Github user chunhui-shi commented on the issue:
https://github.com/apache/drill/pull/702
MongoGroupScan tries to read first record to guess record width. So this
toJson is called for only first record.
Notice that MongoClient.getDefaultCodecRegistry() contains
DBRefCodecProvider:
Shown in MongoClient.java(In MongoClient.java):
private static final CodecRegistry DEFAULT_CODEC_REGISTRY =
fromProviders(asList(new ValueCodecProvider(),
new DBRefCodecProvider(),
new DocumentCodecProvider(new
DocumentToDBRefTransformer()),
new DBObjectCodecProvider(),
new BsonValueCodecProvider(),
new GeoJsonCodecProvider()));
but a default DocumentCodec constructor does not. And it was this
constructor get called if we don't set a codec for toJson function to use. The
list of CodecProviders is shown in DocumentCodec.java in MongoDb's code:
public class DocumentCodec implements CollectibleCodec<Document> {
private static final String ID_FIELD_NAME = "_id";
private static final CodecRegistry DEFAULT_REGISTRY =
fromProviders(asList(new ValueCodecProvider(),
new BsonValueCodecProvider(),
new DocumentCodecProvider()));
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