SELECT 
  prod_cat_id 
FROM 
  cscart_cp_blog_post_product_categories 
WHERE 
  post_id = 4

Query time 0.00019

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_cp_blog_post_product_categories",
      "access_type": "ref",
      "possible_keys": ["post_id"],
      "key": "post_id",
      "key_length": "3",
      "used_key_parts": ["post_id"],
      "ref": ["const"],
      "rows": 1,
      "filtered": 100,
      "using_index": true
    }
  }
}