QSQLite Error: Database is locked

Question

I am new to Qt development, the way it handles threads (signals and slots) and databases (and SQLite at that). It has been 4 weeks that I have started working on the mentioned technologies. This is the first time I'm posting a question on SO and I feel I have done research before coming to you all. This may look a little long and possibly a duplicate, but I request you all to read it thoroughly once before dismissing it off as a duplicate or tl;dr.

Context:

I am working on a Windows application that performs a certain operation X on a database. The application is developed in Qt and uses QSQLite as database engine. It's a single threaded application, i.e., the tables are processed sequentially. However, as the DB size grows (in number of tables and records), this processing becomes slower. The result of this operation X is written in a separate results table in the same DB. The processing being done is immaterial to the problem, but in basic terms here's what it does:

Read a row from Table_X_1
Read a row from Table_X_2
Do some operations on the rows (only read)
Push the results in Table_X_Results table (this is the only write being performed on the DB)

Table_X_1 and Table_X_2 are identical in number and types of columns and number of rows, only the data may differ.

What I'm trying to do:

In order to improve the performance, I am trying to make the application multi-threaded. Initially I am spawning two threads (using QtConcurrentRun). The two tables can be categorized in two types, say A and B. Each thread will take care of the tables of two types. Processing within the threads remains same, i.e., within each thread the tables are being processed sequentially.

The function is such that it uses SELECT to fetch rows for processing and INSERT to insert result in results table. For inserting the results I am using transactions.

I am creating all the intermediate tables, result tables and indices before starting my actual operation. I am opening and closing connections everytime. For the threads, I create and open a connection before entering the loop (one for each thread).

THE PROBLEM:

Inside my processing function, I get following (nasty, infamous, stubborn) error:

QSqlError(5, "Unable to fetch row", "database is locked")

I am getting this error when I'm trying to read a row from DB (using SELECT). This is in the same function in which I'm performing my INSERTs into results table. The SELECT and the INSERT are in the same transaction (begin and commit pair). For INSERT I'm using prepared statement (SQLiteStatement).

Reasons for seemingly peculiar things that I am doing:

1. I am using QtConcurrentRun to create the threads because it is straightforward to do! I have tried using QThread (not subclassing QThread, but the other method). That also leads to same problem.
2. I am compiling with DSQLITE_THREADSAFE=0 to avoid application from crashing. If I use the default (DSQLITE_THREADSAFE=1), my application crashes at SQLiteStatement::recordSet->Reset(). Also, with the default option, internal SQLITE sync mechanism comes into play which may not be reliable. If the need be, I'll employ explicit sync.
3. Making the application multi-threaded to improve performance, and not doing this. I'm taking care of all the optimizations recommended there.
4. Using QSqlDatabase::setConnectOptions with QSQLITE_BUSY_TIMEOUT=0. A link suggested that it will prevent the DB to get locked immediately and hence may give my thread(s) appropriate amount of time to "die peacefully". This failed: the DB got locked much frequently than before.

Observations:

1. The database goes into lock only and as soon as when one of the threads return. This behavior is consistent.
2. When compiling with DSQLITE_THREADSAFE=1, the application crashes when one of the threads return. Call stack points at SQLiteStatement::recordSet->Reset() in my function, and at winMutexEnter() (called from EnterCriticalSection()) in sqlite3.c. This is consistent as well.
3. The threads created using QtConcurrentRun do not die immediately.
4. If I use QThreads, I can't get them to return. That is to say, I feel the thread never returns even though I have connected the signals and the slots correctly. What is the correct way to wait for threads and how long it takes them to die?
5. The thread that finishes execution never returns, it has locked the DB and hence the error.
6. I checked for SQLITE_BUSY and tried to make the thread sleep but could not get it to work. What is the correct way to sleep in Qt (for threads created with QtConcurrentRun or QThreads)?

7. When I close my connections, I get this warning:

   QSqlDatabasePrivate::removeDatabase: connection 'DB_CONN_CREATE_RESULTS' is still in use, all queries will cease to work.

   Is this of any significance? Some links suggested that this warning arises because of using local QSqlDatabase, and will not arise if the connection is made a class member. However, could it be the reason for my problem?

Further experiments:

1. I am thinking of creating another database which will only contain results table (Table_X_Results). The rationale is that while the threads will read from one DB (the one that I have currently), they will get to write to another DB. However, I may still face the same problem. Moreover, I read on the forums and wikis that it IS possible to have two threads doing read and write on same DB. So why can I not get this scenario to work?
2. I am currently using SQLITE version 3.6.17. Could that be the problem? Will things be better if I used version 3.8.5?

I was trying to post the web resources that I have already explored, but I get a message saying "I'd need 10 reps to post more than 2 links". Any help/suggestions would be much appreciated.