SELECT 
  storefront_id 
FROM 
  cscart_hybrid_auth_storefronts_providers 
WHERE 
  provider_id = 1

Query time 0.00005

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_hybrid_auth_storefronts_providers",
          "access_type": "index",
          "key": "PRIMARY",
          "key_length": "8",
          "used_key_parts": ["storefront_id", "provider_id"],
          "rows": 3,
          "filtered": 100,
          "attached_condition": "cscart_hybrid_auth_storefronts_providers.provider_id = 1",
          "using_index": true
        }
      }
    ]
  }
}