Use a prepared statement for all records during the CSV import

Don't build a separate SQL statement per row to insert during CSV import
but use a single prepared statement which can be reused for each row.
This should speed up the CSV import noticeably.

src/ImportCsvDialog.cpp

@@ -442,11 +442,20 @@ void ImportCsvDialog::importCsv(const QString& fileName, const QString &name)
                else if(f->isInteger() && !f->notnull())        // If this is an integer column and NULL is allowed, insert NULL
                    nullValues << QString();
                else                                            // Otherwise (i.e. if this isn't an integer column), insert an empty string
                    nullValues << "''";
                    nullValues << "";
            }
        }
    }

    // Prepare the INSERT statement. The prepared statement can then be reused for each row to insert
    QString sQuery = QString("INSERT INTO %1 VALUES(").arg(sqlb::escapeIdentifier(tableName));
    for(size_t i=1;i<=csv.columns();i++)
        sQuery.append(QString("?%1,").arg(i));
    sQuery.chop(1); // Remove last comma
    sQuery.append(")");
    sqlite3_stmt* stmt;
    sqlite3_prepare_v2(pdb->_db, sQuery.toUtf8(), sQuery.toUtf8().length(), &stmt, nullptr);

    // now lets import all data, one row at a time
    CSVParser::TCSVResult::const_iterator itBegin = csv.csv().begin();
    if(ui->checkboxHeader->isChecked())     // If the first row contains the field names we should skip it here because this is the data import
@@ -455,52 +464,49 @@ void ImportCsvDialog::importCsv(const QString& fileName, const QString &name)
        it != csv.csv().end();
        ++it)
    {
        QString sql = QString("INSERT INTO %1 VALUES(").arg(sqlb::escapeIdentifier(tableName));

        QStringList insertlist;
        for(int i=0;i<it->size();i++)
        // Bind all values
        unsigned int bound_fields = 0;
        for(int i=0;i<it->size();i++,bound_fields++)
        {
            // Empty values need special treatment, but only when importing into an existing table where we could find out something about
            // its table definition
            if(importToExistingTable && it->at(i).isEmpty() && nullValues.size() > i)
            {
                // This is an empty value. We'll need to look up how to handle it depending on the field to be inserted into.
                QString val = nullValues.at(i);
                if(val.isNull())
                    insertlist << "NULL";
                else
                    insertlist << val;
                if(!val.isNull())       // No need to bind NULL values here as that is the default bound value in SQLite
                    sqlite3_bind_text(stmt, i+1, val.toUtf8(), val.toUtf8().size(), SQLITE_TRANSIENT);
            } else {
                // This is a non-empty value. Just add it to the list. The sqlite3_mprintf call with the %Q placeholder takes the value,
                // adds single quotes around it and doubles all single quotes inside it. This means it'll be safe to be inserted into the SQL
                // statement.
                char* formSQL = sqlite3_mprintf("%Q", (const char*)it->at(i).toUtf8());
                insertlist << formSQL;
                if(formSQL)
                    sqlite3_free(formSQL);
                // This is a non-empty value. Just add it to the statement
                sqlite3_bind_text(stmt, i+1, static_cast<const char*>(it->at(i).toUtf8()), it->at(i).toUtf8().size(), SQLITE_TRANSIENT);
            }
        }

        // add missing fields with empty values
        for(unsigned int i = insertlist.size(); i < csv.columns(); ++i)
        // Insert row
        if(sqlite3_step(stmt) != SQLITE_DONE)
        {
            qWarning() << "ImportCSV" << tr("Missing field for record %1").arg(std::distance(itBegin, it) + 1);
            insertlist << "NULL";
            sqlite3_finalize(stmt);
            return rollback(this, pdb, progress, restorepointName, std::distance(itBegin, it) + 1, tr("Inserting row failed: %1").arg(pdb->lastError()));
        }

        sql.append(insertlist.join(QChar(',')));
        sql.append(");");

        if(!pdb->executeSQL(sql, false, false))
            return rollback(this, pdb, progress, restorepointName, std::distance(itBegin, it) + 1, tr("Inserting row failed: %1").arg(pdb->lastError()));
        // Reset statement for next use. Also reset all bindings to NULL. This is important, so we don't need to bind missing columns or empty values in NULL
        // columns manually.
        sqlite3_reset(stmt);
        sqlite3_clear_bindings(stmt);

        // Update progress bar and check if cancel button was clicked
        unsigned int prog = std::distance(csv.csv().begin(), it);
        if(prog % 100 == 0)
            progress.setValue(prog);
        if(progress.wasCanceled())
        {
            sqlite3_finalize(stmt);
            return rollback(this, pdb, progress, restorepointName, std::distance(itBegin, it) + 1, "");
        }
    }

    // Clean up prepared statement
    sqlite3_finalize(stmt);
}

void ImportCsvDialog::setQuoteChar(const QChar& c)