What's "consistent gets - examination"?

Question

http://www.dba-oracle.com/m_consistent_gets.htm
says it is a special consistent gets which only need one latch instead of 2 latches(for normal consistent gets). Could you anyone put more information on this? Under what condition which this stats happens? such as access the root block of the index?

Answer 1

user646745,
Thanks for supplying your test case to demonstrate your point.
My original comment was:
Blocks from a suitably defined single table hash cluster make another case because (if there have been no hash collisions) the location of the correct row-pointer is implied by the hash-key value requested.
You also need a unique index on the hash key (which won't be used for the access, even though it exists) and have to query with equality on the hash key.
I modified your test case to demonstrate this - running on 9.2.0.8:
SQL> set echo on
SQL> @temp
SQL> drop table t_hashed;
Table dropped.
SQL> drop cluster hash_cluster;
Cluster dropped.
SQL>
SQL> create cluster hash_cluster
  2  ( hash_key number(10) )
  3  hashkeys 100
  4  size 60
  5  single table
  6  hash is HASH_KEY;
Cluster created.
SQL>
SQL> create table t_hashed
  2  cluster hash_cluster(a)
  3  as
  4  select cast( rownum as number(10) ) a from all_objects where rownum <= 100;
Table created.
SQL>
SQL> create unique index t_pk on t_hashed(a);
Index created.
SQL> alter table t_hashed add constraint t_pk primary key (a);
Table altered.
SQL>
SQL> execute dbms_stats.gather_schema_stats('test_user', cascade=>true);
PL/SQL procedure successfully completed.
SQL>
SQL> select name,value from v$mystat a , v$statname b where a.STATISTIC#=b.STATISTIC# and b.NAME like 'consistent%';
NAME                                                                  VALUE
consistent gets                                                        7393
consistent changes                                                       24
consistent gets - examination                                           611
3 rows selected.
SQL>
SQL> declare
  2            a int;
  3  begin
  4            for i in 1 .. 10000 loop
  5                 select a into a from t_hashed where a = 1 + mod(i,100);
  6            end loop;
  7  end;
  8  /
PL/SQL procedure successfully completed.
SQL>
SQL> select name,value from v$mystat a , v$statname b where a.STATISTIC#=b.STATISTIC# and b.NAME like 'consistent%';
NAME                                                                  VALUE
consistent gets                                                       17417
consistent changes                                                       24
consistent gets - examination                                         10620
3 rows selected.
SQL>
SQL>
SQL> ed temp.lst
SQL> spool offThe "examination" seems to be about doing a minimal amount of work while holding the latch - so reading every row from the block doesn't count.
Regards
Jonathan Lewis
http://jonathanlewis.wordpress.com
http://www.jlcomp.demon.co.uk
"The greatest enemy of knowledge is not ignorance,
it is the illusion of knowledge." Stephen Hawking.