SQL: incorrect DATE format when converting and GROUPing BY

[bpintea]

ES/SQL will return a DATETIME value (instead of a DATE) when GROUP BY is applied:
SELECT {fn CONVERT("@timestamp", SQL_DATE)} FROM kibana_sample_data_logs GROUP BY 1
will return:

"columns" : [
    {
      "name" : """CONVERT("@timestamp", SQL_DATE)""",
      "type" : "date"
    }
  ],
  "rows" : [
    [
      "2018-07-22T00:39:02.912Z"
    ],
...

It works as expected without the GROUP BY clause (that is "2018-07-22T00:00:00Z")

[elasticmachine]

Pinging @elastic/es-search

[astefan]

master (8.0.0): 3e25db2
7.x (7.5.0): 40e9353
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