Question

Suppose I have two tables:

Group
(
    id integer primary key,
    someData1 text,
    someData2 text
)

GroupMember
(
    id integer primary key,
    group_id   foreign key to Group.id,
    someData   text
)

I’m aware that my SQL syntax is not correct 🙂 Hopefully is clear enough. My problem is this: I want to load a group record and all the GroupMember records associated with that group. As I see it, there are two options.

A single query:

SELECT Group.id, Group.someData1, Group.someData2 GroupMember.id, GroupMember.someData
FROM Group INNER JOIN GroupMember ...
WHERE Group.id = 4;

Two queries:

SELECT id, someData2, someData2
FROM Group
WHERE id = 4;

SELECT id, someData
FROM GroupMember
WHERE group_id = 4;

The first solution has the advantage of only being one database round trip, but has the disadvantage of returning redundant data (All group data is duplicated for every group member)

The second solution returns no duplicate data but involves two round trips to the database.

What is preferable here? I suppose there’s some threshold such that if the group sizes become sufficiently large, the cost of returning all the redundant data is going to be greater than the overhead involved with an additional database call. What other things should I be thinking about here?

Thanks,
Jordan

Answer 1

If you actually want the results joined, I believe it is always more efficient to do the joining at the server level. The SQL processor is designed to match sets of data.
If you really want the results of 2 sql statements, you can always send two statements in one batch separated by a semicolon, and get two resultsets back with one round trip to the DB.