How to combine two select statements in c++ - c++

Question

For an assignment, I'm looking to make my code faster. I'm using the sqlite3 c++ API to perform tasks in order to eventually build an r-tree and b-tree.
I am doing the assignment's tasks correctly, but unfortunately it's extremely slow. For my question, I'll first show simple mock tables, then show a simple flow of my program.
Simplified table schema's:
areaTable (id int, closed int)
middleTable (nodeid int, areaid int)
nodeTable (id int, x float, y float)
The flow of my program is as follows:
query1
SELECT id FROM areaTable WHERE closed = 1;
Using query1 I save the resulting id's into an vector array (we'll call it query1ResultsArray).
Then using sqlite3_prepare_v2 I prepare a new select query:
query2
SELECT MIN(x), MIN(y)
FROM nodeTable
WHERE id IN
(
SELECT nodeid
FROM middleTable
WHERE areaid = ?
);
The idea of query 2 is that we find will find the minimum values of the nodes that get grouped together by middleTable and areaTable. I bind individual results from query1 into query2 using a for loop like the following:
prepare query2
begin transaction (not sure if this helps)
for (auto &id : query1ResultsArray) {
bind(id)
step(stmt)
x = column 0
y = column 1
cout << "INSERT INTO ...."
reset(stmt)
}
end transaction
finalize(stmt)
This solution appears to work. It get's the proper results I need to continue with the assignment's tasks (building insert statements), but it's very very slow. I doubt the professor expects our programs to be this slow.
This was context for my question. The question itself is essentially:
Am I able to combine my two select statements? By combining the select statements I would be able to circumvent the constant binding and resetting which I hope (with no knowledge to back it up) will speed up my program.
I've tried the following:
SELECT MIN(x), MIN(y), MAX(x), MAX(y)
FROM nodeCartesian
WHERE id IN
(
SELECT nodeid
FROM waypoint
WHERE wayid IN
(
SELECT id
FROM way
WHERE closed = 1
)
);
But this gets the minimum of all nodes since they don't get properly grouped together into their respective 'areas'.
P.S. I am dealing with a 2D r-tree, so I know what I wrote isn't correct, but I just wrote what I'm having difficulty with. Also, I tried researching how to apply inner joins to my statement, but couldn't figure out how :(, so if you think that may help my performance as well, I would love to hear it. Another thing is that query1 deals with 2+ million rows, while query2 deals with approximately 340,000 rows, and I estimated that it will take about 1 day for query2 to finish.
Thanks

Answer 1

I am not sure about your schema; however, I think that something like this by including a group by your area should do it
SELECT m.areaid, MIN(n.x), MIN(n.y), MAX(n.x), MAX(n.y)
FROM
nodeCartesian n
INNER JOIN waypoint wp ON n.id = wp.nodeid
INNER JOIN way w ON wp.wayid = w.id
INNER JOIN middleTable m ON n.id = m.nodeid
WHERE
w.closed = 1
GROUP BY
m.areaid
Note: calling a SELECT query multiple times in a loop is a bad idea, because each call has a great overhead which makes it really slow. Making a single query returning all the relevant rows and then looping through them in code is much faster.