create table projection_fields (
  label text,
  description text,
  source ,
  fields 
);
comment on table projection_fields is '
  A single field of a projection schema.
';

comment on column projection_fields.label is '
  A human-readable label for the field.
';
comment on column projection_fields.description is '
  A human-readable description for the field.
';
comment on column projection_fields.source is '
  The source of the field. Either `source` or `fields` must be set.
';
comment on column projection_fields.fields is '
  Defines nested fields for this projection. Either `source` or `fields` must be set.
';