Bulk iteration over a store or an index

[darrachequesne]

Is your feature request related to a problem? Please describe.

Iterating with a cursor over a lot of documents is quite slow and can be made faster with store.getAll(query, count) (but only in ascending order):

function bulkIterate (storeOrIndex, query, direction, bulkSize) {
  if (direction === "next") {
    let offset = query;

    return (async function*() {
      while (true) {
        const items = await storeOrIndex.getAll(offset, bulkSize);

        if (items.length === 0) {
          break;
        }

        for (const item of items) {
          yield { value: item };
        }

        const lastItem = items[items.length - 1];
        const lastItemId = Array.isArray(storeOrIndex.keyPath) ?
          storeOrIndex.keyPath.map((k) => lastItem[k]) : lastItem[storeOrIndex.keyPath];
        if (query instanceof IDBKeyRange && query.upper !== undefined) {
          offset = IDBKeyRange.bound(lastItemId, query.upper, true, query.upperOpen);
        } else {
          offset = IDBKeyRange.lowerBound(lastItemId, true);
        }
      }
    })();
  }

  return storeOrIndex.iterate(query, direction);
}

// usage
for await (const cursor of bulkIterate(store, null, "next", 101)) {
  console.log(cursor.value);
}

Do you see anything wrong with this approach? Else, do you think this could/should be added in the iterate() method here? Or simply mentioned in the README?

Thanks in advance!

Related: w3c/IndexedDB#130