The Art of PostgreSQL/

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Fixing the Behavior

Fixing the Behavior

Learn how to fix errors generated by writing the erroneous code for triggers.

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While it’s easy to maintain cache in an event-driven fashion thanks to PostgreSQL and its trigger support. Turning an insert into an update with contention on a single row is never a good idea. It’s even a classic anti-pattern.

Script for triggers

Here’s a modern way to fix the problem with the previous trigger implementation, this time applied to a per-message counter of Retweet and favorite actions:

begin;
create table twcache.counters
 (
   messageid  bigint not null references tweet.message(messageid),
   rts        bigint,
   favs       bigint,
   unique(messageid)
 );
create or replace function twcache.tg_update_counters ()
 returns trigger
 language plpgsql
as $$
declare
begin
   insert into twcache.counters(messageid, rts, favs)
        select NEW.messageid,
               case when NEW.action = 'rt' then 1 else 0 end,
               case when NEW.action = 'fav' then 1 else 0 end
   on conflict (messageid)
     do update
           set rts = case when NEW.action = 'rt'
                          then counters.rts + 1
                          when NEW.action = 'de-rt'
                          then counters.rts - 1
                          else counters.rts
                      end,
               favs = case when NEW.action = 'fav'
                           then counters.favs + 1
                           when NEW.action = 'de-fav'
                           then counters.favs - 1
                           else counters.favs
                       end
         where counters.messageid = NEW.messageid;
  RETURN NULL;
end;
$$;
CREATE TRIGGER update_counters
         AFTER INSERT
            ON tweet.activity
      FOR EACH ROW
       EXECUTE PROCEDURE twcache.tg_update_counters();
insert into tweet.activity(messageid, action)
     values (7, 'rt'),
            (7, 'fav'),
            (7, 'de-fav'),
            (8, 'rt'),
            (8, 'rt'),
            (8, 'rt'),
            (8, 'de-rt'),
            (8, 'rt');
select messageid, rts, favs
  from twcache.counters;
rollback;

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