Kai Kang created SPARK-29721:
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Summary: Spark SQL reads unnecessary nested fields from Parquet
after using explode
Key: SPARK-29721
URL: https://issues.apache.org/jira/browse/SPARK-29721
Project: Spark
Issue Type: Improvement
Components: SQL
Affects Versions: 2.4.4
Reporter: Kai Kang
This is a follow up for SPARK-4502. SPARK-4502 correctly addressed column
pruning for nested structures. However, when explode() is called on a nested
field, all columns for that nested structure is still fetched from data source.
We are working on a project to create a parquet store for a big pre-joined
table between two tables that has one-to-many relationship, and this is a
blocking issue for us.
The following code illustrates the issue.
Part 1: loading some nested data
{quote}{{import spark.implicits._}}
{{val jsonStr = """{}}
{{ "items": [}}
{{ {}}
{{ "itemId": 1,}}
{{ "itemData": "a"}}
{{ },}}
{{ {}}
{{ "itemId": 1,}}
{{ "itemData": "b"}}
{{ }}}
{{ ]}"""}}
{{val df = spark.read.json(Seq(jsonStr).toDS)}}
{{df.write.format("parquet").mode("overwrite").saveAsTable("persisted")}}
{quote}
Part 2: reading it back and explaining the queries
{quote}val read = spark.table("persisted")
spark.conf.set("spark.sql.optimizer.nestedSchemaPruning.enabled", true)
read.select($"items.itemId").explain(true) // pruned, only loading itemId
read.select(explode($"items.itemId")).explain(true) // not pruned, loading both
itemId and itemData
{quote}
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