PrestoDB Mongo query taking too much time - mongodb

Question

I am running a query in PrestoDB through a MongoDB connector. The query fetches data from a single collection in MongoDB. The query is something like:
SELECT studentId, classId, sum(date_diff('DAY', entryTime, (CASE WHEN (exitTime <= TIMESTAMP '2018-04-15 23:59:59 UTC') THEN exitTime ELSE TIMESTAMP '2018-04-15 23:59:59 UTC' END))) as timeSpent
FROM mongodb.school.student WHERE entryTime BETWEEN TIMESTAMP '2017-10-30 00:00:00 UTC' AND TIMESTAMP '2018-05-15 23:59:59 UTC' AND contains(classId, '1234') AND subject = 'Maths'
GROUP BY classId, studentId
ORDER BY timeSpent DESC;
I have about 8 million records in the collection and this query takes about 45 seconds to execute.
My PrestoDB is set up on a single Ubuntu instance acting as coordinator and worker with total RAM of 8GB. The jvm.config file looks like:
-server
-Xmx8G
-XX:+UseG1GC
-XX:G1HeapRegionSize=32M
-XX:+UseGCOverheadLimit
-XX:+AggressiveOpts
-XX:+ExplicitGCInvokesConcurrent
-XX:+HeapDumpOnOutOfMemoryError
-XX:+ExitOnOutOfMemoryError
The config.properties file has the following config:
coordinator=true
node-scheduler.include-coordinator=true
http-server.http.port=8080
query.max-memory=5GB
query.max-memory-per-node=1GB
discovery-server.enabled=true
discovery.uri=http://localhost:8080
The -Xmx8G was earlier -Xmx4G but I changed it to -Xmx8G to try but performance was almost the same. Am I:
Using instance with too low RAM (8GB)?
Should I try running PrestoDB as a cluster? What configuration is expected if there are to be about 60 million records in that collection with this query?
Or is it something with my current configuration itself?

Answer 1

Please run EXPLAIN ANALYZE for your query in Presto and show us the output.
It should be clear which part of the query takes most of the time.