SELECT 
  category_id 
FROM 
  cscart_products_categories 
WHERE 
  product_id = 31361 
  AND link_type = 'M'

Query time 0.00043

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "0.45"
    },
    "table": {
      "table_name": "cscart_products_categories",
      "access_type": "ref",
      "possible_keys": [
        "link_type",
        "pt",
        "idx_product_category_lookup",
        "idx_products_categories_composite"
      ],
      "key": "idx_products_categories_composite",
      "used_key_parts": [
        "product_id"
      ],
      "key_length": "3",
      "ref": [
        "const"
      ],
      "rows_examined_per_scan": 2,
      "rows_produced_per_join": 1,
      "filtered": "50.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "0.25",
        "eval_cost": "0.10",
        "prefix_cost": "0.45",
        "data_read_per_join": "16"
      },
      "used_columns": [
        "product_id",
        "category_id",
        "link_type"
      ],
      "attached_condition": "(`cscart_db`.`cscart_products_categories`.`link_type` = 'M')"
    }
  }
}