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https://issues.apache.org/jira/browse/SPARK-35386?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17345821#comment-17345821
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Rafal Wojdyla commented on SPARK-35386:
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{quote}
IIRC, it's not documented. We might think about documenting this behaviour.
{quote}
I don't really have the context for this, and also consider the current
behaviour to be a bug.
{quote}
You could clip the schema from the file and user specified schema, and compare
by recursively traversing.
{quote}
I mean, I certainly could, I'm just thinking what is the good user experience
for this. Assuming post-read schema comparison:
{code:python}
my_read_schema = ...
df = spark.read.parquet(..., schema=my_read_schema)
# custom code to compare df.schema to my_read_schema, it would:
# ignore metadata, and fail on nullable -> required "promotion"
# but for example allow required -> nullable promotion
{code}
This is obviously not a great user experience, and likely won't handle all
corner cases.
I think as a user I would prefer if:
* if the user specified schema has a {{nullable=False}}/{{required}} column
that doesn't exist in the data, the read should fail
* if the user specified schema has a {{nullable=True}} column that doesn't
exist in the data, the read would create a new "empty" column
This allows for schema evolution, where you can promote {{required}} to
{{nullable}}. And allows for to user to control the read op, and avoid extra
complicated schema comparison code.
[~hyukjin.kwon] wdyt?
> parquet read with schema should fail on non-existing columns
> ------------------------------------------------------------
>
> Key: SPARK-35386
> URL: https://issues.apache.org/jira/browse/SPARK-35386
> Project: Spark
> Issue Type: Bug
> Components: Input/Output, PySpark
> Affects Versions: 3.0.1
> Reporter: Rafal Wojdyla
> Priority: Major
>
> When read schema is specified as I user I would prefer/like if spark failed
> on missing columns.
> {code:python}
> from pyspark.sql.dataframe import DoubleType, StructType
> spark: SparkSession = ...
> spark.read.parquet("/tmp/data.snappy.parquet")
> # inferred schema, includes 3 columns: col1, col2, new_col
> # DataFrame[col1: bigint, col2: bigint, new_col: bigint]
> # let's specify a custom read_schema, with **non nullable** col3 (which is
> not present):
> read_schema = StructType(fields=[StructField("col3",DoubleType(),False)])
> df = spark.read.schema(read_schema).parquet("/tmp/data.snappy.parquet")
> df.schema
> # we get a DataFrame with **nullable** col3:
> # StructType(List(StructField(col3,DoubleType,true)))
> df.count()
> # 0
> {code}
> Is this a feature or a bug? In this case there's just a single parquet file,
> I have also tried {{option("mergeSchema", "true")}}, which doesn't help.
> Similar read pattern would fail on pandas (and likely dask).
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