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https://issues.apache.org/jira/browse/HUDI-915?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Balaji Varadarajan updated HUDI-915:
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Priority: Blocker (was: Major)
> Partition Columns missing in files upserted after Metadata Bootstrap
> --------------------------------------------------------------------
>
> Key: HUDI-915
> URL: https://issues.apache.org/jira/browse/HUDI-915
> Project: Apache Hudi
> Issue Type: Sub-task
> Components: Common Core
> Reporter: Udit Mehrotra
> Assignee: Udit Mehrotra
> Priority: Blocker
>
> 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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