This PR fixes a performance problem when reviewing a pull request in a
big instance which have many records in the `review` table.
Traditionally, we should add more indexes in that table. But since
dismissed reviews of 1 pull request will not be too many as expected in
a common repository. Filtering reviews in the memory should be more
quick .
---------
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
This PR rewrites `GetReviewer` function and move it to service layer.
Reviewers should not be watchers, so that this PR removed all watchers
from reviewers. When the repository is under an organization, the pull
request unit read permission will be checked to resolve the bug of
#32394Fix#32394
