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SELECT 
  sku_columns 
FROM 
  cscart_products 
WHERE 
  product_id = 64993

Query time 0.00028

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
{"22646":7,"22649":8,"22650":8,"22648":8,"22643":10,"22644":10,"22645":12,"23144":10,"23137":10,"23136":10,"22339":1,"21927":2,"23146":3,"22012":4,"21930":4,"21931":4,"22254":6,"21938":7,"21939":7,"23132":9,"23149":1,"23148":3,"23327":1,"23739":11}