Add Index on post_categories.post_id to Accelerate Query

Created by: wtune

Problem

Missing index on table post_categories column post_id might make the underlying query issued via eg. PostCategoryRepository#findAllByPostId slow.Since it's also easy to find that we always read post_categories by post_id or category_id and sometimes insert or update, the performance impact for write operations is acceptable under common cases.

Possible Solution

Add Index on post_categories.post_id