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Multithread program to update different parts of table with different cursors

I have a table of one million records. on a column of each record I have to do a math operation on java that takes a fair amount of time. To improve total time, I created threads. If I create 10 threads, I make a query on each thread of 100.000 records by using a SQL offset and a first fetch so I end up operating in the whole one million along all the threads.
It does improve the processing. I have 12 free processors. But I realized that if I create for example 4 threads, it is much better than one thread (half the total time), but if I create 8 threads, the time is almost the same as 4 threads.
On each thread, I am using something like
Connection conn=DriverManager.getConnection(...);
Statement stmt= conn.createStatement(ResultSet.TYPE_SCROLL_SENSITIVE, ResultSet.CONCUR_UPDATABLE);
ResultSet rs= stmt.executeQuery("select COL from DATA OFFSET X FETCH FIRST Y");
while(rs.next()){
//do math stuff on col
rs.updateString("COL", new_math_stuff);
rs.updateRow();
}
I was wondering if having different cursors on different connection on different parts of the table, still can cause too much IO on the table, hence having more threads doesn't help, or if the problem might by somewhere else. (I have very limited profiling capabilities because I am working on a remote computer and to compile the code I have to send it to some guy to approve it and do it.)

Reading from Database through multiple threads in java

I am reading data from vertica database using multiple threads in java.
I have around 20 million records and I am opening 5 different threads having select queries like this....
start = threadnum;
while (start*20000<=totalRecords){
select * from tableName order by colname limit 20000 offset start*20000.
start +=5;
}
The above query assigns 20K distinct records to read from db to each thread.
for eg the first thread will read first 20k records then 20K records starting from 100 000 position,etc
But I am not getting performance improvement. In fact using a single thread if it takes x seconds to read 20 million records then it is taking almost x seconds for each thread to read from database.
Shouldn't there be some improvement from x seconds (was expecting x/5 seconds)?
Can anybody pinpoint what is going wrong?

Your database presumably lies on a single disk; that disk is connected to a motherboard using a single data cable; if the database server is on a network, then it is connected to that network using a single network cable; so, there is just one path that all that data has to pass through before it can arrive at your different threads and be processed.
The result is, of course, bad performance.
The lesson to take home is this:
Massive I/O from the same device can never be improved by multithreading.
To put it in different terms: parallelism never increases performance when the bottleneck is the transferring of the data, and all the data come from a single sequential source.
If you had 5 different databases stored on 5 different disks, that would work better.
If transferring the data was only taking half of the total time, and the other half of the time was consumed in doing computations with the data, then you would be able to halve the total time by desynchronizing the transferring from the processing, but that would require only 2 threads. (And halving the total time would be the best that you could achieve: more threads would not increase performance.)
As for why reading 20 thousand records appears to perform almost as bad as reading 20 million records, I am not sure why this is happening, but it could be due to a silly implementation of the database system that you are using.
What may be happening is that your database system is implementing the offset and limit clauses on the database driver, meaning that it implements them on the client instead of on the server. If this is in fact what is happening, then all 20 million records are being sent from the server to the client each time, and then the offset and limit clauses on the client throw most of them away and only give you the 20 thousand that you asked for.
You might think that you should be able to trick the system to work correctly by turning the query into a subquery nested inside another query, but my experience when I tried this a long time ago with some database system that I do not remember anymore is that it would result in an error saying that offset and limit cannot appear in a subquery, they must always appear in a top-level query. (Precisely because the database driver needed to be able to do its incredibly counter-productive filtering on the client.)
Another approach would be to assign an incrementing unique integer id to each row which has no gaps in the numbering, so that you can select ... where unique_id >= start and unique_id <= (start + 20000) which will definitely be executed on the server rather than on the client.
However, as I wrote above, this will probably not allow you to achieve any increase in performance by parallelizing things, because you will still have to wait for a total of 20 million rows to be transmitted from the server to the client, and it does not matter whether this is done in one go or in 1000 gos of 20 thousand rows each. You cannot have two stream of rows simultaneously flying down a single wire.

I will not repeat what Mike Nakis says as it is true and well explained :
I/O from a physical disk cannot be improved by multithreading
Nevertheless I would like to add something.
When you execute a query like that :
select * from tableName order by colname limit 20000 offset start*20000.
from the client side you may handle the result of the query that you could improve by using multiple threads.
But from the database side you have not the hand on the processing of the query and the Vertica database is probably designed to execute your query by performing parallel tasks according to the machine possibilities.
So from the client side you may split the execution of your query in one, two or three parallel threads, it should not change many things finally as a professional database is designed to optimize the response time according to the number of requests it receives and machine possibilities.

