Copying records in a table with self referencing ids - postgresql

Question

I have a table with records which can reference another row in the same table so there is a parent-child relationship between rows in the same table.
What I am trying to achieve is to create the same data for another user so that they can see and manage their own version of this structure through the web ui where these rows are displayed as a tree.
Problem is when I bulk insert this data by only changing user_id, I lose the relation between rows because the parent_id values will be invalid for these new records and they should be updated as well with the newly generated ids.
Here is what I tried: (did not work)
Iterate over main_table
copy-paste the static values after each
do another insert on a temp table for holding old and new ids
update old parent_ids with new ids after loop ends
My attempt at doing such thing(last step is not included here)
create or replace function test_x()
returns void as
$BODY$
declare
r RECORD;
userId int8;
rowPK int8;
begin
userId := (select 1)
create table if not exists id_map (old_id int8, new_id int8);
create table if not exists temp_table as select * from main_table;
for r in select * from temp_table
loop
rowPK := insert into main_table(id, user_id, code, description, parent_id)
values(nextval('hibernate_sequence'), userId, r.code, r.description, r.parent_id) returning id;
insert into id_map (old_id, new_id) values (r.id, rowPK);
end loop;
end
$BODY$
language plpgsql;
My PostgreSQL version is 9.6.14.
DDL below for testing.
create table main_table(
id bigserial not null,
user_id int8 not null,
code varchar(3) not null,
description varchar(100) not null,
parent_id int8 null,
constraint mycompkey unique (user_id, code, parent_id),
constraint mypk primary key (id),
constraint myfk foreign key (parent_id) references main_table(id)
);
insert into main_table (id, user_id, code, description, parent_id)
values(0, 0, '01', 'Root row', null);
insert into main_table (id, user_id, code, description, parent_id)
values(1, 0, '001', 'Child row 1', 0);
insert into main_table (id, user_id, code, description, parent_id)
values(2, 0, '002', 'Child row 2', 0);
insert into main_table (id, user_id, code, description, parent_id)
values(3, 0, '002', 'Grand child row 1', 2);
How to write a procedure to accomplish this?
Thanks in advance.

Answer 1

It appears your task is coping all data for a given user to another while maintaining the hierarchical relationship within the new rows. The following accomplishes that.
It begins creating a new copy of the existing rows with the new user_id, including the old row parent_id. That will be user in the next (update) step.
The CTE logically begins with the new rows which have parent_id and joins to the old parent row. From here it joins to the old parent row to the new parent row using the code and description. At that point we have the new id along with the new parent is. At that point just update with those values. Actually for the update the CTE need only select those two columns, but I've left the intermediate columns so you trace through if you wish.
create or replace function copy_user_data_to_user(
source_user_id bigint
, target_user_id bigint
)
returns void
language plpgsql
as $$
begin
insert into main_table ( user_id,code, description, parent_id )
select target_user_id, code, description, parent_id
from main_table
where user_id = source_user_id ;
with n_list as
(select mt.id, mt.code, mt.description, mt.parent_id
, mtp.id p_id,mtp.code p_code,mtp.description p_des
, mtc.id c_id, mtc.code c_code, mtc.description c_description
from main_table mt
join main_table mtp on mtp.id = mt.parent_id
join main_table mtc on ( mtc.user_id = target_user_id
and mtc.code = mtp.code
and mtc.description = mtp.description
)
where mt.parent_id is not null
and mt.user_id = target_user_id
)
update main_table mt
set parent_id = n_list.c_id
from n_list
where mt.id = n_list.id;
return;
end ;
$$;
-- test
select * from copy_user_data_to_user(0,1);
select * from main_table;

Answer 2

CREATE TABLE 'table name you want to create' SELECT * FROM myset
but new table and myset column name should be equal and you can also
use inplace of * to column name but column name exist in new table
othwerwise getting errors