Usage of MAX Function.

Question

Hi,
One of our customers is facing an error while executing a PL/SQL block like,
declare
l_flag varchar2(30);
begin
select max('Y') into l_flag from v$database; -- Can use any table here
end;
however, when the select query is changed to as follows, no error is seen.
select substr(max('Y'),1,30) into l_flag from v$database;
Can someone please help me in identifying what could be issue?
Thanks,
Dola

Answer 1

We are actually trying to check if the table contains any rows.Hmmm, I would have thought that using a MAX() function to do that is rather expensive (if it's a large table)...
SQL> set autotrace traceonly stat
SQL> select max('Y') from tt1
  2  /
Statistics
          0  recursive calls
          0  db block gets
        432  consistent gets
        428  physical reads
          0  redo size
        378  bytes sent via SQL*Net to client
        499  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed
SQL>          
SQL> select count(*) from tt1
  2  /
Statistics
          0  recursive calls
          0  db block gets
        432  consistent gets
        423  physical reads
          0  redo size
        381  bytes sent via SQL*Net to client
        499  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed
SQL>          
SQL> select null  from tt1
  2  where rownum = 1
  3  /
Statistics
          0  recursive calls
          0  db block gets
          4  consistent gets
          5  physical reads
          0  redo size
        372  bytes sent via SQL*Net to client
        499  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed
SQL> Cheers, APC