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[SPARK-17356][SQL] Fix out of memory issue when generating JSON for T…
…reeNode ## What changes were proposed in this pull request? class `org.apache.spark.sql.types.Metadata` is widely used in mllib to store some ml attributes. `Metadata` is commonly stored in `Alias` expression. ``` case class Alias(child: Expression, name: String)( val exprId: ExprId = NamedExpression.newExprId, val qualifier: Option[String] = None, val explicitMetadata: Option[Metadata] = None, override val isGenerated: java.lang.Boolean = false) ``` The `Metadata` can take a big memory footprint since the number of attributes is big ( in scale of million). When `toJSON` is called on `Alias` expression, the `Metadata` will also be converted to a big JSON string. If a plan contains many such kind of `Alias` expressions, it may trigger out of memory error when `toJSON` is called, since converting all `Metadata` references to JSON will take huge memory. With this PR, we will skip scanning Metadata when doing JSON conversion. For a reproducer of the OOM, and analysis, please look at jira https://issues.apache.org/jira/browse/SPARK-17356. ## How was this patch tested? Existing tests. Author: Sean Zhong <[email protected]> Closes #14915 from clockfly/json_oom. (cherry picked from commit 6f13aa7) Signed-off-by: Wenchen Fan <[email protected]>
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