## What changes were proposed in this pull request?
When running FlatMapGroupsInPandasExec or AggregateInPandasExec the shuffle uses a default number of partitions of 200 in "spark.sql.shuffle.partitions". If the data is small, e.g. in testing, many of the partitions will be empty but are treated just the same.
This PR checks the `mapPartitionsInternal` iterator to be non-empty before calling `ArrowPythonRunner` to start computation on the iterator.
## How was this patch tested?
Existing tests. Ran the following benchmarks a simple example where most partitions are empty:
```python
from pyspark.sql.functions import pandas_udf, PandasUDFType
from pyspark.sql.types import *
df = spark.createDataFrame(
[(1, 1.0), (1, 2.0), (2, 3.0), (2, 5.0), (2, 10.0)],
("id", "v"))
pandas_udf("id long, v double", PandasUDFType.GROUPED_MAP)
def normalize(pdf):
v = pdf.v
return pdf.assign(v=(v - v.mean()) / v.std())
df.groupby("id").apply(normalize).count()
```
**Before**
```
In [4]: %timeit df.groupby("id").apply(normalize).count()
1.58 s ± 62.8 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
In [5]: %timeit df.groupby("id").apply(normalize).count()
1.52 s ± 29.5 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
In [6]: %timeit df.groupby("id").apply(normalize).count()
1.52 s ± 37.8 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
```
**After this Change**
```
In [2]: %timeit df.groupby("id").apply(normalize).count()
646 ms ± 89.9 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
In [3]: %timeit df.groupby("id").apply(normalize).count()
408 ms ± 84.6 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
In [4]: %timeit df.groupby("id").apply(normalize).count()
381 ms ± 29.9 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
```
Closes#24926 from BryanCutler/pyspark-pandas_udf-map-agg-skip-empty-parts-SPARK-28128.
Authored-by: Bryan Cutler <cutlerb@gmail.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>
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