Is there a way to generate composite key efficiently in a PostgreSQL database? - sql

Question

Suppose I have a student table with id as primary key, which holds information about all students.
id | name
---+------
1 | aaron
2 | bob
In addition there is a table where id and tid form a composite key, which holds the scores for each test.
id | tid | score
---| --- | -----
Note: Different students have different tests with different numbers and no correlation. tid does not represent a specific test at all, but for a student the test serial number. id=1 and id=2, if tid=1, does not mean it is the same test.
There are two ways to generate tid, one is globally unique and increases by 1 for each record inserted, e.g.
id | tid | score
-- | --- | -----
1 | 1 | 99
1 | 2 | 98
2 | 3 | 97
2 | 4 | 96
The other is unique within a specific id, and different ids can have the same tid take value, for example
id | tid | score
-- | --- | -----
1 | 1 | 99
1 | 2 | 98
2 | 1 | 97
2 | 2 | 96
In the previous way, a student with id=2 could probably guess how many tests roughly the whole school went through in between based on his tid change. Since the tid of each student changes globally, this is something I don't want. Of course I could consider using a non-repeating use of random numbers or scheme. But I would prefer a slightly more compact incremental integer to describe it.
For the latter, is there a more efficient and simple way to implement it?

Answer 1

Option 1.
create table student(id integer PRIMARY KEY, name varchar);
create table test(tid integer PRIMARY KEY, name varchar, test_date date);
create table test_score(sid integer, tid integer references test, score integer, PRIMARY KEY(sid, tid);
UPDATE
Option 2.
Create an BEFORE INSERT trigger that uses a function that does roughly:
CREATE OR REPLACE FUNCTION tid_incr()
RETURNS trigger
LANGUAGE plpgsql
AS $function$
DECLARE
max_ct integer;
BEGIN
SELECT INTO max_ct max(tid) FROM score WHERE id = NEW.id;
NEW.tid = max_ct + 1;
RETURN NEW;
END;
$function$

Answer 2

The correct design is option 2, because tid should refer (ie be a foreign key) to the test (ie a collection of questions) being taken (whether the test is custom for each student or not).
If students can take the same test more than once, add a date column (or timestamp if multiple attempts may be made of the same day) to distinguish the results of repeat attempts at the same test.