rsdb/prefer_union_all is a SAP Parameter attribute which is used to control set if UNION ALL shall be used for FOR ALL ENTRIES information. This is available within R/3 SAP systems depending on the version and release level.
Below is the standard documentation available and a few details of the attributes values .
This parameter is only intended for internal use and must not be changed
without explicit instruction from SAP.
An internal table [itab] with entry data is always returned to an ABAP
SELECT query with the addition FOR ALL ENTRIES. Depending on whether
this parameter is set to "0" or "1", a link to whole statements with
UNION or an OR connection of condition is generated in the WHERE clause.
Each of the linked part-statements/part-conditions represent an entrey
of the input table [itab].
Example
The open SQL statement
SELECT ... FOR ALL ENTRIES IN itab WHERE f = itab-f.
is displayed depending on the parameter rsdb/prefer_union_all as follows
on standard-compliant SQL:
rsdb/prefer_union_all = 0 (and rsdb/prefer_in_itab = 0)
SELECT ... WHERE f = itab[1]-f
OR f = itab[2]-f
...
OR f = itab[N]-f
rsdb/prefer_union_all = 1
SELECT ... WHERE f = itab[1]-f
UNION ALL SELECT ... WHERE f = itab[2]-f
...
UNION ALL SELECT ... WHERE f = itab[N]-f
Here N refers to the number of lines in itab and itab[i]-f the value of
the field f in the i table row.
See SAP Parameter Documentation for full SAP documentation for this profile parameter.