SELECT 
  products.product_id 
FROM 
  cscart_products AS products 
WHERE 
  products.master_product_id = 68350

Query time 0.00025

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "0.75"
    },
    "table": {
      "table_name": "products",
      "access_type": "ref",
      "possible_keys": [
        "idx_master_product_id",
        "idx_products_master_status",
        "idx_products_status_company_master",
        "idx_products_main_query",
        "idx_products_query_opt",
        "idx_products_composite_query"
      ],
      "key": "idx_master_product_id",
      "used_key_parts": [
        "master_product_id"
      ],
      "key_length": "4",
      "ref": [
        "const"
      ],
      "rows_examined_per_scan": 1,
      "rows_produced_per_join": 1,
      "filtered": "100.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "0.65",
        "eval_cost": "0.10",
        "prefix_cost": "0.75",
        "data_read_per_join": "5K"
      },
      "used_columns": [
        "product_id",
        "master_product_id"
      ]
    }
  }
}