Question

I wrote the following UPDATE command, but there’s redundancy in the sub-selects. I’m not an expert in SQL and would appreciate help in making this query more efficient. Thanks ahead of time.

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)
;

Below are the relevant table definitions. There are approximately 72K
trips, 8K stops, and 2 million stop_times. Maybe an average of 20? stops
per trip (just guessing).

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

First add a index on stop_times to include the trip_id and the stop_sequence columns

ALTER TABLE stop_times ADD PRIMARY KEY(trip_id, stop_sequence)

Then, try running this update:

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

Note: changing trip_id to a INT will give you better performance

Also, the trips table should store the origin_id and destination_id, which can later be joined to the stops table to find the name, instead of storing the name in all the rows