[Qt-interest] QSqlQuery::isNull() behavior on MySQL

[Jiri Horky]

Hello again,

this was my fault :-) Don't pass -1 as index to isNull() to get a 
reasonable result....

JH

On 02/27/2011 02:17 PM, Jiri Horky wrote:
> Hello,
>
> digging into it a bit with valgrind I discovered that the
>
> "query.isNull(index)" call [ see example from the previous post]
> depends on uninitialized value allocated at mysql code:
>
> ==25356==  Uninitialised value was created by a heap allocation
> ==25356==    at 0x4025C4C: malloc (vg_replace_malloc.c:195)
> ==25356==    by 0x6110B5D: my_malloc (my_malloc.c:35)
> ==25356==    by 0x6114084: alloc_root (my_alloc.c:217)
> ==25356==    by 0x6137EE7: cli_read_rows (client.c:1414)
> ==25356==    by 0x6135F2F: mysql_store_result (client.c:2954)
> ==25356==    by 0x4040762: QMYSQLResult::reset(QString const&)
> (qsql_mysql.cpp:701)
> ==25356==    by 0x4083FE0: QSqlQuery::exec(QString const&)
> (qsqlquery.cpp:374)
> ....
> ....
>
> It seems to me as a bug either in mysql or in Qt [it it's mysql's
> expected behavior on NULL values].
>
> I am using Qt 4.6.3 and mysql-5.1.51 on a Gentoo host.
>
> Best regards
> Jiri Horky
>
> On 02/27/2011 12:41 PM, Jiri Horky wrote:
>> Hello,
>>
>> I am experiencing strange problem with QSqlQuery and isNull() function
>> using MYSQL backend.
>>
>> I have an application that returns some data from db formated in XML
>> upon user's request. Right now, I have only one record in database, so I
>> always return the same data. The query's result consists of many
>> columns, some of which can be NULL. Prior to returning the data to
>> client, I want to filter out these NULL values.
>>
>> Getting the data from one column looks like this (query is a local
>> variable of the function):
>>
>>            index = query.record().indexOf("country");
>>            if ( ! query.isNull(index)) {
>>                t->setCountry(query.value(index).toString());
>>            }
>>
>>
>> When a first client connects, the isNull() function correctly returns
>> true on columns where the NULL value really is. However, when a second
>> clients make the request, it returns false on such columns and thus
>> query.value(index).toString() triggers a Warning:
>>
>> WARNING: QSqlQuery::value: not positioned on a valid record
>>
>> Please note that it returns correct values on columns with non-NULL values.
>>
>> If it matter, I am in multiple threads environment, but I am pretty sure
>> I access the database only from one thread that made the connection to it.
>>
>> The execution of my application is not affected, but I would like to get
>> rid of these warnings.
>>
>> According to documentation, the isNull() function can behave incorrectly
>> on some DB engines, but sadly, it doesn't say on which ones. If this is
>> the case for MYSQL, what is the correct way to solve my problem?
>>
>> Thank you in advance
>> Jiri Horky
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