Udit Mehrotra created HUDI-915:
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Summary: Partition Columns missing in files upserted after
Metadata Bootstrap
Key: HUDI-915
URL: https://issues.apache.org/jira/browse/HUDI-915
Project: Apache Hudi (incubating)
Issue Type: Sub-task
Components: Common Core
Reporter: Udit Mehrotra
This issue happens in when the source data is partitioned using _*hive-style
partitioning*_ which is also the default behavior of spark when it writes the
data. With this partitioning, the partition column/schema is never stored in
the files but instead retrieved on the fly from the file paths which have
partition folder in the form *_partition_key=partition_value_*.
Now, during metadata bootstrap we store only the metadata columns in the hudi
table folder. Also the *bootstrap schema* we are computing directly reads
schema from the source data file which does not have the *partition column
schema* in it. Thus it is not complete.
All this manifests into issues when we ultimately do *upserts* on these
bootstrapped files and they are fully bootstrapped. During upsert time the
schema evolves because the upsert dataframe needs to have partition column in
it for performing upserts. Thus ultimately the *upserted rows* have the correct
partition column value stored, while the other records which are simply copied
over from the metadata bootstrap file have missing partition column in them.
Thus, we observe a different behavior here with *bootstrapped* vs
*non-bootstrapped* tables.
While this is not at the moment creating issues with *Hive* because it is able
to determine the partition columns becuase of all the metadata it stores,
however it creates a problem with other engines like *Spark* where the
partition columns will show up as *null* when the upserted files are read.
Thus, the proposal is to fix the following issues:
* When performing **bootstrap, figure out the partition schema and store it in
the *bootstrap schema* in the commit metadata file. This would provide the
following benefits:
** From a completeness perspective this is good so that there is no behavioral
changes between bootstrapped vs non-bootstrapped tables.
** In spark bootstrap relation and incremental query relation where we need to
figure out the latest schema, once can simply get the accurate schema from the
commit metadata file instead of having to determine whether or not partition
column is present in the schema obtained from the metadata file and if not
figure out the partition schema everytime and merge (which can be expensive).
* When doing upsert on files that are metadata bootstrapped, the partition
column values should be correctly determined and copied to the upserted file to
avoid missing and null values.
** Again this is consistent behavior with non-bootstrapped tables and even
though Hive seems to somehow handle this, we should consider other engines like
*Spark* where it cannot be automatically handled.
** Without this it will be significantly more complicated to be able to
provide the partition value on read side in spark, to be able to determine
everytime whether partition value is null and somehow filling it in.
** Once the table is fully bootstrapped at some point in future, and the
bootstrap commit is say cleaned up and spark querying happens through *parquet*
datasource instead of *new bootstrapped datasource*, the *parquet datasource*
will return null values wherever it find the missing partition values. In that
case, we have no control over the *parquet* datasource as it is simply reading
from the file.
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