用qt从postgres读写图像（大对象）

Question

I want to read and write large object like image or video in postgres with oid type with framework qt. In query i can using lo_import and lo_export to read and write largeobject with specific local address for import and export. But question is how can i read and write with qt and qsqlquery?

Answer 1

You may use the server side SQL functions for large objects, callable by SELECT statements through QSqlQuery::exec() .

To read or write a large object in one go, see lo_get and lo_put .

If the binary data are large enough that you'd rather process them by chunks, start a transaction, then use lo_open , loread / lowrite in a loop, lo_close ...

The PostgreSQL type of the binary data sent and retrieved is bytea . QSql::Binary should be used when binding input parameters with QSqlQuery , for the driver to escape the data accordingly when needed.

Answer 2

Let's assume you have a table named "my_table" . For storing large binary objects with PostgreSQL it is recommended to use bytea type for a datafield, so name it "binary_data" .

To insert data I'd use QSqlRecord , like this:

QSqlTableModel *model;
QSqlDatabase m_database;
m_database = ...//connect to the database here
model = new QSqlTableModel(0, m_database);
model->setTable("my_table");
model->select();
QSqlRecord rec;
rec = model->record();
QFile f("path_to_a_big_file");
if(f.open(QIODevice::ReadOnly))
{
    QByteArray ba = f.readAll();
    rec.setValue("binary_data",ba.toBase64());
}
model->insertRecord(-1,rec);
model->submitAll();

To update the data QSqlQuery will do:

QSqlQuery query(m_database);
query.prepare("update my_table set binary_data = :binary_data where [condition]....");
QFile f("path_to_a_big_file");
if(f.open(QIODevice::ReadOnly))
{
    QByteArray ba = f.readAll();
    query.bindValue(":binary_data",ba.toBase64());
}
query.exec();

And finally to read the data you'll need to do something like this:

QSqlTableModel *model;
model = new QSqlTableModel(0, m_database);
model->setTable("my_table");
model->select();
 while(model->canFetchMore())
    model->fetchMore();
for (int i = 0; i < model->rowCount(); ++i)
{
    QByteArray ba = QByteArray::fromBase64(model->record(i).value("binary_data").toByteArray())
    //do something with the retrieved data
}

Also note that it is your responsibility to remember the type of a file you put inside the table.