How do I represent an array of tuples in postgresql? - postgresql

Question

Here's the easiest way I can think of to explain this. Imagine a user wants to bookmark a bunch of webpages. There's a url table with a UrlID and the actual url. I'd like the user to have a list of UrlIDs which are unique (but I don't need the constraint) and a 32bit int value such as an epoch date. The only two things I care about is 1) being to check if UrlID is in this list or not and 2) get the entire list and sort it by date (or second value)
If it helps I'm expecting no more than 8K bookmarks but most likely it will be <128

Answer 1

If you really want to avoid the extra table to express the relationship, you can do something like that:
CREATE TABLE "user" (
id integer primary key,
name text not null,
bookmarks integer[] not null
);
CREATE TABLE url (
id integer primary key,
time timestamp with time zone not null,
val text not null
);
Then finding all bookmarks for a particular user (say with id 66) would involve doing something like that:
SELECT url,time
FROM (SELECT bookmarks FROM "user" WHERE id=66) u
JOIN url ON url.id=ANY(bookmarks)
ORDER BY TIME;
Now here's why I don't like this schema. First, adding a new bookmark would require to rewrite the bookmarks array and hence the entire user row (so adding n bookmarks, one after the other, would require Θ(n^2) time). Secondly, you cannot use foreign keys on the elements of the array. Thridly, many queries will become more complicated to write, e.g. in order to retrieve all bookmarks for all users, you have to do something like that:
SELECT "user".id,"user".name,url.val,url.time
FROM "user",
LATERAL unnest((SELECT bookmarks)) b
LEFT JOIN url ON b = url.id;
Edit: So here's the schema I would use and which I think fits best with the relational paradigm
CREATE TABLE "user" (
id integer primary key,
name text not null
);
CREATE TABLE url (
id integer primary key,
val text not null
);
CREATE TABLE bookmark (
user_id integer not null REFERENCES "user",
url_id integer REFERENCES url,
time timestamp with time zone not null,
UNIQUE (user_id,url_id)
);