查看IfeatureLayer的Search方法的帮助，如下：

If there is a definition query set on the layer, the Search method will work on the subset of features in the layer that meet the definition criteria. Otherwise, it will search the FeatureClass associated with the layer.

This Search method will not work on joined fields. If the FeaureLayer has any joins, you should use the IGeoFeatureLayer::SeachDisplayFeatures method instead.

值得注意的是，如果图层没有设置definition query ，那么两者的结果没有差别，但是如果图层设置了definition query ，那么就有差别了，呵呵。