Exception of multi thread reading QSqlTableModel
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I created the QSqlTableModel in the main thread and connected it to the database correctly. I want to quickly read the data in the model in the child thread
You are not allowed to perform SQL operations in a different thread from where the connection/model were created/live.
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@JonB
In other words, it is necessary to copy the data in the model into memory. Is my understanding correct please?wrote on 3 Dec 2020, 09:32 last edited by@tovax said in Exception of multi thread reading QSqlTableModel:
In other words, it is necessary to copy the data in the model into memory. Is my understanding correct please?
I don't really understand what this question means? A SQL operation like fetching the data must be performed in the thread owning the SQL connection. That thread could pre-read the result set, or a child thread could send it a signal asking it to do so when it wants it. I don't know why you are using child thread(s) here, but if you do your reading in another thread you might be best putting the
QSql...variables there, or moving them to that thread. -
@tovax said in Exception of multi thread reading QSqlTableModel:
In other words, it is necessary to copy the data in the model into memory. Is my understanding correct please?
I don't really understand what this question means? A SQL operation like fetching the data must be performed in the thread owning the SQL connection. That thread could pre-read the result set, or a child thread could send it a signal asking it to do so when it wants it. I don't know why you are using child thread(s) here, but if you do your reading in another thread you might be best putting the
QSql...variables there, or moving them to that thread.wrote on 3 Dec 2020, 10:11 last edited by@JonB
The data of QSqlTableModel is a global parameter, which needs to be used by multiple child threads. Your reply made me know the reason. According to your help, copying the data in the model to memory should solve my problem. I am modifying the code.
It should also be a good way to reestablish a connection between the model and the database at each child thread.
Thank you again. -
Hi,
If you want multithreaded access you should create one connection per thread.
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wrote on 4 Dec 2020, 03:43 last edited by
Main thread: create QTableView to modify the database
Child thread1: only read database
Child thread2: read database by "tableModel->data()", and write database by "tableModel->setData()"
Child thread3: only read database
All threads must be synchronized. -
wrote on 4 Dec 2020, 07:12 last edited by
In other words, different threads connect to the same database. When one thread updates the data in the database, how do other threads know that the database has been updated? The method of using Signal-Slots between threads seems troublesome.
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In other words, different threads connect to the same database. When one thread updates the data in the database, how do other threads know that the database has been updated? The method of using Signal-Slots between threads seems troublesome.
wrote on 4 Dec 2020, 07:28 last edited by JonB 12 Apr 2020, 07:31@tovax said in Exception of multi thread reading QSqlTableModel:
The method of using Signal-Slots between threads seems troublesome.
Nonetheless this is indeed the paradigm Qt wants you to use! Why do you find it "troublesome"?
Although doubtless he is correct --- he usually is! --- I do not share @SGaist's immediate alacrity to move to separate connections per thread. It may be required, but it could be costly on resources/memory/speed/consistency. You might also consider either working on an in-memory, shared copy of the data already read in, or signals/slots to/from the main thread. It depends on what you are doing where.
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@JonB said in Exception of multi thread reading QSqlTableModel:
It may be required,
It is required - QSqlDatabase database connection is not thread-safe.
how do other threads know that the database has been updated?
Use signals/slots
The method of using Signal-Slots between threads seems troublesome.
Why? It's much easier than using semaphores and wait conditions.
But still don't see why you need threads in the first place.
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@tovax said in Exception of multi thread reading QSqlTableModel:
The method of using Signal-Slots between threads seems troublesome.
Nonetheless this is indeed the paradigm Qt wants you to use! Why do you find it "troublesome"?
Although doubtless he is correct --- he usually is! --- I do not share @SGaist's immediate alacrity to move to separate connections per thread. It may be required, but it could be costly on resources/memory/speed/consistency. You might also consider either working on an in-memory, shared copy of the data already read in, or signals/slots to/from the main thread. It depends on what you are doing where.
wrote on 4 Dec 2020, 07:36 last edited by@JonB said in Exception of multi thread reading QSqlTableModel:
gm Qt wants you to use! Why do you find it "troubles
If there are more threads that modify the database, then it is necessary to establish signal slot connections from these threads to all other threads.
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@tovax said in Exception of multi thread reading QSqlTableModel:
then it is necessary to establish signal slot connections from these threads to all other threads.
That's also true when you don't use signals and slots...
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@JonB said in Exception of multi thread reading QSqlTableModel:
It may be required,
It is required - QSqlDatabase database connection is not thread-safe.
how do other threads know that the database has been updated?
Use signals/slots
The method of using Signal-Slots between threads seems troublesome.
Why? It's much easier than using semaphores and wait conditions.
