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".
Dave Li
Gian Merlino
linbo.jin
Himanshu Gupta