SELECT 
  category_id 
FROM 
  cscart_cp_blog_categories 
WHERE 
  posts_count > 0

Query time 0.00018

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_cp_blog_categories",
      "access_type": "ALL",
      "rows": 7,
      "filtered": 100,
      "attached_condition": "cscart_cp_blog_categories.posts_count > 0"
    }
  }
}