SELECT 
  * 
FROM 
  cscart_product_blocks 
WHERE 
  short_code = 'SHORT_CODE_5'

Query time 0.00066

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_product_blocks",
          "access_type": "const",
          "possible_keys": ["product_short_code"],
          "key": "product_short_code",
          "key_length": "386",
          "used_key_parts": ["short_code"],
          "ref": ["const"],
          "rows": 1,
          "filtered": 100
        }
      }
    ]
  }
}