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Posted to issues@spark.apache.org by "Wenchen Fan (JIRA)" <ji...@apache.org> on 2019/04/05 05:36:00 UTC
[jira] [Assigned] (SPARK-27356) File source V2: return actual
schema in method `FileScan.readSchema`
[ https://issues.apache.org/jira/browse/SPARK-27356?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]
Wenchen Fan reassigned SPARK-27356:
-----------------------------------
Assignee: Gengliang Wang
> File source V2: return actual schema in method `FileScan.readSchema`
> --------------------------------------------------------------------
>
> Key: SPARK-27356
> URL: https://issues.apache.org/jira/browse/SPARK-27356
> Project: Spark
> Issue Type: Improvement
> Components: SQL
> Affects Versions: 3.0.0
> Reporter: Gengliang Wang
> Assignee: Gengliang Wang
> Priority: Major
>
> The method `Scan.readSchema` returns the actual schema of this data source scan.
> In the current file source V2 framework, the schema is not returned correctly. The result should be `readDataSchema + partitionSchema`, while the framework returns the schema which is pushed down in `SupportsPushDownRequiredColumns.requiredSchema`.
> This is normally OK. But if there are overlap columns between `dataSchema` and `partitionSchema`, the result of row-base scan will be wrong. The actual schema should be
> `dataSchema - overlapSchema + partitionSchema`, which is different from from the pushed down `requiredSchema`.
> This PR is to:
> 1. Bug fix: fix the corner case that `dataSchema` overlaps with `partitionSchema`.
> 2. Improvement: Prune partition column values if part of the partition columns are not required.
> 3. Behavior change: To make it simple, the schema of `FileTable` is `dataSchema - overlapSchema + partitionSchema`, instead of mixing data schema and partitionSchema (see `PartitioningUtils.mergeDataAndPartitionSchema`)
> For example, the data schema is [a,b,c], the partition schema is [b,d],
> In V1, the schema of `HadoopFsRelation` is [a, b, c, d]
> in File source V2 , the schema of `FileTable` is [a, c, b, d]
> Putting all the partition columns to the end of table schema is more reasonable. Also, when there is `select *` operation and there is no schema pruning, the schema of `FileTable` and `FileScan` still matches.
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