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

CHAPTER 4 ■ MEMORY STRUCTURES 117
sort is done. The SORT_AREA_SIZE-SORT_AREA_RETAINED_SIZE is generally allocated out of your
PGA, and the SORT_AREA_RETAINED_SIZE will be in your UGA. You can discover your current
usage of PGA and UGA memory and monitor its size by querying special Oracle V$ views, also
referred to as dynamic performance views.
For example, let’s run a small test whereby in one session we’ll sort lots of data and, from a
second session, we’ll monitor the UGA/PGA memory usage in that first session. To do this in a
predicable manner, we’ll make a copy of the ALL_OBJECTS table, with about 45,000 rows in this
case, without any indexes (so we know a sort has to happen):
ops$tkyte@ORA10G> create table t as select * from all_objects;
Table created.
ops$tkyte@ORA10G> exec dbms_stats.gather_table_stats( user, 'T' );
PL/SQL procedure successfully completed.
To remove any side effects from the initial hard parsing of queries, we’ll run the following
script, but for now ignore its output. We’ll run the script again in a fresh session so as to see
the effects on memory usage in a controlled environment. We’ll use the sort area sizes of 64KB,
1MB, and 1GB in turn:
create table t as select * from all_objects;
exec dbms_stats.gather_table_stats( user, 'T' );
alter session set workarea_size_policy=manual;
alter session set sort_area_size = 65536;
set termout off
select * from t order by 1, 2, 3, 4;
set termout on
alter session set sort_area_size=1048576;
set termout off
select * from t order by 1, 2, 3, 4;
set termout on
alter session set sort_area_size=1073741820;
set termout off
select * from t order by 1, 2, 3, 4;
set termout on
■Note When we process SQL in the database, we must first “parse” the SQL statement. There are two
types of parses available. The first is a hard parse, which is what happens the first time a query is parsed
by the database instance and includes query plan generation and optimization. The second is a soft parse,
which can skip many of the steps a hard parse must do. We hard parse the previous queries so as to not
measure the work performed by that operation in the following section.
Now, I would suggest logging out of that SQL*Plus session and logging back in before continuing,
in order to get a consistent environment, or one in which no work has been done yet.