SELECT 
  cscart_bm_layouts.* 
FROM 
  cscart_bm_layouts 
WHERE 
  1 
  AND is_default = 1 
  AND theme_name = 'nova_theme' 
  AND storefront_id = 1

Query time 0.01796

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_bm_layouts",
          "access_type": "system",
          "possible_keys": ["is_default"],
          "rows": 1,
          "filtered": 100
        }
      }
    ]
  }
}