Please help me about joins

Question

Can anyone please tell me in real environment, generally people use 'joins' or 'for all entries' in select stmt ..... Eventhough 'for all entries' is used for performance in which case 'joins' will be used? Pleaseeee
give me the info..
Regards
Yuktha

Answer 1

Hi Yuktha,
Here is the information that could give you some idea in regard of FOR ALL ENTRIES VS JOINS:
3 major drawbacks of the "for all entries" clause:
1. duplicate rows are automatically removed
2. if the itab used in the clause is empty , all the rows in the source table will be selected .
3. performance degradation when using the clause on big tables.
First of let us see the use of for all entries clause as amean to join an ABAP code ::
Say for example you have the following abap code:
Select * from mara
For all entries in itab
Where matnr = itab-matnr.
If the actual source of the material list (represented here by itab) is actually another database table, like:
select matnr from mseg
into corresponding fields of table itab
where ?.
Then you could have used one sql statement that joins both tables.
Select t1.*
From mara t1, mseg t2
Where t1.matnr = t2.matnr
And T2?..
So what are the drawbacks of using the "for all entires" instead of a join ?
At run time , in order to fulfill the "for all entries " request, the abap engine will generate several sql statements (for detailed information ). Regardless of which method the engine uses (union all, "or" or "in" predicates) . If the itab is bigger then a few records, the abap engine will break the itab into parts, and rerun an sql statement several times in a loop. This rerun of the same sql statement , each time with different host values, is a source of resource waste because it may lead to re-reading of data pages.
In other words , when you use the "for all entries " clause instead of coding a join , you are depriving the database of important information needed to select the best access path for your application. Moreover, you are depriving your DBA of the same vital information. When the DBA monitors & tunes the system, he (or she) is less likely to recognize this kind of resource waste. The DBA will see a simple statement that uses an index , he is less likely to realize that this statement is executed in a loop unnecessarily.
In conclusion I suggest to "think twice" before using the "for all entries" clause and to evaluate the use of database views as a means to:
a. simplify sql
b. simplify abap code
c. get around open sql limitations.
Here in this issues we can approach JOINS as an alternate option.
Hope this onfirmation is useful to you.
Reward points if helpful,
Thanks & Regards,
Madhavi.M