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SELECT 
  sku_columns 
FROM 
  cscart_products 
WHERE 
  product_id = 65536

Query time 0.00022

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
{"22816":1,"22805":2,"22815":5,"22797":5,"22796":5,"22794":5,"22795":5,"22798":7,"21941":9,"23591":1,"22927":5,"22928":5,"23592":1,"22931":5,"22932":5,"22924":6,"22923":6,"23593":1,"23857":4}