Fix issue in union read

[luoyuxia]

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• I searched in the issues and found nothing similar.

Fluss version

0.8.0 (latest release)

Please describe the bug 🐞

During testing union read paimon, I found some critial issues:

• for partitioned table, if partition key is the first field of primary key, union read will return wrong result
• if some buckets don't have lake data, exception will be thrown, and the batch union read will never finish

Solution

No response

Are you willing to submit a PR?

• I'm willing to submit a PR!