REP-6503 Avoid $type in partition queries unless needed (#129)

The $type checks that REP-6129 (PR #118) added to fix type bracketing have impeded performance against 4.4 clusters.

This changeset makes partition queries use $type only when the range requires it, which should only be true for a handful of partitions anyhow. The queries are simplified, too, so that instead of ANDing together predicates like `_id >= 12 OR $type in ["string", "symbol", ...])` and `id <= 99 OR $type in ["null", "minkey"]` we just have a single $type invocation (if it’s even there, which it usually won’t be now).

The existing checks for type-bracketing accommodation are expanded a bit here as well.

Author: FGasper

internal/partitions/bson.go

@@ -85,13 +85,13 @@ var bsonTypeString = map[bsontype.Type]string{
 // This returns BSON types that the server’s type bracketing excludes from
 // query results when matching against the given value.
 //
-// The returned slices are types before & after, respectively. They are
-// strings rather than bsontype.Type to facilitate easy insertion into queries.
+// The returned slices are types before & after, respectively. They require
+// transformation (e.g., via typesToStrings()) before insertion into queries.
 //
 // This is kind of like strings.Cut() but against the sort-ordered list of BSON
 // types, except that if the given value is a number or string-like, then other
 // “like” types will not be in the returned slices.
-func getTypeBracketExcludedBSONTypes(val any) ([]string, []string, error) {
+func getTypeBracketExcludedBSONTypes(val any) ([]bsontype.Type, []bsontype.Type, error) {
 	bsonType, _, err := bson.MarshalValue(val)
 	if err != nil {
 		return nil, nil, errors.Wrapf(err, "marshaling min value (%v)", val)
@@ -120,7 +120,7 @@ func getTypeBracketExcludedBSONTypes(val any) ([]string, []string, error) {
 		later = lo.Without(later, stringTypes...)
 	}
 
-	return typesToStrings(earlier), typesToStrings(later), nil
+	return earlier, later, nil
 }
 
 func typesToStrings(in []bsontype.Type) []string {