mongo4.4 mapreduce

db.runCommand({
“mapreduce” : “column_info”,
“map” : function() {
for (var key in this) { emit(key, null); }
},
“reduce” : function(key, stuff) { return null; },
“out”: “column_info_key2”
})

获取collection下的所有key，run后，没有生成column_info_key2的collection