If the table on which the For All Entries IN clause is 
based is empty, all rows are selected into the destination 
table. Hence it is advisable to check before-hand that the 
first table is not empty.

all data will upload to main internal table with out any 
checking

when you are using for all enteries always check whether 
IF ITAB NOT INITIAL.THis internal table is the one 
used 'for all enteries in itab'.this will prevent from the 
query fetching all data.

IF ITAB NOT INITIAL.
select ... for all enteries in itab.
endif.

If the itab used in for all entries does not have records 
the program will get trapped in an infinite loop.

Hi All,
  If we use FOR ALL ENTRIES, first internal table should
have at least one record ( ie, it should not initial ).The
data will fetch from database based on second select
statement,irrespective of first internal table records ,
because first internal table is empty.

   It is best practice to use IF NOT I_TAB1[] IS INITIAL.
Because we use FOR ALL ENTRIES statement to avoid the
duplicates entries.

If they is no data in itab , the for all entries itab1 will 
not display any data..
because 1st itab should be not initial .
check for initial before going for for all entries.

Please Ignore my earlier Answer ....I regret for that 
answer...Please check  this one..

If no data is there in ITAB which you are applying for FOR 
ALL ENTRIES then select query will fetch all the values 
from the DB table which can be real performance issue.

so before making for allentires for a table make a check 
that its not empty.

it will retrive all entries of the table on which you are 
performing the for all entries into internal table.