async function testGroupBy(dbName, storeName) 
{
	const db = await databaseOpen(dbName);
	const rows = await runTx(db, storeName, "readonly", (store) => 
	{
		return new Promise((resolve, reject) => {
		const index = store.index("f1_f2");
		const groupedData = [];
		const req = index.openCursor();
		let key = "";
		req.onsuccess = (event) => 
		{
			const cursor = event.target.result;
			if (cursor) 
			{
				const tmp = cursor.value;
				//const category = record.f1_f2;
				const category = tmp.f1 + "" + tmp.f2;
				//console.log("i:"+idx+" category:"+category);
				if (!groupedData[category]) 
				{
					groupedData.push(category);
				}
				//next record
				cursor.continue();
			} 
			else 
			{
				//cursor end
				resolve(groupedData);
			}
		};
			req.onerror = () => reject(req.error);
		});
	});
	
	db.close();

	return rows;
}
function waitForTx(tx) 
{
	return new Promise((resolve, reject) => 
	{
		tx.oncomplete = () => resolve();
		tx.onabort = () => reject(tx.error ?? new Error("Transaction aborted"));
		tx.onerror = () => reject(tx.error);
	});
}

async function runTx(db, storeName, mode, work) 
{
	const tx = db.transaction([storeName], mode);
	const store = tx.objectStore(storeName);
	const result = await work(store);
	await waitForTx(tx);
	return result;
}

function openDatabase(dbName, version, upgrade) 
{
	return new Promise((resolve, reject) => 
	{
		const req = indexedDB.open(dbName, version);
		req.onupgradeneeded = (event) => 
		{
			const db = event.target.result;
			upgrade?.(db, event);
		};
		req.onsuccess = (event) => 
		{
			resolve(event.target.result);
		};
		req.onerror = () => reject(req.error);
	});
}
testGroupBy("aaa", "t2")
.then(rows => 
{
	for(i=0;i < rows.length;i++)
	{
		console.log("i:"+i+" rows:"+rows[i]);
	}
})
.catch(error => {
	console.error("group by error:", error);
});