“Diffing” objects from a relational database

Question

Our win32 application assembles objects from the data in a number of tables in a MySQL relational database. Of such an object, multiple revisions are stored in the database.

Answer 1

In your situation in PostgreSQL I used a difference tables with the schema:

history_columns (
    column_id smallint primary key,
    column_name text not null,
    table_name text not null,
    unique (table_name, column_name)
);
create temporary sequence column_id_seq;
insert into history_columns
select nextval('column_id_seq'), column_name, table_name
    from information_schema.columns
    where
        table_name in ('table1','table2','table3')
        and table_schema=current_schema() and table_catalog=current_database();

create table history (
    column_id smallint not null references history_columns,
    id int not null,
    change_time timestamp with time zone not null
        constraint change_time_full_second -- only one change allowed per second
            check (date_trunc('second',change_time)=change_time),
    primary key (column_id,id,change_time),
    value text
);

And on the tables I used a trigger like this:

create or replace function save_history() returns trigger as
$$
    if (tg_op = 'DELETE') then
        insert into historia values (
            find_column_id('id',tg_relname), OLD.id,
            date_trunc('second',current_timestamp),
            OLD.id );
        [for each column_name] {
            if (char_length(OLD.column_name)>0) then
                insert into history values (
                    find_column_id(column_name,tg_relname), OLD.id,
                    OLD.change_time, OLD.column_name
                )
        }
    elsif (tg_op = 'UPDATE') then
        [for each column_name] {
            if (OLD.column_name is distinct from NEW.column_name) then
                insert into history values (
                    find_column_id(column_name,tg_relname), OLD.id,
                    OLD.change_time, OLD.column_name
                );
            end if;
        }
    end if;
$$ language plpgsql volatile;

create trigger save_history_table1
    before update or delete on table1
    for each row execute procedure save_history();