The following high-CPU stress test query gave an interesting result. SYSCOLUMN contains only 2013 rows.
184.108.40.20698 SELECT COUNT(*) FROM SYSCOLUMN AS A CROSS JOIN SYSCOLUMN AS B CROSS JOIN SYSCOLUMN AS C; There was an error reading the results of the SQL statement. The displayed results may be incorrect or incomplete. Value SUM() out of range for destination SQLCODE=-158, ODBC 3 State="22003"
FWIW COUNT(*) seems to have INTEGER as its data type...
SELECT EXPRTYPE ( 'SELECT COUNT(*)', 1 ); EXPRTYPE('SELECT COUNT(*)',1) int
...so that might explain why 2013 * 2013 * 2013 = 8,157,016,197 causes difficulties (an INTEGER is limited to 2,147,483,647)
When intra-query parallelism was turned off, the query failed in a different fashion: an invalid result -432918395...
SET TEMPORARY OPTION MAX_QUERY_TASKS = '1'; SELECT COUNT(*) FROM SYSCOLUMN AS A CROSS JOIN SYSCOLUMN AS B CROSS JOIN SYSCOLUMN AS C; Execution time: 2433.895 seconds COUNT() -432918395
The first query used 8 CPUs, and took 622 seconds to fail... whether it was "finished" or not is unknown, but it probably was getting close since it had been using fewer and fewer CPUs for the last minute or so.