Treat multiple tables separated by comma as COMMA JOIN (CROSS JOIN)

[abbashus]

Right now queries such as SELECT firstname, lastname FROM table1, table2 WHERE lastname = 'ABC' are treated a single query applied on multiple tables instead of Cross Join which the user would expect as per SQL standard.

--- Current behavior ---

/table1,table2/_search?
{
  "from" : 0,
  "size" : 200,
  "query" : {
    "bool" : {
      "filter" : [
        {
          "bool" : {
            "must" : [
              {
                "term" : {
                  "firstname" : {
                    "value" : "ABC",
                    "boost" : 1.0
                  }
                }
              }
            ],
            "adjust_pure_negative" : true,
            "boost" : 1.0
          }
        }
      ],
      "adjust_pure_negative" : true,
      "boost" : 1.0
    }
  },
  "_source" : {
    "includes" : [
      "firstname",
      "lastname"
    ],
    "excludes" : [ ]
  }
}

Either we should treat and implement it as Cross Join (Comma Join) or should throw 4XX exception when multiple tables are passed.