SELECT 
  a.*, 
  b.description 
FROM 
  cscart_currencies as a 
  LEFT JOIN cscart_currency_descriptions as b ON a.currency_id = b.currency_id 
  AND lang_code = 'de' 
  LEFT JOIN cscart_storefronts_currencies AS storefronts_currencies ON storefronts_currencies.currency_id = a.currency_id 
WHERE 
  1 
  AND a.status IN ('A', 'H') 
  AND (
    storefronts_currencies.storefront_id IN (1) 
    OR storefronts_currencies.storefront_id IS NULL
  ) 
ORDER BY 
  a.position

Query time 0.00298

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "9.72"
    },
    "ordering_operation": {
      "using_temporary_table": true,
      "using_filesort": true,
      "cost_info": {
        "sort_cost": "2.50"
      },
      "nested_loop": [
        {
          "table": {
            "table_name": "a",
            "access_type": "ALL",
            "rows_examined_per_scan": 3,
            "rows_produced_per_join": 1,
            "filtered": "50.00",
            "cost_info": {
              "read_cost": "2.33",
              "eval_cost": "0.30",
              "prefix_cost": "2.63",
              "data_read_per_join": "156"
            },
            "used_columns": [
              "currency_id",
              "currency_code",
              "after",
              "symbol",
              "coefficient",
              "is_primary",
              "position",
              "decimals_separator",
              "thousands_separator",
              "decimals",
              "status"
            ],
            "attached_condition": "(`danishecarter_latest`.`a`.`status` in ('A','H'))"
          }
        },
        {
          "table": {
            "table_name": "b",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "currency_id",
              "lang_code"
            ],
            "key_length": "9",
            "ref": [
              "danishecarter_latest.a.currency_id",
              "const"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 1,
            "filtered": "100.00",
            "cost_info": {
              "read_cost": "1.50",
              "eval_cost": "0.30",
              "prefix_cost": "4.43",
              "data_read_per_join": "1K"
            },
            "used_columns": [
              "currency_id",
              "description",
              "lang_code"
            ]
          }
        },
        {
          "table": {
            "table_name": "storefronts_currencies",
            "access_type": "index",
            "key": "PRIMARY",
            "used_key_parts": [
              "storefront_id",
              "currency_id"
            ],
            "key_length": "8",
            "rows_examined_per_scan": 3,
            "rows_produced_per_join": 2,
            "filtered": "55.56",
            "using_index": true,
            "using_join_buffer": "Block Nested Loop",
            "cost_info": {
              "read_cost": "2.29",
              "eval_cost": "0.50",
              "prefix_cost": "7.22",
              "data_read_per_join": "40"
            },
            "used_columns": [
              "storefront_id",
              "currency_id"
            ],
            "attached_condition": "(<if>(found_match(storefronts_currencies), ((`danishecarter_latest`.`storefronts_currencies`.`storefront_id` = 1) or isnull(`danishecarter_latest`.`storefronts_currencies`.`storefront_id`)), true) and <if>(is_not_null_compl(storefronts_currencies), (`danishecarter_latest`.`storefronts_currencies`.`currency_id` = `danishecarter_latest`.`a`.`currency_id`), true))"
          }
        }
      ]
    }
  }
}