Oracle 基于性能的SQL查询

SQL> create table t as
  2  select d.owner, d.object_name, d.object_type from dba_objects d, ( select 1 from dual connect by level <= 200 ) ;

Table created.

SQL>
SQL> select count(*) from t;

  COUNT(*)
----------
  15753000

1 row selected.

SQL>
SQL> set timing on
SQL> begin
  2    for i in ( select owner, object_name, object_type from t )
  3    loop
  4       null;
  5    end loop;
  6  end;
  7  /

PL/SQL procedure successfully completed.

Elapsed: 00:00:06.71
SQL>
SQL> begin
  2    for i in ( select object_type, object_name, owner from t )
  3    loop
  4       null;
  5    end loop;
  6  end;
  7  /

PL/SQL procedure successfully completed.

Elapsed: 00:00:06.70
SQL>
SQL> begin
  2    for i in ( select * from t )
  3    loop
  4       null;
  5    end loop;
  6  end;
  7  /

PL/SQL procedure successfully completed.

Elapsed: 00:00:06.73