How to apply LIMIT only to parent rows - sql

Question

In my Postgres database I have a table that holds a simply hierarchy, something like this:
id | parent_id
---------------
When an item in the table is a "top-level" item, its parent_id is set to NULL
However, when I query my table I retrieve the top-level items and the child items that belong to those items. E.g. if there is a single top-level item with two children my query returns three rows. My query is super simple, it looks something like this:
SELECT
*
FROM
my_table
LIMIT
_limit
OFFSET
_offset
;
When the above returns the three rows, in my business logic I then transform that result into a JSON structure that is then serialized to the client. It looks something like this:
items: [
{
id: 1,
parent_id: null,
items: [
{
id: 2,
parent_id: 1
},
{
id: 3,
parent_id: 1
}
]
}
]
However, as you can see my query has OFFSET and LIMIT for, you guessed it, pagination. The table is quite large and I want to restrict the amount of items that can be requested in a single request.
The problem is that, and continuing to use my single top-level item as an example, if the LIMIT is set to 1 then the children of the top-level item will never be returned.
What I am basically looking for is a way to exclude child rows from counting towards the LIMIT, or, to expand the LIMIT with the total number of child rows found.

Answer 1

You're going to have to do two things:
Get the top level entries to include (paginated)
Run another query for the descendants of the top level
This is a fully recursive example
create table t (id int primary key, parent_id int);
insert into t (id, parent_id) values
(1, null), (2, null), (3, null), (4, 1),
(5, 1), (6, 4), (7, 2), (8, 2),
(9, 8), (10, 3), (11, null), (12, null);
with recursive entries (id, parent_id) as (
(
select
id, parent_id
from t
where parent_id is null
order by id limit 2 -- add offset N here
)
union all
(
select
t.id, t.parent_id
from entries inner join t on (t.parent_id = entries.id)
)
)
select * from entries;
https://www.db-fiddle.com/f/g3G2t3mVo7fBhQa9QCA71P/0