Optimize DAG request TypeTableScan executor.

[birdstorm]

Issue by Novemser
Wed Dec 13 02:52:14 2017
Originally opened as pingcap/tikv-client-lib-java#195

---

In some cases, TypeTableScan executor doesn't need to push down all the columns.

Like this, we only need to push down some of the columns for this request.

executors {
  tp: TypeTableScan
  tbl_scan {
    table_id: 363
    columns {
      column_id: 1
      tp: 3
      collation: 63
      columnLen: 11
      decimal: 0
      flag: 4227
      pk_handle: true
    }
    columns {
      column_id: 2
      tp: 15
      collation: 83
      columnLen: 45
      decimal: 0
      flag: 0
      pk_handle: false
    }
    columns {
      column_id: 3
      tp: 12
      collation: 63
      columnLen: 19
      decimal: 0
      flag: 128
      pk_handle: false
    }
    columns {
      column_id: 4
      tp: 252
      collation: 63
      columnLen: 65535
      decimal: 0
      flag: 128
      pk_handle: false
    }
    columns {
      column_id: 5
      tp: 254
      collation: 63
      columnLen: 2
      decimal: 0
      flag: 128
      pk_handle: false
    }
    columns {
      column_id: 6
      tp: 10
      collation: 63
      columnLen: 10
      decimal: 0
      flag: 128
      pk_handle: false
    }
    columns {
      column_id: 7
      tp: 7
      collation: 63
      columnLen: 19
      decimal: 0
      flag: 1153
      pk_handle: false
    }
    columns {
      column_id: 8
      tp: 13
      collation: 63
      columnLen: 4
      decimal: 0
      flag: 96
      pk_handle: false
    }
    columns {
      column_id: 9
      tp: 8
      collation: 63
      columnLen: 20
      decimal: 0
      flag: 128
      pk_handle: false
    }
    columns {
      column_id: 10
      tp: 246
      collation: 63
      columnLen: 11
      decimal: 0
      flag: 128
      pk_handle: false
    }
    columns {
      column_id: 11
      tp: 5
      collation: 63
      columnLen: 22
      decimal: -1
      flag: 128
      pk_handle: false
    }
    columns {
      column_id: 12
      tp: 4
      collation: 63
      columnLen: 12
      decimal: -1
      flag: 128
      pk_handle: false
    }
    columns {
      column_id: 13
      tp: 3
      collation: 63
      columnLen: 11
      decimal: 0
      flag: 128
      pk_handle: false
    }
    columns {
      column_id: 14
      tp: 9
      collation: 63
      columnLen: 9
      decimal: 0
      flag: 128
      pk_handle: false
    }
    columns {
      column_id: 15
      tp: 5
      collation: 63
      columnLen: 22
      decimal: -1
      flag: 128
      pk_handle: false
    }
    columns {
      column_id: 16
      tp: 2
      collation: 63
      columnLen: 6
      decimal: 0
      flag: 128
      pk_handle: false
    }
    columns {
      column_id: 17
      tp: 1
      collation: 63
      columnLen: 4
      decimal: 0
      flag: 128
      pk_handle: false
    }
    columns {
      column_id: 18
      tp: 254
      collation: 83
      columnLen: 10
      decimal: 0
      flag: 0
      pk_handle: false
    }
    columns {
      column_id: 19
      tp: 15
      collation: 83
      columnLen: 40
      decimal: 0
      flag: 0
      pk_handle: false
    }
    columns {
      column_id: 20
      tp: 251
      collation: 83
      columnLen: -1
      decimal: 0
      flag: 0
      pk_handle: false
    }
    columns {
      column_id: 21
      tp: 250
      collation: 83
      columnLen: 16777215
      decimal: 0
      flag: 0
      pk_handle: false
    }
    columns {
      column_id: 22
      tp: 252
      collation: 83
      columnLen: 65535
      decimal: 0
      flag: 0
      pk_handle: false
    }
    columns {
      column_id: 23
      tp: 249
      collation: 83
      columnLen: 255
      decimal: 0
      flag: 0
      pk_handle: false
    }
    columns {
      column_id: 24
      tp: 16
      collation: 63
      columnLen: 1
      decimal: 0
      flag: 32
      pk_handle: false
    }
    columns {
      column_id: 25
      tp: 11
      collation: 63
      columnLen: 10
      decimal: 0
      flag: 128
      pk_handle: false
    }
    columns {
      column_id: 26
      tp: 247
      collation: 83
      columnLen: -1
      decimal: 0
      flag: 0
      elems: "1"
      elems: "2"
      elems: "3"
      elems: "4"
      pk_handle: false
    }
    columns {
      column_id: 27
      tp: 248
      collation: 83
      columnLen: -1
      decimal: 0
      flag: 0
      elems: "a"
      elems: "b"
      elems: "c"
      elems: "d"
      pk_handle: false
    }
  }
}
executors {
  tp: TypeSelection
  selection {
    conditions {
      tp: ScalarFunc
      children {
        tp: ScalarFunc
        children {
          tp: ColumnRef
          val: "\200\000\000\000\000\000\000\005"
        }
        sig: TimeIsNull
        field_type {
          tp: 2
        }
      }
      sig: UnaryNot
      field_type {
        tp: 2
      }
    }
  }
}
time_zone_offset: 0
flags: 0
output_offsets: 5
}

[Novemser]

Fixed by #143 and pingcap/tikv-client-lib-java#203