Trivial project?

[Julian Hyde]

I would like to generate a trivial project operator that returns the entire 
input record. What expression should I use?

When implementing "select * from donuts", here is the logical plan I am 
currently generating:

{
  "head" : {
    "type" : "apache_drill_logical_plan",
    "version" : 1,
    "generator" : {
      "type" : "manual",
      "info" : "na"
    }
  },
  "storage" : [ {
    "type" : "classpath",
    "name" : "donuts-json"
  }, {
    "type" : "queue",
    "name" : "queue"
  } ],
  "query" : [ {
    "op" : "scan",
    "@id" : 1,
    "memo" : "initial_scan",
    "storageengine" : "donuts-json",
    "selection" : {
      "path" : "/donuts.json",
      "type" : "JSON"
    },
    "ref" : "donuts"
  }, {
    "op" : "project",
    "@id" : 2,
    "input" : 1,
    "exprs" : [ {
      "ref" : "output._MAP",
      "expr" : null
    } ]
  }, {
    "op" : "store",
    "@id" : 3,
    "memo" : "output sink",
    "input" : 2,
    "target" : {
      "number" : 0
    },
    "partition" : null,
    "storageEngine" : "queue"
  } ]
}

The

      "expr" : null

causes the reference implementation to barf (not surprisingly). What should I 
put in place of "null"?

Julian