Suppose we have an Alternative$-2$ unclustered index on $($assignment id$,$ student id$)$ with a depth of $3($the root is at depth $1).$ Here$$s the schema:

CREATE TABLE Submissions $($

record$\_$id integer UNIQUE,

assignment$\_$id integer,

student$\_$id integer,

time$\_$submitted integer,

grade$\_$received byte,

comment text,

regrade$\_$request text,

PRIMARY KEY$($assignment$\_$id$,$ student$\_$id$)$

$)$;

CREATE INDEX SubmissionLookupIndex ON

Submissions $($assignment$\_$id$,$ student$\_$id$)$;