Ondrej Kokes created SPARK-32989:
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Summary: Performance regression when selecting from str_to_map
Key: SPARK-32989
URL: https://issues.apache.org/jira/browse/SPARK-32989
Project: Spark
Issue Type: Bug
Components: SQL
Affects Versions: 3.0.1
Reporter: Ondrej Kokes
When I create a map using str_to_map and select more than a single value, I
notice a notable performance regression in 3.0.1 compared to 2.4.7. When
selecting a single value, the performance is the same. Plans are identical
between versions.
It seems like in 2.x the map from str_to_map is preserved for a given row, but
in 3.x it's recalculated for each column. One hint that it might be the case is
that when I tried forcing materialisation of said map in 3.x (by a coalesce,
don't know if there's a better way), I got the performance roughly to 2.x
levels.
Here's a reproducer:
{code}
$ head regression.csv
foo
foo=bar&baz=bak&bar=foo
foo=bar&baz=bak&bar=foo
foo=bar&baz=bak&bar=foo
foo=bar&baz=bak&bar=foo
foo=bar&baz=bak&bar=foo
... (10M more rows)
{code}
{code:python}
import time
import pyspark
from pyspark.sql import SparkSession
import pyspark.sql.functions as f
if __name__ == '__main__':
print(pyspark.__version__)
spark = SparkSession.builder.getOrCreate()
df = spark.read.option('header', True).csv('regression.csv')
t = time.time()
dd = (df
.withColumn('my_map', f.expr('str_to_map(foo, "&", "=")'))
.select(
f.col('my_map')['foo'],
)
)
dd.write.mode('overwrite').csv('tmp')
t2 = time.time()
print('selected one', t2 - t)
dd = (df
.withColumn('my_map', f.expr('str_to_map(foo, "&", "=")'))
# .coalesce(100) # forcing evaluation before selection speeds it up
in 3.0.1
.select(
f.col('my_map')['foo'],
f.col('my_map')['bar'],
f.col('my_map')['baz'],
)
)
dd.explain(True)
dd.write.mode('overwrite').csv('tmp')
t3 = time.time()
print('selected three', t3 - t2)
{code}
Results for 2.4.7 and 3.0.1, both installed from PyPI, Python 3.7, macOS (times
are in seconds)
{code}
# 3.0.1
# selected one 6.375471830368042
# selected three 14.847578048706055
# 2.4.7
# selected one 6.679579019546509
# selected three 6.5622029304504395
{code}
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