Apress.Expert.Oracle.Database.Architecture.9i.and.10g.Programming.Techniques.and.Solutions.Sep.2005

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CHAPTER 4 ■ MEMORY STRUCTURES
NAME
VALUE
------------------------------ ------------
session uga memory 1,212,872
session uga memory max 7,418,680
session pga memory 1,612,364
session pga memory max 7,838,284
The same 7.5MB of RAM we observed earlier. Now, we will proceed to fill up that CHAR
array we have in the package (a CHAR datatype is blank-padded so each of these array elements
is exactly 2,000 characters in length):
ops$tkyte@ORA10G> begin
2 for i in 1 .. 100000
3 loop
4 demo_pkg.g_data(i) := 'x';
5 end loop;
6 end;
7 /
PL/SQL procedure successfully completed.
Upon measuring our session’s current PGA utilization after that, we find something similar
to the following:
ops$tkyte@ORA10GR1> select a.name, to_char(b.value, '999,999,999') value
2 from v$statname a, v$mystat b
3 where a.statistic# = b.statistic#
4 and a.name like '%ga memory%';
NAME
VALUE
------------------------------ ------------
session uga memory 312,952,440
session uga memory max 312,952,440
session pga memory 313,694,796
session pga memory max 313,694,796
Now, that is memory allocated in the PGA that the database itself cannot control. We
already exceeded the PGA_AGGREGATE_TARGET and there is quite simply nothing the database
can do about it—it would have to fail our request if it did anything, and it will do that only
when the OS reports back that there is no more memory to give. If we wanted, we could allocate
more space in that array and place more data in it, and the database would just have to
do it for us.
However, the database is aware of what we have done. It does not ignore the memory it
cannot control; rather, it recognizes that the memory is being used and backs off the size of
memory allocated for work areas accordingly. So if we rerun the same sort query, we see that
this time we sorted to disk—the database did not give us the 7MB or so of RAM needed to do
this in memory since we had already exceeded the PGA_AGGREGATE_TARGET:
ops$tkyte@ORA10GR1> set autotrace traceonly statistics;
ops$tkyte@ORA10GR1> select * from big_table order by 1,2,3,4;
50000 rows selected.