[jira] [Created] (SPARK-5446) Parquet column pruning should work for Map and Struct

["Jianshi Huang (JIRA)" <ji...@apache.org>]

Jianshi Huang created SPARK-5446:
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             Summary: Parquet column pruning should work for Map and Struct
                 Key: SPARK-5446
                 URL: https://issues.apache.org/jira/browse/SPARK-5446
             Project: Spark
          Issue Type: Improvement
          Components: SQL
    Affects Versions: 1.2.0, 1.3.0
            Reporter: Jianshi Huang


Consider the following query:

{code:sql}
select stddev_pop(variables.var1) stddev
from model
group by model_name
{code}

Where variables is a Struct containing many fields, similarly it can be a Map with many key-value pairs.

During execution, SparkSQL will shuffle the whole map or struct column instead of extracting the value first. The performance is very poor.

The optimized version could use a subquery:
{code:sql}
select stddev_pop(var) stddev
from (select variables.var1 as var, model_name from model) m
group by model_name
{code}

Where we extract the field/key-value only in the mapper side, so data being shuffled is small.

Parquet already supports reading a single field/key-value in the storage level, but SparkSQL currently doesn’t have optimization for it. This will be very useful optimization for tables having Map or Struct with many columns. 

Jianshi



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