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https://issues.apache.org/jira/browse/SPARK-20413?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Apache Spark reassigned SPARK-20413:
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Assignee: Apache Spark
> New Optimizer Hint to prevent collapsing of adjacent projections
> ----------------------------------------------------------------
>
> Key: SPARK-20413
> URL: https://issues.apache.org/jira/browse/SPARK-20413
> Project: Spark
> Issue Type: Improvement
> Components: Optimizer, PySpark, SQL
> Affects Versions: 2.1.0
> Reporter: Michael Styles
> Assignee: Apache Spark
>
> I am proposing that a new optimizer hint called NO_COLLAPSE be introduced.
> This hint is essentially identical to Oracle's NO_MERGE hint.
> Let me first give an example of why I am proposing this.
> df1 = sc.sql.createDataFrame([(1, "abc")], ["id", "user_agent"])
> df2 = df1.withColumn("ua", user_agent_details(df1["user_agent"]))
> df3 = df2.select(df2["ua"].device_form_factor.alias("c1"),
> df2["ua"].browser_version.alias("c2"))
> df3.explain(True)
> == Parsed Logical Plan ==
> 'Project [ua#85[device_form_factor] AS c1#90, ua#85[browser_version] AS
> c2#91]
> +- Project [id#80L, user_agent#81, UDF(user_agent#81) AS ua#85]
> +- LogicalRDD [id#80L, user_agent#81]
> == Analyzed Logical Plan ==
> c1: string, c2: string
> Project [ua#85.device_form_factor AS c1#90, ua#85.browser_version AS c2#91]
> +- Project [id#80L, user_agent#81, UDF(user_agent#81) AS ua#85]
> +- LogicalRDD [id#80L, user_agent#81]
> == Optimized Logical Plan ==
> Project [UDF(user_agent#81).device_form_factor AS c1#90,
> UDF(user_agent#81).browser_version AS c2#91]
> +- LogicalRDD [id#80L, user_agent#81]
> == Physical Plan ==
> *Project [UDF(user_agent#81).device_form_factor AS c1#90,
> UDF(user_agent#81).browser_version AS c2#91]
> +- Scan ExistingRDD[id#80L,user_agent#81]
> user_agent_details is a user-defined function that returns a struct. As can
> be seen from the generated query plan, the function is being executed
> multiple times which could lead to performance issues. This is due to the
> CollapseProject optimizer rule that collapses adjacent projections.
> I'm proposing a hint that prevent the optimizer from collapsing adjacent
> projections. A new function called 'no_collapse' would be introduced for this
> purpose. Consider the following example and generated query plan.
> df1 = sc.sql.createDataFrame([(1, "abc")], ["id", "user_agent"])
> df2 = F.no_collapse(df1.withColumn("ua",
> user_agent_details(df1["user_agent"])))
> df3 = df2.select(df2["ua"].device_form_factor.alias("c1"),
> df2["ua"].browser_version.alias("c2"))
> df3.explain(True)
> == Parsed Logical Plan ==
> 'Project [ua#69[device_form_factor] AS c1#75, ua#69[browser_version] AS
> c2#76]
> +- NoCollapseHint
> +- Project [id#64L, user_agent#65, UDF(user_agent#65) AS ua#69]
> +- LogicalRDD [id#64L, user_agent#65]
> == Analyzed Logical Plan ==
> c1: string, c2: string
> Project [ua#69.device_form_factor AS c1#75, ua#69.browser_version AS c2#76]
> +- NoCollapseHint
> +- Project [id#64L, user_agent#65, UDF(user_agent#65) AS ua#69]
> +- LogicalRDD [id#64L, user_agent#65]
> == Optimized Logical Plan ==
> Project [ua#69.device_form_factor AS c1#75, ua#69.browser_version AS c2#76]
> +- NoCollapseHint
> +- Project [UDF(user_agent#65) AS ua#69]
> +- LogicalRDD [id#64L, user_agent#65]
> == Physical Plan ==
> *Project [ua#69.device_form_factor AS c1#75, ua#69.browser_version AS c2#76]
> +- *Project [UDF(user_agent#65) AS ua#69]
> +- Scan ExistingRDD[id#64L,user_agent#65]
> As can be seen from the query plan, the user-defined function is now
> evaluated once per row.
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