Understanding DISTINCT vs DISTINCT ON vs Group by - database

Question

I have a query which returns a set of 'records'.
The result is always from the same table, and should always be unique. It has a set of inner joins to filter the rows down to the appropriate subset.
The query is returning roughly 10 columns.
However, I found that it was returning duplicate rows, so I added select distinct to the query, which solved the duplication problem but has significant performance issues.
My understanding is that select distinct on (records.id), id... will return the same result in this case, as all duplicates would have the same primary key, and seems to be about twice as fast.
My other tests show that group by records.id is even faster again, and seems to do the same thing?
Am I correct that all three of these approaches will always return the same set of single table records?
Also, is there an easy way to compare the results of different approaches to ensure the set is being returned?
Here is my query:
SELECT DISTINCT records.*
FROM records
INNER JOIN records parents on parents.path #> records.path
INNER JOIN record_types ON record_types.id = records.record_type_id
INNER JOIN user_roles ON user_roles.record_id = parents.id AND user_roles.user_id = _user_id
INNER JOIN memberships ON memberships.role_id = user_roles.role_id
INNER JOIN roles ON roles.id = memberships.role_id
INNER JOIN groups ON memberships.group_id = groups.id AND
groups.id = record_types.view_group_id
Any individual record can have tree of 'parent' records. This is done using the ltree plugin. Effectively, we are looking to see if the user has a role which is in a group which is defined as the 'view group' for either the current record, or any of the parents. The query is actually a function, and _user_id is being passed in.

Answer 1

Since you are only selecting from records, you don't need DISTINCT; the records are already distinct (I presume).
So the duplicates you encounter could be caused by all the joins, for instance if more than one role or group membership matches one of your records, the same record will be combined with each of these references.
SELECT *
FROM records r
WHERE EXISTS (
SELECT *
FROM records pa on pa.path #> r.path
JOIN record_types typ ON typ.id = r.record_type_id
JOIN user_roles ur ON ur.record_id = pa.id AND ur.user_id = _user_id
JOIN memberships mem ON mem.role_id = ur.role_id
JOIN roles ON roles.id = mem.role_id
JOIN groups gr ON mem.group_id = gr.id AND gr.id = typ.view_group_id
)
;