Add Index on post_categories.category_id to Accelerate Query

Created by: wtune

Problem

Missing index on table post_categories column category_id might make the underlying query issued via eg. PostCategoryRepository#findAllByCategoryId 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.category_id