niebayes opened a new issue, #14974:
URL: https://github.com/apache/datafusion/issues/14974
### Is your feature request related to a problem or challenge?
In scalar aggregation queries, the result is always a single row. As a
result, applying a LIMIT operator above the Aggregate operator has no effect
unless the limit is set to 0. Therefore, I propose we can safely eliminate
unnecessary LIMIT operators in scalar aggregation queries.
For example, given a sql
``` sql
select count(*) from t limit 10
```
The `count` UDF always returns a single row and applying a `limit 10` is
definitely unnecessary. However, the execution plan has a top `GlobalLimitExec`
operator.
```
+---------------+------------------------------------------------------------------------------+
| plan_type | plan
|
+---------------+------------------------------------------------------------------------------+
| logical_plan | Limit: skip=0, fetch=10
|
| | Aggregate: groupBy=[[]], aggr=[[count(Int64(1)) AS
count(*)]] |
| | TableScan: information_schema.tables projection=[]
|
| physical_plan | GlobalLimitExec: skip=0, fetch=10
|
| | AggregateExec: mode=Final, gby=[], aggr=[count(*)]
|
| | CoalescePartitionsExec
|
| | AggregateExec: mode=Partial, gby=[], aggr=[count(*)]
|
| | RepartitionExec: partitioning=RoundRobinBatch(8),
input_partitions=1 |
| | StreamingTableExec: partition_sizes=1
|
| |
|
+---------------+------------------------------------------------------------------------------+
```
### Describe the solution you'd like
I think we can design an optimizer rule (logical or physical ??) , to
eliminate such unnecessary LIMIT operators.
### Describe alternatives you've considered
_No response_
### Additional context
_No response_
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