Github user maropu commented on the pull request:
https://github.com/apache/spark/pull/10898#issuecomment-175992128
I found that it's not easy to support this type in postgresql in the
current interface of `JdbcDialect`.
This is because the `postgresql-jdbc` implementation cannot handle a
decimal type name with precision and scale such as `DECIMAL(xx, xx)` in
`Connection#createArrayOf`.
To fix this issue, we can add a specific entry in
`PostgresDialect#getJDBCType`
(https://github.com/apache/spark/pull/10898/files#diff-23da60722bc6bfc160cbb59bd99f5925R64).
However, this fix makes things worse; `JdbcUtils#schemaString` cannot pass
precision and scale for `Decimal` when relation defined in postgresql through
`DataFrameWriter`.
I'm not sure how to fix this though, it seems okay to just add TODO
comments and throw an unsupported exception for this type for the time being.
cc: @marmbrus
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