[SPARK-10731][SQL] Delegate to Scala's DataFrame.take implementation in Python DataFrame. #8876
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…in Python DataFrame.
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Test build #42876 has finished for PR 8876 at commit
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Test build #42891 has finished for PR 8876 at commit
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Test build #1793 has finished for PR 8876 at commit
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LGTM |
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Test build #42920 has finished for PR 8876 at commit
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asfgit
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Sep 23, 2015
… in Python DataFrame. Python DataFrame.head/take now requires scanning all the partitions. This pull request changes them to delegate the actual implementation to Scala DataFrame (by calling DataFrame.take). This is more of a hack for fixing this issue in 1.5.1. A more proper fix is to change executeCollect and executeTake to return InternalRow rather than Row, and thus eliminate the extra round-trip conversion. Author: Reynold Xin <rxin@databricks.com> Closes #8876 from rxin/SPARK-10731. (cherry picked from commit 9952217) Signed-off-by: Reynold Xin <rxin@databricks.com>
asfgit
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Sep 14, 2016
… same in Python ## What changes were proposed in this pull request? In PySpark, `df.take(1)` runs a single-stage job which computes only one partition of the DataFrame, while `df.limit(1).collect()` computes all partitions and runs a two-stage job. This difference in performance is confusing. The reason why `limit(1).collect()` is so much slower is that `collect()` internally maps to `df.rdd.<some-pyspark-conversions>.toLocalIterator`, which causes Spark SQL to build a query where a global limit appears in the middle of the plan; this, in turn, ends up being executed inefficiently because limits in the middle of plans are now implemented by repartitioning to a single task rather than by running a `take()` job on the driver (this was done in #7334, a patch which was a prerequisite to allowing partition-local limits to be pushed beneath unions, etc.). In order to fix this performance problem I think that we should generalize the fix from SPARK-10731 / #8876 so that `DataFrame.collect()` also delegates to the Scala implementation and shares the same performance properties. This patch modifies `DataFrame.collect()` to first collect all results to the driver and then pass them to Python, allowing this query to be planned using Spark's `CollectLimit` optimizations. ## How was this patch tested? Added a regression test in `sql/tests.py` which asserts that the expected number of jobs, stages, and tasks are run for both queries. Author: Josh Rosen <joshrosen@databricks.com> Closes #15068 from JoshRosen/pyspark-collect-limit. (cherry picked from commit 6d06ff6) Signed-off-by: Davies Liu <davies.liu@gmail.com>
asfgit
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Sep 14, 2016
… same in Python ## What changes were proposed in this pull request? In PySpark, `df.take(1)` runs a single-stage job which computes only one partition of the DataFrame, while `df.limit(1).collect()` computes all partitions and runs a two-stage job. This difference in performance is confusing. The reason why `limit(1).collect()` is so much slower is that `collect()` internally maps to `df.rdd.<some-pyspark-conversions>.toLocalIterator`, which causes Spark SQL to build a query where a global limit appears in the middle of the plan; this, in turn, ends up being executed inefficiently because limits in the middle of plans are now implemented by repartitioning to a single task rather than by running a `take()` job on the driver (this was done in #7334, a patch which was a prerequisite to allowing partition-local limits to be pushed beneath unions, etc.). In order to fix this performance problem I think that we should generalize the fix from SPARK-10731 / #8876 so that `DataFrame.collect()` also delegates to the Scala implementation and shares the same performance properties. This patch modifies `DataFrame.collect()` to first collect all results to the driver and then pass them to Python, allowing this query to be planned using Spark's `CollectLimit` optimizations. ## How was this patch tested? Added a regression test in `sql/tests.py` which asserts that the expected number of jobs, stages, and tasks are run for both queries. Author: Josh Rosen <joshrosen@databricks.com> Closes #15068 from JoshRosen/pyspark-collect-limit.
wgtmac
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Sep 19, 2016
… same in Python ## What changes were proposed in this pull request? In PySpark, `df.take(1)` runs a single-stage job which computes only one partition of the DataFrame, while `df.limit(1).collect()` computes all partitions and runs a two-stage job. This difference in performance is confusing. The reason why `limit(1).collect()` is so much slower is that `collect()` internally maps to `df.rdd.<some-pyspark-conversions>.toLocalIterator`, which causes Spark SQL to build a query where a global limit appears in the middle of the plan; this, in turn, ends up being executed inefficiently because limits in the middle of plans are now implemented by repartitioning to a single task rather than by running a `take()` job on the driver (this was done in apache#7334, a patch which was a prerequisite to allowing partition-local limits to be pushed beneath unions, etc.). In order to fix this performance problem I think that we should generalize the fix from SPARK-10731 / apache#8876 so that `DataFrame.collect()` also delegates to the Scala implementation and shares the same performance properties. This patch modifies `DataFrame.collect()` to first collect all results to the driver and then pass them to Python, allowing this query to be planned using Spark's `CollectLimit` optimizations. ## How was this patch tested? Added a regression test in `sql/tests.py` which asserts that the expected number of jobs, stages, and tasks are run for both queries. Author: Josh Rosen <joshrosen@databricks.com> Closes apache#15068 from JoshRosen/pyspark-collect-limit.
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Python DataFrame.head/take now requires scanning all the partitions. This pull request changes them to delegate the actual implementation to Scala DataFrame (by calling DataFrame.take).
This is more of a hack for fixing this issue in 1.5.1. A more proper fix is to change executeCollect and executeTake to return InternalRow rather than Row, and thus eliminate the extra round-trip conversion.