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SELECT 
  sku_columns 
FROM 
  cscart_products 
WHERE 
  product_id = 65430

Query time 0.00036

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
{"23136":10,"23137":10,"23138":10,"23143":10,"22644":12,"22645":13,"23141":11,"22254":8,"23427":6,"23428":6,"23435":9,"23436":9}