Dumpling: The SortByPk option is missing a flag

[dveeden]

Bug Report

1. Minimal reproduce step (Required)

Inspect the code: There is a SortByPk option, but no flag to set it.

This causes issues when exporting data from Vitess as that doesn't support queries like this if the table is sharded:

SELECT * FROM tbl ORDER BY col1;

However it does support:

SELECT col1, col2, ... FROM tbl ORDER BY col1;

Or:

SELECT * FROM tbl

The column in the ORDER BY has to be explicitly named in the SELECT-list.

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

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

[dveeden]

Related: pingcap/docs#5783

[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.