Query Flattening
The ANSI Entry SQL-92 standard specifies that query flattening is not performed. By default, Ingres uses query flattening to improve the performance of queries.
ANSI/ISO Entry SQL-92
No Flattening:
dbmsinfo('flatten_singleton') returns N
dbmsinfo('flatten_aggregate') returns N
Ingres Default
Flattening used to optimize queries, including queries involving aggregate subselects or singleton subselects:
dbmsinfo('flatten_singleton') returns Y
dbmsinfo('flatten_aggregate') returns Y