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Happens generally when field marked as recordKey or partitionKey is not present in some incoming record. Please cross verify your incoming record once. 

2.4 if it is possible to use a nullable field that contains null records as a primary key when creating hudi table

No, will throw HoodieKeyException

Caused by: org.apache.hudi.exception.HoodieKeyException: recordKey value: "null" for field: "name" cannot be null or empty.
  at org.apache.hudi.keygen.SimpleKeyGenerator.getKey(SimpleKeyGenerator.java:58)
  at org.apache.hudi.HoodieSparkSqlWriter$$anonfun$1.apply(HoodieSparkSqlWriter.scala:104)
  at org.apache.hudi.HoodieSparkSqlWriter$$anonfun$1.apply(HoodieSparkSqlWriter.scala:100)

Hive Sync

3.1 Caused by: java.sql.SQLException: Error while processing statement: FAILED: Execution Error, return code 1 from org.apache.hadoop.hive.ql.exec.DDLTask. Unable to alter table. The following columns have types incompatible with the existing columns in their respective positions :
__col1,__col2

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