rewrite Load plan

[jianwan0214]

What type of PR is this?

• [ ] API-change
• [ ] BUG
• [ ] Improvement
• [ ] Documentation
• [x] Feature
• [ ] Test and CI
• [ ] Code Refactoring

Which issue(s) this PR fixes:

issue #3421

What this PR does / why we need it:

rewrite the load plan as insert into table select from external table; add sql to support external table as we can select info from external table;