QSqlQuery Memory issues. QSqlQuery::exec() and QSqlDatabase::open()/close();

Question

I'm checking the memory usage of an application I've made. It makes numerous calls to read and write values to and from a database (SQLite 3). I've observed the following:

QSqlQuery::exec() uses some KB of RAM to execute a given query, but does not release the memory after it goes out of scope.
QSqlDatabase:: open() & close() do not help free resources as the documentation suggest. If anything, close() causes resources (at least memory) to remain 'trapped' on the heap/stack.

For example, here is a typical segment of code I've been using to access my database.

QStringList values;
db.open();
QString strQuery = "SELECT DISTINCT " + field + " FROM " + table + str;

QSqlQuery query(db);
query.prepare(strQuery);

if(query.exec() == true)
{
  while(query.next())
  {
    values.push_back(query.value(0).toString());
  }
}

db.close();

Having experimented with I find the code below 'traps' less memory:

QStringList values;
QString strQuery = "SELECT DISTINCT " + field + " FROM " + table + str;

QSqlQuery query(strQuery, db);

  while(query.next())
  {
    values.push_back(query.value(0).toString());
  }

However, a small amount of memory is still not released. Has anyone else experienced anything like this?

Can I some how release this memory?

P.s. Same happens here, some memory is never released:

db.open();
QSqlQuery query(db);

query.exec("DELETE FROM table1");
query.exec("DELETE FROM table2");
query.exec("DELETE FROM table3");
query.exec("DELETE FROM table4");
...

db.close();

nf313743 · Accepted Answer · 2011-07-18 11:09:55Z

2

It seems that in order to release this memory you must create the QSqlQuery variable as a pointer, and delete this pointer before you close the database:

QStringList values;
db.open();
QString strQuery = "SELECT DISTINCT " + field + " FROM " + table + str;

QSqlQuery *query = new QSqlQuery(db);
query->prepare(strQuery);

if(query->exec() == true)
{
  while(query->next())
  {
    values.push_back(query->value(0).toString());
  }
}

delete query;
db.close();

The memory is then released after the database closes.

answered Jul 18, 2011 at 11:09

nf313743

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1 Comment

Parthiv Shah Over a year ago

Here memory is not getting deleted for query. so the issue remains same as memory leak.

Rav · Accepted Answer · 2011-09-05 15:16:39Z

1

You have to use QSqlQuery.finish () or QSqlQuery.clear before you close the database. Otherwise residual memory is left out in the Query object. It is mentioned in the document that Query object can be used for multiple query. You will noticed the "memory leak".. when you query for 10,000 records. The memory usage goes up drastically.

answered Sep 5, 2011 at 15:16

Rav

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1 Comment

arthur Over a year ago

finish() or clear() is a lot better idea as the "solution" to delete the QSqlQuery object and closing the DB. I am not willing to close the DB at all, and deleting the QSqlQuery object didn't release the memory at all. Only finish cleared the resources. And since I am using very fat select statements, this halfed the memory usage of my program.

hmuelner · Accepted Answer · 2011-12-29 04:12:19Z

1

From the documentation of QSqlDatabase::addDatabase and QSqlDatabase::database() one can deduce that there is a global variable that manages the database connections. If you look into qsqldatabase.cpp you will find a QConnectionDict.

BTW: Do not construct your SQL queries by concatenating strings, always use prepare and bindValue (SQL injecttion!), if there is any chance that parts of the query come from user input.

edited Dec 29, 2011 at 4:12

answered Jul 14, 2011 at 9:40

hmuelner

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