aggregate (spec/04 §4.10): one row per distinct value of by, in order of first
appearance (a list of fields groups by every one, a nil value a group of its own),
holding by's columns and, under as, the named aggregate (fn) of the group's
readings of field, nils dropped; without by, one row of as alone. A pair field
[x, y] reads each row as the point [x, y], x in seconds when it is a time.
The columns are read from the table's own storage, a packed numeric column without building a row list.