fix: left table hash mismatch

[donghanz]

Summary

source in the compute_left_table is overwritten in the join part task, resulting in a change in left_source hash To fix this issue, we compare the hash first and then overwrite the left source. This should ensure that left_source hash is not modified unexpectedly.

Test Plan

• [x] Covered by existing CI
• [x] Integration tested

Reviewers

@ezvz