Adam Budde created SPARK-19455:
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Summary: Add option for case-insensitive Parquet field resolution
Key: SPARK-19455
URL: https://issues.apache.org/jira/browse/SPARK-19455
Project: Spark
Issue Type: Improvement
Components: SQL
Affects Versions: 2.1.0
Reporter: Adam Budde
[SPARK-16980|https://issues.apache.org/jira/browse/SPARK-16980] removed the
schema inferrence from the HiveMetastoreCatalog class when converting a
MetastoreRelation to a LoigcalRelation (HadoopFsRelation, in this case) in
favor of simply using the schema returend by the metastore. This results in an
optimization as the underlying file status no longer need to be resolved until
after the partition pruning step, reducing the number of files to be touched
significantly in some cases. The downside is that the data schema used may no
longer match the underlying file schema for case-sensitive formats such as
Parquet.
This change initially included a [patch to
ParquetReadSupport|https://github.com/apache/spark/blob/6ce1b675ee9fc9a6034439c3ca00441f9f172f84/sql/core/src/main/scala/org/apache/spark/sql/execution/datasources/parquet/ParquetReadSupport.scala#L270-L284]
that attempted to remedy this conflict by using a case-insentive fallback
mapping when resolving field names during the schema clipping step.
[SPARK-1833|https://issues.apache.org/jira/browse/SPARK-18333] later removed
this patch after
[SPARK-17183|https://issues.apache.org/jira/browse/SPARK-17183] added support
for embedding a case-sensitive schema as a Hive Metastore table property. AFAIK
the assumption here was that the data schema obtained from the Metastore table
property will be case sensitive and should match the Parquet schema exactly.
The problem arises when dealing with Parquet-backed tables for which this
schema has not been embedded as a table attributes and for which the underlying
files contain case-sensitive field names. This will happen for any Hive table
that was not created by Spark or created by a version prior to 2.1.0. We've
seen Spark SQL return no results for any query containing a case-sensitive
field name for such tables.
The change we're proposing is to introduce a configuration parameter that will
re-enable case-insensitive field name resolution in ParquetReadSupport. This
option will also disable filter push-down for Parquet, as the filter predicate
constructed by Spark SQL contains the case-insensitive field names which
Parquet will return 0 records for when filtering against a case-sensitive
column name. I was hoping to find a way to construct the filter on-the-fly in
ParquetReadSupport but Parquet doesn't propegate the Configuration object
passed to this class to the underlying InternalParquetRecordReader class.
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