Add Index on comments.post_id to Accelerate Query

Created by: wtune

Problem

Missing index on table comments column post_id might make the underlying query slow (eg. BaseCommentRepository#findAllByPostId).It's easy to find out that column post_id is always in the condition when we read table comments.It's worth adding the index on comments, although it is inserted somtimes.

Possible Solution

Add Index on comments.post_id