SELECT 
  product_id, 
  group_id 
FROM 
  tmall_product_variation_group_products 
WHERE 
  group_id IN (10)

Query time 0.00802

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.006758432,
    "nested_loop": [
      {
        "table": {
          "table_name": "tmall_product_variation_group_products",
          "access_type": "index",
          "possible_keys": ["idx_group_id"],
          "key": "PRIMARY",
          "key_length": "6",
          "used_key_parts": ["product_id", "group_id"],
          "loops": 1,
          "rows": 7,
          "cost": 0.006758432,
          "filtered": 100,
          "attached_condition": "tmall_product_variation_group_products.group_id = 10",
          "using_index": true
        }
      }
    ]
  }
}