帶有多個類似子選擇的mysql update命令

Question

我編寫了以下UPDATE命令，但子選擇中存在冗余。 我不是SQL方面的專家，希望能幫助您提高查詢效率。 提前謝謝。

update trips
  set origin = 
  (select stop_name 
    from stops 
    inner join stop_times
    on stops.stop_id = stop_times.stop_id
    where stop_times.trip_id = trips.trip_id
    order by stop_sequence asc
    limit 1) 
  ,
  destination = 
  (select stop_name 
    from stops 
    inner join stop_times
    on stops.stop_id = stop_times.stop_id
    where stop_times.trip_id = trips.trip_id
    order by stop_sequence desc
    limit 1)
  ,
  starts = 
  (select arrival_time
    from stop_times
    where stop_times.trip_id = trips.trip_id
    order by stop_sequence asc
    limit 1) 
  ,
  ends = 
  (select arrival_time
    from stop_times
    where stop_times.trip_id = trips.trip_id
    order by stop_sequence desc
    limit 1)
;

以下是相關的表定義。 大約有72K次旅行，8000次停靠和200萬次停靠時間。 也許平均20？ 每次旅行停止（只是猜測）。

create table stop_times (
  trip_id varchar(255),
  arrival_time time,
  stop_id varchar(255),
  stop_sequence int unsigned,
) type=MyISAM;

alter table stop_times add index stop_id (stop_id(5));
alter table stop_times add index trip_id (trip_id(5));

create table stops (
  stop_id varchar(255),
  stop_name varchar(255),
  stop_lat float,
  stop_lon float,
  primary key (stop_id)
) type=MyISAM;

create table trips (
  route_id varchar(255),
  trip_id varchar(255), /* primary key is here */
  /* denormalized fields */
  origin varchar(255),
  destination varchar(255),
  starts time,
  ends time,
  primary key(trip_id)
) type=MyISAM;
alter table trips add index route_id (route_id(5));

Answer 1

首先在stop_times上添加一個索引，以包含trip_id和stop_sequence列

ALTER TABLE stop_times ADD PRIMARY KEY(trip_id, stop_sequence)

然后，嘗試運行此更新：

update trips t JOIN (
    SELECT trip_id, MIN(stop_sequence) minS, MAX(stop_sequence) maxS 
    FROM stop_times
    GROUP BY trip_id
) tg ON t.trip_id = tg.trip_id
JOIN stop_times stFirst ON tg.trip_id = stFirst.trip_id AND stFirst.stop_sequence = tg.minS
JOIN stop_times stLast ON tg.trip_id = stLast.trip_id AND stLast.stop_sequence = tg.maxS
JOIN stops stFirstStop ON stFirst.stop_id = stFirstStop.stop_id
JOIN stops stLastStop ON stLast.stop_id = stLastStop.stop_id
SET t.origin = stFirstStop.stop_name,
    t.destination = stLastStop.stop_name,
    t.starts = stFirst.arrival_time,
    t.ends = stLast.arrival_time

注意：將trip_id更改為INT可以提高性能

另外，行程表應該存儲origin_id和destination_id，以后可以將它們與stops表連接以查找名稱，而不是在所有行中存儲名稱