Acquiring offset of some row in SQL - sql

Question

TL;DR: Is there a possibility to get OFFSET position of a particular, known row in SQL, considering some ORDER BY is applied?
So consider a schema like this (simplified):
CREATE TABLE "public"."painting" (
"uuid" uuid NOT NULL DEFAULT uuid_generate_v4(),
"name" varchar NOT NULL,
"score" int4 NOT NULL,
"approvedAt" timestamp,
PRIMARY KEY ("uuid")
);
Like
abc1,test1,10,10:00
abc2,test2,9,11:00
abc3,test3,8,8:00
abc4,test4,8,12:00
abc5,test5,6,7:00
I want to make a request sorted by score and limited with 3 items, and I should emphasize that multiple entities might have the same score.
Because of a dynamic nature of that table, while traversing through those items, sorted by score, some new item might appear somewhere in the list.
If I use SQL OFFSET statement, that means this new entity will shift all entities below to one row, so that the new selection will have an item, that was last on previous 3 items selection.
abc1,test1,10,10:00
abc2,test2,9,11:00
abc6,test6,8,15:00 (new item)
CURRENT OFFSET = 3
abc3,test3,8,8:00 (was in previous select)
abc4,test4,8,12:00
abc5,test5,6,7:00
To avoid that, instead of using OFFSET, I can remember the UUID of the item I fetched last, so it'll be abc3. On next request, I can use it's score to add an extra WHERE SCORE < 8 statement, but this will skip abc4, because it's too having score of 8.
If I use WHERE SCORE <= 8 this will again return abc3 which is already traversed. I can't use another field in WHERE clause, because this will affect the results. Additional ORDER BY won't help either.
It seems to me that it is a very common problem in database selection, yet I can't find one comprehensive answer.
So, my question then, if it's possible to do some kind of request like following:
SELECT * FROM "painting" WHERE "score" <= :score ORDER BY "score" DESC OFFSET %position of `abc3`% LIMIT 3
Or alternatively
SELECT OFFSET OF (`abc3`) FROM "painting" WHERE SCORE <= :score ORDER BY "score" DESC LIMIT 3
That will return 2 (because it's the second row with such score), then do
SELECT * FROM "painting" WHERE "score" <= :score ORDER BY "score" DESC OFFSET :offset LIMIT 3
where :score is the score of last received item and :offset is the result of SELECT OFFSET - 1
My own assumption is that we have to SELECT WHERE "score" = :score, and get offset position outside the SQL (or make a very complex SQL query). Though, if we have a lot of items with similar ORDER BY attribute, this helper request might end up being heavier than the data fetch itself.
Yet, I feel like that there's a much more clever SQL way of doing what I'm trying to do.

Answer 1

Good question. Accurate Backend Pagination requires the underlying data to use an ordering criteria with a set of columns that represent a UNIQUE key.
In your case your ordering criteria can be made unique by adding the column uuid to it. With that in mind you can increase the page size by 1 behind the scenes to 4. That 4th row won't be displayed but only used to retrieve the next page.
For example, you can get:
select *
from painting
order by -score, approvedAt, uuid
limit 4
Now you would display the first three rows:
abc1,test1,10,10:00
abc2,test2,9,11:00
abc3,test3,8,8:00
The client app (most likely the UI) will remember -- not display -- the 4th row (the "key") to retrieve the next page:
abc4,test4,8,12:00
Then, to get the next page the query will add a WHERE clause with the "key" and take the form:
select *
from painting
where (-score, approvedAt, uuid) >= (-8, '12:00', 'abc4')
order by -score, approvedAt, uuid
limit 4
This query won't display the new row being inserted, but the original 4th row.
To get blazing fast data retrieval you could create the index:
create index ix1 on painting ((-score), approvedAt, uuid);
See example at DB Fiddle.