* SQL support for nested groupBys.
Allows, for example, doing exact count distinct by writing:
SELECT COUNT(*) FROM (SELECT DISTINCT col FROM druid.foo)
Contrast with approximate count distinct, which is:
SELECT COUNT(DISTINCT col) FROM druid.foo
* Add deeply-nested groupBy docs, tests, and maxQueryCount config.
* Extract magic constants into statics.
* Rework rules to put preconditions in the "matches" method.
* sortByDimsFirst flag for groupBy query
* Remove need for KeyType in Grouper<KeyType> to be Comparable<KeyType>
* fix review comments
* fix review comments regarding removing code duplication of dim/time comparison
* move comparator for KeyType object to KeySerdeFactory so that creation of comparator does not need KeySerde
* remove unnecessary system.out.println
* make access static var NATURAL_NULLS_FIRST directly
* further review comments addressing
Also change defaults:
- bufferGrouperMaxLoadFactor from 0.75 to 0.7.
- maxMergingDictionarySize to 100MB from 25MB, should be more appropriate
for most heaps.
This fixes a potential issue where groupBy resources could be allocated to
create a Sequence, but then the Sequence is never used, and thus the resources
are never freed.
Also simplifies how groupBy handles config overrides (this made the new
unit test easier to write).
This is actually reasonable for a groupBy or lexicographic topNs that is
being used to do a "COUNT DISTINCT" kind of query. No aggregators are
needed for that query, and including a dummy aggregator wastes 8 bytes
per row.
It's kind of silly for timeseries, but why not.
This patch introduces a GroupByStrategy concept and two strategies: "v1"
is the current groupBy strategy and "v2" is a new one. It also introduces
a merge buffers concept in DruidProcessingModule, to try to better
manage memory used for merging.
Both of these are described in more detail in #2987.
There are two goals of this patch:
1. Make it possible for historical/realtime nodes to return larger groupBy
result sets, faster, with better memory management.
2. Make it possible for brokers to merge streams when there are no order-by
columns, avoiding materialization.
This patch does not do anything to help with memory management on the broker
when there are order-by columns or when there are nested queries. That could
potentially be done in a future patch.
* docs: replace OR by AND inside topnquery docs about multi value dimensions
* docs: replace OR by AND inside groupby docs about multi value dimensions
- Add central doc for multi-value dimensions, with some content from other docs.
- Link to multi-value dimension doc from topN and groupBy docs.
- Fixes a broken link from dimensionspecs.md, which was presciently already
linking to this nonexistent doc.
- Resolve inconsistent naming in docs & code (sometimes "multi-valued", sometimes
"multi-value") in favor of "multi-value".
Jihoon Son
Gian Merlino
Himanshu
Dave Li
linbo.jin