Support simple query pushdown for Tigris

[seeforschauer]

The goal of this task is to enable a simple query pushdown for querier that looks like {_id: <ObjectID>}.
Only that field name (_id) is supported, and only that value type (ObjectID).

In the future, we will add support for other fields (starting with simple scalar fields),
other values (starting with strings, numbers, and other simple scalar values),
other conditions like {_id: <ObjectID>, field: value} (so only the first part is pushed down).

Tigris

Support tables where the primary key is only one field.

The tjson package ensures that primary key is always ["_id"] and always 1 length.
So no need to check:

• it's not necessary to get the schema description to check Primary Key fields and it's count.
• if len(schema.PrimaryKey) > 1 raise error.
  • because user expects single one value in _id.
  • to comply and be compatible with MongoDB.
• if len(schema.PrimaryKey) == 0 raise error.

But need to check if valaue type if not ObjectID (raise error in that case).

Proof of concept for a {_id: <ObjectID>} pushdown query, Tigris, see example.

_, err = db.Insert(ctx, "coll1", []driver.Document{[]byte(`{"$k":["_id","int32"],"_id":"AAECBAUGBwgJCgsM","int32":42}`)})
if err != nil {
	panic(err)
}
doc, _ := types.NewDocument("_id", types.ObjectID{0x00, 0x01, 0x02, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c})
id := must.NotFail(doc.Get("_id")).(types.ObjectID)
f := must.NotFail(filter.Eq("_id", tjson.ObjectID(id)).Build())
it, err := db.Read(ctx, "coll1", f, nil)
if err != nil {
	panic(err)
}

var tdoc driver.Document
for it.Next(&tdoc) {
	fmt.Println("found:")
	fmt.Printf("%s\n", string(tdoc))
}
if err := it.Err(); err != nil {
	panic(err)
}

Test

Integration tests.
Provide a new test flag that enables a query pushdown.
Run with and without flag: results must be the same.
When explain feature will be ready, compare the query plan.

Links

Issue analysis: #847