But still don't see why you need threads in the first place.
wrote on 4 Dec 2020, 07:44 last edited by@Christian-Ehrlicher said in Exception of multi thread reading QSqlTableModel:
It may be required,
It is required - QSqlDatabase database connection is not thread-safe.
Yes, I was the first person to say that if you want to access a
QSqlDatabasedatabase connection you must only do so from its owning thread. If you read what I was saying, I was talking about not having multiple database connections, as an alternative.If, for whatever reason, the user wants to have 100 threads running accessing data read from the database, having 100 separate database connections is not an advisable approach. IMHO.
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@JonB said in Exception of multi thread reading QSqlTableModel:
It may be required,
It is required - QSqlDatabase database connection is not thread-safe.
how do other threads know that the database has been updated?
Use signals/slots
The method of using Signal-Slots between threads seems troublesome.
Why? It's much easier than using semaphores and wait conditions.
But still don't see why you need threads in the first place.
wrote on 4 Dec 2020, 07:47 last edited by@Christian-Ehrlicher said in Exception of multi thread reading QSqlTableModel:
But still don't see why you need threads in the first place.
The time-consuming trajectory algorithm and real-time motion control algorithm in the application program need to be processed by separate threads, which will use the parameter data in the database.
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@JonB said in Exception of multi thread reading QSqlTableModel:
gm Qt wants you to use! Why do you find it "troubles
If there are more threads that modify the database, then it is necessary to establish signal slot connections from these threads to all other threads.
wrote on 4 Dec 2020, 07:48 last edited by@tovax said in Exception of multi thread reading QSqlTableModel:
If there are more threads that modify the database, then it is necessary to establish signal slot connections from these threads to all other threads.
As @Christian-Ehrlicher already written,
QSqlDatabaseis not thread-safe, so you have 2 options:- do all DB stuff in one thread, by using signals/slots it is quit easy.
- use as many
QSqlDatabaseconnections as you have threads ==> take a look here for example: https://forum.qt.io/topic/103626/problem-with-sqlite-database-and-threads-database-is-locked/14
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@Christian-Ehrlicher said in Exception of multi thread reading QSqlTableModel:
It may be required,
It is required - QSqlDatabase database connection is not thread-safe.
Yes, I was the first person to say that if you want to access a
QSqlDatabasedatabase connection you must only do so from its owning thread. If you read what I was saying, I was talking about not having multiple database connections, as an alternative.If, for whatever reason, the user wants to have 100 threads running accessing data read from the database, having 100 separate database connections is not an advisable approach. IMHO.
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@tovax said in Exception of multi thread reading QSqlTableModel:
If there are more threads that modify the database, then it is necessary to establish signal slot connections from these threads to all other threads.
As @Christian-Ehrlicher already written,
QSqlDatabaseis not thread-safe, so you have 2 options:- do all DB stuff in one thread, by using signals/slots it is quit easy.
- use as many
QSqlDatabaseconnections as you have threads ==> take a look here for example: https://forum.qt.io/topic/103626/problem-with-sqlite-database-and-threads-database-is-locked/14
wrote on 4 Dec 2020, 08:07 last edited by@KroMignon said in Exception of multi thread reading QSqlTableModel:
do all DB stuff in one thread, by using signals/slots it is quit easy.
It's easy to write data by using signals/slots, but is it necessary to copy a memory copy for reading data? Because DB data cannot be read directly from other threads...
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@KroMignon said in Exception of multi thread reading QSqlTableModel:
do all DB stuff in one thread, by using signals/slots it is quit easy.
It's easy to write data by using signals/slots, but is it necessary to copy a memory copy for reading data? Because DB data cannot be read directly from other threads...
wrote on 4 Dec 2020, 08:14 last edited by JonB 12 Apr 2020, 08:16@tovax
Yes, this is the issue. As you say, emitting a signal to ask the main thread to do a write (setData()) for your thread is not so difficult. Your problem comes on trying to synchronise read access (data()).It is easier to maintain per-thread connections for that purpose. But that comes with other overheads, as discussed. Depends on how much of what you need to do when.
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Again: why do you think you need threads for simple reading data from the database? What heaviy computation do you do with those values?
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Again: why do you think you need threads for simple reading data from the database? What heaviy computation do you do with those values?
wrote on 4 Dec 2020, 08:21 last edited by@Christian-Ehrlicher
The main purpose of thread is to process time-consuming algorithms and real-time control. Reading data from the database is only to obtain the global parameters needed in the algorithm and control process. -
Lifetime Qt Championwrote on 4 Dec 2020, 08:22 last edited by Christian Ehrlicher 12 Apr 2020, 08:23
Then read your data in one thread, start/inform the workers that there is data and let them do the work. When you a mutex to protect read/write access to your data you don't even need to copy it. But that's plain threading stuff which you should read about before trying to actually writing something. Threading is not easy and should be avoided when not really needed for basic programmers.
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