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Grouping Algorithm

In this guide we will learn about the heuristics used for trimming results in Pinot's grouping algorithm (used when processing GROUP BY queries) to make sure that the server doesn't run out of memory.
Within segment
When grouping rows within a segment, Pinot keeps a maximum of <numGroupsLimit> groups per segment. This value is set to 100,000 by default and can be configured by the pinot.server.query.executor.num.groups.limit property.
If the number of groups of a segment reaches this value, the extra groups will be ignored and the results returned may not be completely accurate. The numGroupsLimitReached property will be set to true in the query response if the value is reached.
Trimming tail groups
After the inner segment groups have been computed, the Pinot query engine optionally trims tail groups. Tail groups are ones that have a lower rank based on the ORDER BY clause used in the query.
This configuration is disabled by default, but can be enabled by configuring the pinot.server.query.executor.min.segment.group.trim.size property. 
When segment group trim is enabled, the query engine will trim the tail groups and keep max(<minSegmentGroupTrimSize>, 5 * LIMIT) groups if it gets more groups. Pinot keeps at least 5 * LIMIT groups when trimming tail groups to ensure the accuracy of results.
This value can be overridden on a query by query basis by passing the following option:
SELECT * 
FROM ...
​
OPTION(minSegmentGroupTrimSize=<minSegmentGroupTrimSize>)
Cross segments
Once grouping has been done within a segment, Pinot will merge segment results and trim tail groups and keep max(<minServerGroupTrimSize>, 5 * LIMIT) groups if it gets more groups.
<minServerGroupTrimSize> is set to 5,000 by default and can be adjusted by configuring the pinot.server.query.executor.min.server.group.trim.size property. When setting the configuration to -1, the cross segments trim can be disabled.
This value can be overridden on a query by query basis by passing the following option:
SELECT * 
FROM ...
​
OPTION(minServerGroupTrimSize=<minServerGroupTrimSize>)
When cross segments trim is enabled, the server will trim the tail groups before sending the results back to the broker. It will also trim the tail groups when the number of groups reaches the <trimThreshold>. 
This configuration is set to 1,000,000 by default and can be adjusted by configuring the pinot.server.query.executor.groupby.trim.threshold property. 
A higher threshold reduces the amount of trimming done, but consumes more heap memory. If the threshold is set to more than 1,000,000,000, the server will only trim the groups once before returning the results to the broker.
GROUP BY behavior
Pinot sets a default LIMIT of 10 if one isn't defined and this applies to GROUP BY queries as well. Therefore, if no limit is specified, Pinot will return 10 groups.
Pinot will trim tail groups based on the ORDER BY clause to reduce the memory footprint and improve the query performance. It keeps at least 5 * LIMIT groups so that the results give good enough approximation in most cases. The configurable min trim size can be used to increase the groups kept to improve the accuracy but has a larger extra memory footprint.
HAVING behavior
If the query has a HAVING clause, it is applied on the merged GROUP BY results that already have the tail groups trimmed. If the HAVING clause is the opposite of the ORDER BY order, groups matching the condition might already be trimmed and not returned. e.g.
SELECT SUM(colA) 
FROM myTable 
GROUP BY colB 
ORDER BY SUM(colA) DESC 
HAVING SUM(colA) < 100 
LIMIT 10
Increase min trim size to keep more groups in these cases.
Configuration Parameters
Parameter
Default
Query Override
pinot.server.query.executor.num.groups.limit
The maximum number of groups allowed per segment.
100,000
N/A
pinot.server.query.executor.min.segment.group.trim.size
The minimum number of groups to keep when trimming groups at the segment level.
-1 (trim disabled)
OPTION(minSegmentGroupTrimSize=<minSegmentGroupTrimSize>)
pinot.server.query.executor.min.server.group.trim.size
The minimum number of groups to keep when trimming groups at the server level.
5,000
OPTION(minServerGroupTrimSize=<minServerGroupTrimSize>)
pinot.server.query.executor.groupby.trim.threshold
The number of groups to trigger the server level trim.
1,000,000
N/A
pinot.server.query.executor.max.execution.threads
The maximum number of execution threads (parallelism of segment processing) used per query.
-1 (use all execution threads)
OPTION(maxExecutionThreads=<maxExecutionThreads>)
​

Parameter	Default	Query Override
`pinot.server.query.executor.num.groups.limit` The maximum number of groups allowed per segment.	100,000	N/A
`pinot.server.query.executor.min.segment.group.trim.size` The minimum number of groups to keep when trimming groups at the segment level.	-1 (trim disabled)	`OPTION(minSegmentGroupTrimSize=<minSegmentGroupTrimSize>)`
`pinot.server.query.executor.min.server.group.trim.size` The minimum number of groups to keep when trimming groups at the server level.	5,000	`OPTION(minServerGroupTrimSize=<minServerGroupTrimSize>)`
`pinot.server.query.executor.groupby.trim.threshold` The number of groups to trigger the server level trim.	1,000,000	N/A
`pinot.server.query.executor.max.execution.threads` The maximum number of execution threads (parallelism of segment processing) used per query.	-1 (use all execution threads)	`OPTION(maxExecutionThreads=<maxExecutionThreads>)`

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