictionary, find it at global id 1, all is good.
Now, we check the long dictionary for `1`, which due to `-(insertionpoint + 1)`
gives us `-(1 + 2) = -2`. Since the global id space is actually stacked
dictionaries, global ids for long and double values must be "adjusted" by the
size of string dictionary, and size of string + size of long for doubles.
Back to indexOf for longs and doubles. Since prior to this patch we were not
checking that the globalId is 0 or larger, we then immediately looked up the
`localDictionary.indexOf(-2 + adjustLong) = localDictionary.indexOf(-2 + 2) =
localDictionary.indexOf(0)` ... which is an actual value contained in the
dictionary! We should have skipped the longs completely since there were no
global matches, but instead we randomly picked the 'null' value index since it
is 0.
On to doubles, `-(insertionPoint + 1)` gives us `-(0 + 1) = -1`. The double
adjust value is '3' since 2 strings and 1 long, so `localDictionary.indexOf(-1
+ 3)` = `localDictionary.indexOf(2)` which is also a real value in our local
dictionary that is definitely not '1'.
So in this one case, looking for '1' actually ended up matching every row,
fun! Anyway, my bad, nested column indexes are pretty complicated 😅
<hr>
This PR has:
- [ ] been self-reviewed.
- [ ] using the [concurrency
checklist](https://github.com/apache/druid/blob/master/dev/code-review/concurrency.md)
(Remove this item if the PR doesn't have any relation to concurrency.)
- [ ] added documentation for new or modified features or behaviors.
- [ ] a release note entry in the PR description.
- [ ] added Javadocs for most classes and all non-trivial methods. Linked
related entities via Javadoc links.
- [ ] added or updated version, license, or notice information in
[licenses.yaml](https://github.com/apache/druid/blob/master/dev/license.md)
- [ ] added comments explaining the "why" and the intent of the code
wherever would not be obvious for an unfamiliar reader.
- [ ] added unit tests or modified existing tests to cover new code paths,
ensuring the threshold for [code
coverage](https://github.com/apache/druid/blob/master/dev/code-review/code-coverage.md)
is met.
- [ ] added integration tests.
- [ ] been tested in a test Druid cluster.
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