2010YOUY01 opened a new issue, #18407:
URL: https://github.com/apache/datafusion/issues/18407
### Is your feature request related to a problem or challenge?
In a Nested Loop Join, the selectivity metric is defined as `output_rows /
possible_combinations`. I believe this provides useful application-level
insight.
### Example
In the below query (run in `datafusion-cli`)
```
> set datafusion.explain.analyze_level = summary;
0 row(s) fetched.
Elapsed 0.000 seconds.
> explain analyze select *
from generate_series(10) as t1(a)
join generate_series(10) as t2(b)
on (t1.a + t2.b) = 20;
> explain analyze select *
from generate_series(10) as t1(a)
join generate_series(10) as t2(b)
on (t1.a + t2.b) = 20;
+-------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| plan_type | plan
|
+-------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Plan with Metrics | NestedLoopJoinExec: join_type=Inner, filter=a@0 + b@1
= 20, metrics=[output_rows=1, elapsed_compute=709.665µs, output_bytes=128.0 KB]
|
| | ProjectionExec: expr=[value@0 as a],
metrics=[output_rows=11, elapsed_compute=1.958µs, output_bytes=64.0 KB]
|
| | LazyMemoryExec: partitions=1,
batch_generators=[generate_series: start=0, end=10, batch_size=8192],
metrics=[output_rows=11, elapsed_compute=13.083µs, output_bytes=64.0 KB] |
| | RepartitionExec: partitioning=RoundRobinBatch(14),
input_partitions=1, metrics=[]
|
| | ProjectionExec: expr=[value@0 as b],
metrics=[output_rows=11, elapsed_compute=584ns, output_bytes=64.0 KB]
|
| | LazyMemoryExec: partitions=1,
batch_generators=[generate_series: start=0, end=10, batch_size=8192],
metrics=[output_rows=11, elapsed_compute=14.875µs, output_bytes=64.0 KB] |
| |
|
+-------------------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row(s) fetched.
Elapsed 0.003 seconds.
```
Logically NLJ will do
```
for left_row in t1:
for right_row in t2:
if left_row.a + right_row.b = 20:
output(left_row, right_row)
```
The selectivity will be calculated as `output_size / (left_size *
right_size)` => `1 / (10 * 10)` => `1%`
### Describe the solution you'd like
Add a `selectivity` metrics to `NestedLoopJoinExec`.
Reference PR: https://github.com/apache/datafusion/pull/18406
### Describe alternatives you've considered
_No response_
### Additional context
_No response_
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