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SELECT 
  sku_columns 
FROM 
  cscart_products 
WHERE 
  product_id = 64682

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
{"21930":6,"21931":6,"22012":6,"21932":8,"21935":10,"21934":10,"21933":10,"21936":9,"21937":9,"21938":9,"21969":9,"21939":9,"21968":9,"21974":11,"21975":11,"22052":11,"21941":12}