CREATE TABLE dt64test
(`dt64column` DateTime64(3))
ENGINE = MergeTree
PARTITION BY toYYYYMM(dt64column)
ORDER BY dt64column;

INSERT INTO dt64test  VALUES ('2020-01-13 13:37:00');

SELECT 1 FROM dt64test WHERE dt64column = toDateTime('2020-01-13 13:37:00');
DB::Exception: Key expression contains comparison between inconvertible types: DateTime64(3) and DateTime

SELECT 1 FROM dt64test WHERE dt64column = materialize(toDateTime('2020-01-13 13:37:00'));
┌─1─┐
│ 1 │
└───┘