Dynamic evaluation of GroupBy Initial Capacity

[praveenc7]

Summary

For GroupBy queries, the default size of the GroupByResultHolder is set to 10K(or limit length), which can lead to inefficient resource usage in cases where fewer group-by keys are expected, such as in queries with highly selective filters.

select column1, sum(column2) from testTable where column1 in ("123") group by column1 limit 20000

Description

This update dynamically adjusts the initial capacity of the GroupByResultHolder based on the filter predicates for such queries. By aligning the result holder size with the filter, we aim to optimize resource allocation and improve performance for filtered group-by queries.

Testing

We did some perf testing on some prod queries and great improvement in the p99 latency

1. Performance Improvement We observed a 1.5x improvement in P99.9 latency on a set of queries that benefited from the optimization. Below are sample query formats tested:

SELECT column1, SUM(column2) FROM testTable WHERE column1 IN ("123") GROUP BY column1 LIMIT 20000 ``SELECT column1, column2, SUM(column3) FROM testTable WHERE column1 IN ("123") AND column2 = '123' GROUP BY column1, column2 LIMIT 20000

The Flame graph is also much more normal

2. Impact on Queries with Unmet Conditions For queries processed by the new logic but ultimately exiting the optimization due to unmatched conditions (e.g., insufficient GROUP BY clauses), no significant latency overhead was observed. The framegraph results showed similar performance to existing logic:

SELECT column1, column2, SUM(column3) FROM testTable WHERE column1 IN ("123") GROUP BY column1, column2 LIMIT 20000

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 67.16418% with 22 lines in your changes missing coverage. Please review.

Project coverage is 63.78%. Comparing base (59551e4) to head (325121b). Report is 1256 commits behind head on master.

Files with missing lines	Patch %	Lines
...upby/NoDictionaryMultiColumnGroupKeyGenerator.java	25.00%	6 Missing and 3 partials :warning:
...ry/aggregation/groupby/DefaultGroupByExecutor.java	80.00%	1 Missing and 5 partials :warning:
...pby/NoDictionarySingleColumnGroupKeyGenerator.java	33.33%	3 Missing and 1 partial :warning:
...tion/groupby/DictionaryBasedGroupKeyGenerator.java	83.33%	1 Missing and 2 partials :warning:

Additional details and impacted files

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##             master   #14001      +/-   ##
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  Files          2436     2660     +224     
  Lines        133233   145740   +12507     
  Branches      20636    22308    +1672     
============================================
+ Hits          82274    92954   +10680     
- Misses        44911    45915    +1004     
- Partials       6048     6871     +823     

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[Jackie-Jiang]

These queries are very uncommon, and I don't know if it is worth taking the overhead for all group-by queries. Alternatively, we might just add a query option to override the initial capacity if desired

[praveenc7]

These queries are very uncommon, and I don't know if it is worth taking the overhead for all group-by queries. Alternatively, we might just add a query option to override the initial capacity if desired

@Jackie-Jiang Thanks you for your input! We currently optimize this behavior specifically for GROUP BY queries with filters, and based on our initial performance testing, the overhead was comparable.

However, I see your concern and can certainly add a query option to enable this optimization if desired

[praveenc7]

Discussed offline. I think a more general problem here is when we group by on columns that partially/none have constraints (in filter clause), but themselves low cardinality. @praveenc7 could you maybe try to explore if it is easy to do some thing like min(cartesian_product, default_min_value). The cartesian_product part can be product of the in/eq filter size or column cardinality? In this sense I feel it would probably more generally applicable. In https://github.com/apache/pinot/pull/9420/files we made some of the cardinality accessible, you can maybe take a look.

@jasperjiaguo Looking more closer at the code, it looks like this condition is already handled in DictionaryBasedGroupKeyGenerator and when intializing the size we do the min(cardinality_product, default_product) or min(cardinality_product, optimized value) [If our optimization is in place] in DefaultGroupByResultHolder

[jasperjiaguo]

@praveenc7 I see, thanks for checking! I think the only (small) gap here is that when we have

select ... where A in (a1, a2, a3) group by A,B

Do we currently handle this mixture of bounded + unbounded? ideally the effective card estimation could be calculated with cardinality_product/card(A) * filterlen(A)

[praveenc7]

@praveenc7 I see, thanks for checking! I think the only (small) gap here is that when we have

select ... where A in (a1, a2, a3) group by A,B

Do we currently handle this mixture of bounded + unbounded? ideally the effective card estimation could be calculated with cardinality_product/card(A) * filterlen(A)

@jasperjiaguo Agree this might create a more tighter bound, currently the cardinality is available for DictionaryBasedGroupKey so the unbounded case would be mostly applicable to Dictionary based column, might need some more change to support this. Was thinking of doing in a follow up PR

[jasperjiaguo]

Let's think about the edge case of

select ... where (A in (a1, a2, a3) or C > c) group by A

[praveenc7]

Let's think about the edge case of

select ... where (A in (a1, a2, a3) or C > c) group by A

@jasperjiaguo Added condition that checks for top-level AND and also added mixed support for both bounded & unbounded