Default index on _id used even though there are better ones available

[gvuyk]

Case

Bug

Issue

When querying a collection, the default index on _id is used even though there are other more suitable indexes available. Checked on pouchdb directly and the same query does use another index there.

After a little research, noticed that if no sorting is requested, the default is to sort by _id, which seems to enforce the use of the default index.
When explicitly sorting by another field, a better index is used. Same if I remove the line which forces the sort on _id

Info

• Environment: Node.js

Code

To reproduce, set env var DEBUG to "pouchdb:find"
First query does not use the index on passportId
Second query does use the index on passportId

const RxDB = require("rxdb");
RxDB.plugin(require("pouchdb-adapter-memory"));

async function test()
{
	var db = await RxDB.create({
		name: "testdb",
		adapter: "memory",
		multiInstance: false
	});
	var mySchema = {
		version: 0,
		disableKeyCompression: true,
		type: "object",
		properties: {
			name: {type: "string"},
			passportId: {type: "string", index: true}
		}
	};
	let collection = await db.collection({name: "person", schema: mySchema});
	await collection.insert({
		name: "abc",
		passportId: "foobar"
	});
	console.log("\nFirst query");
	await collection.findOne({passportId: "foofbar"}).exec();

	console.log("\nSecond query");
	await collection.findOne({passportId: "foofbar"}).sort("passportId").exec();
}
test();

First query

Second query