apache / apache/datafusion

Push limit into joins

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#18,295 2 comments 0 reactions 1 assignee Claimed by @jonathanc-n View on GitHub
enhancement
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Description

### Is your feature request related to a problem or challenge?

Related to https://github.com/apache/datafusion/issues/15628

Part of https://github.com/apache/datafusion/issues/15885

For queries like
```
select *
from t1
join t2
on expensive_and_selective_predicate(t1.v1, t2.v1)
limit 10
```

The query plan will look like:
```
-- For simpler demonstration
> set datafusion.execution.target_partitions = 1;
0 row(s) fetched.
Elapsed 0.000 seconds.

> explain analyze select *
from generate_series(100000) as t1(v1)
join generate_series(100000) as t2(v1)
on t1.v1 < t2.v1
limit 10;
+-------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| plan_type | plan |
+-------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Plan with Metrics | GlobalLimitExec: skip=0, fetch=10, metrics=[output_rows=10, elapsed_compute=5.917µs] |
| | NestedLoopJoinExec: join_type=Inner, filter=v1@0 < v1@1, metrics=[output_rows=8192, elapsed_compute=582.002µs, build_input_batches=13, build_input_rows=100001, input_batches=1, input_rows=8192, output_batches=1, build_mem_used=853216, build_time=440.208µs, join_time=141.792µs] |
| | ProjectionExec: expr=[value@0 as v1], metrics=[output_rows=100001, elapsed_compute=6.415µs] |
| | LazyMemoryExec: partitions=1, batch_generators=[generate_series: start=0, end=100000, batch_size=8192], metrics=[output_rows=100001, elapsed_compute=223.997µs] |
| | ProjectionExec: expr=[value@0 as v1], metrics=[output_rows=8192, elapsed_compute=792ns] |
| | LazyMemoryExec: partitions=1, batch_generators=[generate_series: start=0, end=100000, batch_size=8192], metrics=[output_rows=8192, elapsed_compute=15.667µs] |
| | |
+-------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row(s) fetched.
Elapsed 0.004 seconds.
```

### Issue
There is certain early termination applied to avoid joining till the end, however such early termination is done at batch level, instead of row level. It's possible to terminate sooner and speed up the above (1st one) workload.

The global limit executor terminates the execution once the target limit is reached, and NLJ will accumulate `batch_size`(default 8192) rows inside before output, instead of stop once limit `10` is reached.

### Potential Optimization
Ideally it can stop once `limit` is reached, and become 800x (8192 buffer size / 10 limit) faster in the best case.

### Describe the solution you'd like

Push limit into nested loop join operator (potentially also other join types) , and it should terminate once the limit is reached.

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

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