Sandbox

SELECT 
  sku_columns 
FROM 
  cscart_products 
WHERE 
  product_id = 61052

Query time 0.00020

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_products",
      "access_type": "const",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["product_id"],
      "ref": ["const"],
      "rows": 1,
      "filtered": 100
    }
  }
}

Result

sku_columns
{"21970":3,"22224":10,"22028":6,"22031":9,"22030":9,"22029":9,"22034":8,"21941":11,"21973":5,"22040":8,"22039":7,"23551":6}