带有多个类似子选择的mysql update命令

Question

我编写了以下UPDATE命令，但子选择中存在冗余。 我不是SQL方面的专家，希望能帮助您提高查询效率。 提前谢谢。

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)
;

以下是相关的表定义。 大约有72K次旅行，8000次停靠和200万次停靠时间。 也许平均20？ 每次旅行停止（只是猜测）。

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上添加一个索引，以包含trip_id和stop_sequence列

ALTER TABLE stop_times ADD PRIMARY KEY(trip_id, stop_sequence)

然后，尝试运行此更新：

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：将trip_id更改为INT可以提高性能

另外，行程表应该存储origin_id和destination_id，以后可以将它们与stops表连接以查找名称，而不是在所有行中存储名称