This CREATE TABLE … AS … WITH NO DATA request creates a NUSI on column_y of target_table. The primary index defaults to column_x and no indexes are copied from subquery_table.
Because the request specifies a subquery and no explicit table kind is specified, the table kind of target_table defaults to the session mode default, not to the table kind of subquery_table. target_table also takes the default definitions for all table options clause attributes.
CREATE TABLE target_table AS (SELECT column_x, column_y FROM subquery_table) WITH NO DATA INDEX (column_y);