Re: Synchronize two similar tables: recursive triggers - Mailing list pgsql-general

If anyone is interested, here is my solution to my problem, which I hope
will be obsolete when the issues with the FDO PostGIS provider are
fixed. I have also successfully tested this setup out with foreign key
constraints in the primary table only --- the secondary tables just use
primary key constraints. I ended up using a third table to store
information for update synchronization operations (rather than query
pg_stat_activity, since I couldn't see the use in it):

CREATE TABLE prim.sync
(
  source text NOT NULL,
  CONSTRAINT sync_pkey PRIMARY KEY (source)
) WITH (OIDS=FALSE);

BEGIN
  IF TG_OP = 'INSERT' THEN
    IF (TG_TABLE_SCHEMA = 'prim') AND
       (SELECT count(id) = 0 FROM second.mytable WHERE id = NEW.id) THEN
      INSERT INTO second.mytable(id, fname, num) VALUES(NEW.id,
NEW.fname, NEW.num);
    ELSIF (TG_TABLE_SCHEMA = 'second') AND
       (SELECT count(id) = 0 FROM prim.mytable WHERE id = NEW.id)  THEN
      INSERT INTO prim.mytable(id, fname, num) VALUES(NEW.id, NEW.fname,
NEW.num);
    END IF;
  ELSIF TG_OP = 'UPDATE' THEN
    IF (SELECT count(*) = 0 FROM prim.sync WHERE source=TG_TABLE_NAME) THEN
      INSERT INTO prim.sync VALUES (TG_TABLE_NAME);--First trigger fire
      IF TG_TABLE_SCHEMA = 'prim' THEN
        UPDATE second.mytable SET fname = NEW.fname, num = NEW.num WHERE
id = OLD.id;
      ELSIF TG_TABLE_SCHEMA = 'second' THEN
        UPDATE prim.mytable SET fname = NEW.fname, num = NEW.num WHERE
id = OLD.id;
      END IF;
    ELSE--This is the second and last trigger fire
      DELETE FROM prim.sync WHERE source = TG_TABLE_NAME;
    END IF;
  ELSIF TG_OP = 'DELETE' THEN
    IF TG_TABLE_SCHEMA = 'prim' THEN
      DELETE FROM second.mytable WHERE id = OLD.id;
    ELSIF TG_TABLE_SCHEMA = 'second' THEN
      DELETE FROM prim.mytable WHERE id = OLD.id;
    END IF;
  ELSE
    RAISE EXCEPTION 'TG_OP %', TG_OP;
  END IF;
  RETURN NEW;
END;

CREATE TRIGGER prim_sync
  AFTER INSERT OR UPDATE OR DELETE
  ON prim.mytable
  FOR EACH ROW
  EXECUTE PROCEDURE prim.sync_mytable_fn();

CREATE TRIGGER second_sync
  BEFORE INSERT OR UPDATE OR DELETE
  ON "second".mytable
  FOR EACH ROW
  EXECUTE PROCEDURE prim.sync_mytable_fn();


Michael Toews wrote:
> The INSERT and DELETE TG_OPs are straightforward (the simplest solution for these is that the existence of the
primarykey can be checked in the other table), however the UPDATE handler is really confusing. 
>
> Is it possible for a trigger function to know where an UPDATE originated (user vs trigger)? I'm not sure how a
triggercould know the first to be fired, or how many times it has passed between. Any other ideas? Thanks again. 
>
> -Mike
>
> Pavel Stehule wrote:
>
>> Hello
>>
>> ad colum that will contains info about source of value
>>
>> like
>>
>> create table a(a integer, from_trigger bool);
>> create table b(a integer, from_trigger bool);
>>
>> create or replace function synchronize_handler_a()
>> returns trigger as $$
>> begin
>>   if not new.from_trigger then
>>     new.from trigger := true;
>>     insert into b values(new.*);
>>   end if;
>>   return new;
>> end;
>> $$ language plpgsql;
>>
>> this is protection under resursive triggers
>>
>> regards
>> Pavel Stehule
>>
>>
>>
>>
>> 2008/9/18 Michael Toews <mwtoews@sfu.ca>:
>>
>>
>>> Hi all,
>>>
>>> I need to have two tables that are mostly synchronized in my database, such that an edit to a row in one is made to
theother, and vice versa. Normally, this is done using views with rules, however my situation does not allow editable
views(http://trac.osgeo.org/fdo/ticket/346). So, I need to have two database tables. 
>>>
>>> The other thing is that the two tables are not identical, as I need to omit columns with "advanced" data types in
oneof the tables (another bug: http://trac.osgeo.org/fdo/ticket/394). The two tables also need to be isolated in
differentschemata. 
>>>
>>> Here are some example tables:
>>>
>>> CREATE SCHEMA prim;
>>> CREATE SCHEMA second;
>>>
>>> CREATE TABLE prim.mytable
>>> (
>>>  id integer,
>>>  fname character varying,
>>>  num real,
>>>  timestmp timestamp with time zone, -- not in second.mytable
>>>  CONSTRAINT mytable_pkey PRIMARY KEY (id)
>>> ) WITH (OIDS=FALSE);
>>>
>>> CREATE TABLE second.mytable
>>> (
>>>  id integer,
>>>  fname character varying,
>>>  num real,
>>>  CONSTRAINT mytable_pkey PRIMARY KEY (id)
>>> ) WITH (OIDS=FALSE);
>>>
>>>
>>> To synchronized the two tables, I plan to use a trigger function to handle INSERT, UPDATE and DELETE events, using
TG_OPand TG_TABLE_SCHEMA. (If there are better solutions that don't use triggers, stop me here and fill me in). 
>>>
>>> What I'm having difficulty designing is how to deal with recursive triggers, since I require two-way communication.
Forexample: 
>>>
>>>   1. change on prim.mytable fires trigger to sync change on second.mytable
>>>   2. change from (1) on second.mytable fires trigger to sync change on prim.mytable
>>>   3. change from (2) on prim.mytable fires trigger ... etc.
>>>
>>> This behaviour is mentioned in the documentation:
>>> http://www.postgresql.org/docs/8.3/interactive/trigger-definition.html
>>> (search for "recurs") however, it doesn't offer an example nor solution.
>>>
>>> Some possible solutions may involve using trigger functions with parameters (I'm yet to see an example of this), or
disablethe second trigger from the first trigger while updating the other table, etc. Perhaps there is a global
variablesomewhere that could indicate the level of recursion. Or, possibly, a "version" column could be kept in each
column,which is incremented on the first trigger fire, and returns 
>>> NULL if OLD.version=NEW.version.
>>>
>>> Any suggestions or references to other examples would be much
>>> appreciated. Thanks in advance.
>>>
>>> -Mike
>>>
>>> --
>>> Sent via pgsql-general mailing list (pgsql-general@postgresql.org)
>>> To make changes to your subscription:
>>> http://www.postgresql.org/mailpref/pgsql-general
>>>