Qt PL/SQL - Assignment Operator - Character String Buffer too small - c++

Question

I have been picking my brain for a while trying to figure this one out.
The problem I am having is that the function I am using in Oracle returns a BLOB. It's a list of items that are concatenated together using ||.
From the research I have done,
In the QSQLQuery docs it says "Stored procedures that uses the return statement to return values, or return multiple result sets, are not fully supported. For specific details see SQL Database Drivers." - which leads me to believe I may need to switch to a different codebase if Qt cannot handle it yet.
The documentation for the QOCI driver mentions this "Binary Large Objects (BLOBs) can be read and written, but be aware that this process may require a lot of memory. You should use a forward only query to select LOB fields (see QSqlQuery::setForwardOnly())."
I did set
query.setForwardOnly(true);
Before I prepared or executed the statement.
However, I still get this error
QSqlError("6502", "Unable to execute statement", "ORA-06502: PL/SQL: numeric or value error: character string buffer too small\nORA-06512: at line 55\n")
I had to scrub the code a bit, I hope this is still helpful to give context to what i'm trying to accomplish
temp_clob clob;
name varchar2(183) := ?;
start varchar2(16) := ?;
end varchar2(16) := ?;
count integer := ?;
return_val named_redacted_table_object; -- Sorry had to remove this, it's a table with more Date, Varchar, etc
begin
dbms_lob.createtemporary(temp_clob, true);
return_val := package_name.function_name (
set_name => name,
start_time => to_date(start, 'yyyy-mm-dd hh24:mi'),
end_time => to_date(end, 'yyyy-mm-dd hh24:mi'),
max_count => count);
-- In here was a loop that would break apart the removed table object and make it into strings along the following lines
-- '' || return_val(i).name || return_val(i).value || etc
-- and would store these into the CLOB / temp_clob
? := temp_clob;
end;
I could not get something as simple as this to work
begin
? := 'test123';
end;
With the assumption I could at least read this string in Qt.
Here is my code for Qt
QString name = "test";
QSqlQuery query(db);
query.setForwardOnly(true);
query.prepare(sql);
QString test_sql_text = ui->comboBox_test_text->currentText();
qDebug() << name;
query.bindValue(0, name);
query.bindValue(1, "2003-03-14 00:00");
query.bindValue(2, "2005-03-14 23:00");
query.bindValue(3, 2);
query.bindValue(4, QString(""), QSql::Out);
bool query_executed_ok = query.exec();
qDebug() << "did it execute?" << query_executed_ok;
// qDebug() << query.executedQuery();
qDebug() << query.boundValues();
qDebug() << query.lastError();
QSqlRecord rec = query.record();
qDebug() << rec;
int record_count = rec.count();
qDebug() << "Records: " << record_count;
while (query.next()) {
for(int i=0;i<record_count;i++) {
qDebug() << query.isValid() << " - " << rec.fieldName(i) << " " << query.value(i).toString();
}
}

Answer 1

The error posted appears to be from within the Oracle code; ORA.... You have stripped so much it's hard to see what is actually happening, especially the are where the error apparently occurred. But perhaps using Oracle supplied code that is specifically designed to handle CLOBs. Instead of
'' || return_val(i).name ...
Try
dbms_lob.append(temp_clob, to_clob(return_val(i).name))

Answer 2

begin
? := 'test123';
end;
Bind variables are used to assign values to variables. You define your variable in your pl/sql code and assign a value to it at runtime by using a bind variable. In that case pl/sql code will compile correctly.
In your code the bind variable is used to replace the pl/sql variable, not the value, which will fail. Your pl/sql block cannot be compiled because it cannot resolve the "?".
A valid use of bind variables would be
BEGIN
l_xyz := ?;
END;
where you assign the value test123 at runtime.

Answer 3

It took some fiddling, and I realize I gave fairly cryptic code. So thank you to belayer and Koen for taking a shot at my mess.
What I was able to determine and get working for anyone else running into this:
Let me start off by saying I am not sure if this is a bug, or if i'm doing something in a way that was not intended by the designers of QSqlQuery (The class for handling SQL calls).
The call would work in SQL developer and I would see the intended CLOB with all characters. I was unable to get DBMS_Output to work, however, I saw this post saying to reserve space on the string before binding it to the query.
It solves my issue and shows the result in the debug window. However, it presents a new problem. What if the string becomes larger than my hard coded reserve value?
Here's the updated code for that
query.prepare(sql);
QString name= ui->comboBox_name->currentText();
qDebug() << project;
query.bindValue(":name", project);
query.bindValue(":start_date", "2005-03-14 00:00");
query.bindValue(":end_date", "2006-03-14 23:00");
query.bindValue(":max_count", 3);
QString testStr ="*****";
//testStr.truncate(0); //Maybe this works too?
testStr.reserve( 1000000 ); // This did it for sure
qDebug() << testStr.capacity();
query.bindValue(":result_clob", testStr, QSql::Out);
bool query_executed_ok = query.exec();
qDebug() << "did it execute?" << query_executed_ok;
if (query_executed_ok) {
testStr = query.boundValue(":result_clob").toString();
qDebug() << testStr;
} else {
qDebug() << "string is empty";
}
I got the idea to do this, from THIS post.