No, you shouldn't get x/5 seconds. You are not thinking about the fact that you are getting 5 times the number of records in the same amount of time. It's about throughput, not about time.

In my opinion, the following is a good solution. It has worked for us to stream and process millions of records without much of a memory and processing overhead.
PreparedStatement pstmt = conn.prepareStatement(sql, ResultSet.TYPE_FORWARD_ONLY, ResultSet.CONCUR_READ_ONLY);
pstmt.setFetchSize(Integer.MIN_VALUE);
ResultSet rs = pstmt.executeQuery();
while(rs.next()) {
// Do the thing
}
Using OFFSET x LIMIT 20000 will result in the same query being executed again and again. For 20 million records and for 20K records per execution, the query will get executed 1000 times.
OFFSET 0 LIMIT 20000 will perform well, but OFFSET 19980000 LIMIT 20000 itself will take a lot of time. As the query will be executed fully and then from the top it will have to ignore 19980000 records and give the last 20000.
But using the ResultSet.TYPE_FORWARD_ONLY, ResultSet.CONCUR_READ_ONLY options and setting the fetch size to Integer.MIN_VALUE will result in the query being executed only ONCE and the records will be streamed in chunks, and can be processed in a single thread.

JDBC Insert after Delete slows down performance

I run JDBC queries in a sequence INSERT, DELETE, INSERT, DELETE etc. I insert one million of records in 1000 batches then delete those million records in single query then insert it again. In this case I am interested only in insertion performance.
When I run in in a loop of i.e. 10 iterations, at 1st iteration the performance is fastest i.e. 11 seconds then after every next iteration performance of insert is few seconds slower then the previous one. However, when I run it not in the loop the insertion time is very similar.
Any idea why?
for(number of iterations){
//process insert of million records here, batch size is 1000
//prepared statement is used and clearBatch is called after every 1000
//inserts,
//at the end prepared statement is closed and connection.commit() is
//called
//Thread.sleep(1000) called here
//everything is inserted now in the DB so delete what has been inserted
//in single query. connection.commit() called again after delete.
//Thread.sleep(1000) and repeat the same actions until loop finishes.
}
Sorry, I don't have the code with me.
Any idea why at every next iteration the insertion is slower?

I can't be sure without the code, but I think you have a memory leak so that the extra time is due to garbage collection. That would explain why it is faster to run the same code several times without a loop. Run the program with GC logging enabled (-XX:+PrintGC) and see what happens.
To eliminate database issues you may want to test with another (new) table and replace the delete with truncate table, just in case.

ResultSet - Cursor : rs.next() Taking lot of time

I have a cursor returned from Database executes in 31ms (milliseconds) .
But when I use this cursor having more than 1500 rows for fetching rows
ResultSet rs = (ResultSet)cstm.getObject(6);
while(rs.next()){
system.out.println("...");
}
Just simple transversing through each row of cursor it's taking more than 40 seconds (40000 ms)
What can be done?

Indeed, by default JDBC use a fetch size of 10.
Thus, if you don't set a greater value, you'll call database for next records exactly 150 times ..., no need to explain drawbacks of round-trips.
All you have to do is to test performance by setting fetchSize to.. 100 for instance :
statement.setFetchSize(100);
You can play with this number to improve performance according to your environnement.

you have more than 1500 rows in your cursor and the rs what is returned by the database is just a reference to that cursor. So when you invoke rs.next(), every time it goes to the database cursr and get you the current record pointed by the cursor pointer.
So obviously it will take some time to go to database every time and fetch a single record for more than 1500 times with every while loop iteration.

Java - DB2 Performance Improvements

We have a SELECT statement which will take approx. 3 secs to execute. We are calling this DB2 query inside a nested While loop.
Ex:
While(hashmap1.hasNext()){
while(hashmap2.hasNext()){
SQL Query
}
}
Problem is, the outer While loop will execute approx. 1200 times and inner While loop will execute 200 times. Which means the SQL will be called 1200*200 = 240,000 times. Approx. each iteration of Outer While loop will take 150 secs. So, 1200 * 150 secs = 50 hrs.
We can afford only around 12-15hrs of time, before we kick off the next process.
Is there any way to do this process quickly? Any new technology which can help us in fetching these records faster from DB2.
Any help would be highly appreciated.
Note: We already looked into all possible ways to cut down the no.of iterations.

Sounds to me like you're trying to use the middle tier for something that the database itself is better suited for. It's a classic "N+1" query problem.
I'd rewrite this logic to execute entirely on the database as a properly indexed JOIN. That'll not only cut down on all that network back and forth, but it'll bring the database optimizer to bear and save you the expense of bringing all that data to the middle tier for processing.