The reason for this is to make all temporary file names similar and
also to be able to figure out from where a #sql-xxx name orginates.

New format is for most cases:
'#sql-name-current_pid-thread_id[-increment]'
Where name is one of subselect, alter, exchange, temptable or backup

The exceptions are:
ALTER PARTITION shadow files:
'#sql-shadow-thread_id-'original_table_name'

Names used with temp pool:
'#sql-name-current_pid-pool_number'