lightning: lightning doesn't check whether target table is empty (behavior changed compared to 5.0.x)

[fubinzh]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Lightning 5.0.1 added a check to ensure target table is empty before import for DBaaS.
Lightning 5.1.0 removed this check due to new feature parallel import / incremental import, this changes previous product behaviors.

Steps:

1. Import a table to TiDB with lightning
2. Import the table again to TiDB again with lightning

2. What did you expect to see? (Required)

Lightning behavior should be consistent for 5.0.1 and 5.1.0+.

3. What did you see instead (Required)

For lightning 5.1.0+, lightning doesn't check target table empty or not before the import, this may result in:

• Duplicate rows inserted into the table, if the import data doesn't have PK/UK
• Import failed with checksum mismatched error, if import data has PK/UK

4. What is your TiDB version? (Required)

5.2.0

[glorv]

In our previous discussion, this is a design feature that we won't forbid user to import data into a table that already contains data. And we will depend on the duplication detection feature which will be release in this sprint to fix potential duplicate data.

/cc @IANTHEREAL @lonng

[github-actions]

Please check whether the issue should be labeled with 'affects-x.y' or 'fixes-x.y.z', and then remove 'needs-more-info' label.