Re: Synchronize two similar tables: recursive triggers

The INSERT and DELETE TG_OPs are straightforward (the simplest solution
for these is that the existence of the primary key 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 trigger could 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(at)sfu(dot)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 the other, 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 one of the tables (another
>> bug: http://trac.osgeo.org/fdo/ticket/394) The two tables also need to
>> be isolated in different schemata.
>>
>> 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_OP and
>> 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. For example:
>>
>> 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 disable the second
>> trigger from the first trigger while updating the other table, etc.
>> Perhaps there is a global variable somewhere 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
>>
>> --
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>>
>
>