SELECT 
  COUNT(*) 
FROM 
  cscart_promotions 
  LEFT JOIN cscart_promotion_descriptions ON cscart_promotion_descriptions.promotion_id = cscart_promotions.promotion_id 
  AND cscart_promotion_descriptions.lang_code = 'ru' 
  LEFT JOIN cscart_storefronts_promotions AS storefronts_promotions ON storefronts_promotions.promotion_id = cscart_promotions.promotion_id 
  LEFT JOIN cscart_promotion_images ON cscart_promotion_images.promotion_id = cscart_promotions.promotion_id 
  AND cscart_promotion_images.lang_code = 'ru' 
WHERE 
  1 
  AND status IN ('A') 
  AND (
    storefronts_promotions.storefront_id = 1 
    OR storefronts_promotions.storefront_id IS NULL
  )

Query time 0.00079

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "4.17"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_promotions",
          "access_type": "ALL",
          "rows_examined_per_scan": 10,
          "rows_produced_per_join": 1,
          "filtered": "10.00",
          "cost_info": {
            "read_cost": "1.65",
            "eval_cost": "0.10",
            "prefix_cost": "1.75",
            "data_read_per_join": "80"
          },
          "used_columns": [
            "promotion_id",
            "status"
          ],
          "attached_condition": "(`zdrowy_db`.`cscart_promotions`.`status` = 'A')"
        }
      },
      {
        "table": {
          "table_name": "cscart_promotion_descriptions",
          "access_type": "eq_ref",
          "possible_keys": [
            "PRIMARY"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "promotion_id",
            "lang_code"
          ],
          "key_length": "9",
          "ref": [
            "zdrowy_db.cscart_promotions.promotion_id",
            "const"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 1,
          "filtered": "100.00",
          "using_index": true,
          "cost_info": {
            "read_cost": "0.25",
            "eval_cost": "0.10",
            "prefix_cost": "2.10",
            "data_read_per_join": "800"
          },
          "used_columns": [
            "promotion_id",
            "lang_code"
          ]
        }
      },
      {
        "table": {
          "table_name": "storefronts_promotions",
          "access_type": "index",
          "key": "PRIMARY",
          "used_key_parts": [
            "storefront_id",
            "promotion_id"
          ],
          "key_length": "8",
          "rows_examined_per_scan": 11,
          "rows_produced_per_join": 2,
          "filtered": "19.00",
          "using_index": true,
          "using_join_buffer": "hash join",
          "cost_info": {
            "read_cost": "1.40",
            "eval_cost": "0.21",
            "prefix_cost": "3.71",
            "data_read_per_join": "33"
          },
          "used_columns": [
            "storefront_id",
            "promotion_id"
          ],
          "attached_condition": "(<if>(found_match(storefronts_promotions), ((`zdrowy_db`.`storefronts_promotions`.`storefront_id` = 1) or (`zdrowy_db`.`storefronts_promotions`.`storefront_id` is null)), true) and <if>(is_not_null_compl(storefronts_promotions), (`zdrowy_db`.`storefronts_promotions`.`promotion_id` = `zdrowy_db`.`cscart_promotions`.`promotion_id`), true))"
        }
      },
      {
        "table": {
          "table_name": "cscart_promotion_images",
          "access_type": "eq_ref",
          "possible_keys": [
            "promo"
          ],
          "key": "promo",
          "used_key_parts": [
            "promotion_id",
            "lang_code"
          ],
          "key_length": "10",
          "ref": [
            "zdrowy_db.cscart_promotions.promotion_id",
            "const"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 2,
          "filtered": "100.00",
          "using_index": true,
          "cost_info": {
            "read_cost": "0.25",
            "eval_cost": "0.21",
            "prefix_cost": "4.17",
            "data_read_per_join": "33"
          },
          "used_columns": [
            "promotion_id",
            "lang_code"
          ],
          "attached_condition": "<if>(is_not_null_compl(cscart_promotion_images), (`zdrowy_db`.`cscart_promotion_images`.`promotion_id` = `zdrowy_db`.`cscart_promotions`.`promotion_id`), true)"
        }
      }
    ]
  }
}