先看看Tom上的一个例子：

ops$tkyte@ORA817DEV.US.ORACLE.COM> begin
  2     execute immediate 'alter session set current_schema=scott';
  3     for x in ( select sys_context('userenv','current_schema') x from dual )
  4     loop
  5          dbms_output.put_line( x.x );
  6     end loop;
  7 
  8 
  9     execute immediate 'alter session set current_schema=ops$tkyte';
10     for x in ( select sys_context('userenv','current_schema') x from dual )
11     loop
12          dbms_output.put_line( x.x );
13     end loop;
14  end;
15  /
SCOTT
OPS$TKYTE

PL/SQL procedure successfully completed.

Now, if your "block of code" is actually a stored procedure and it is further a DEFINERS RIGHTS procedure (the default kind) then expect this (logged in as ops$tkyte by the way):

ops$tkyte@ORA817DEV.US.ORACLE.COM> create or replace procedure p
  2  as
  3  begin
  4     execute immediate 'alter session set current_schema=scott';
  5     for x in ( select sys_context('userenv','current_schema') x from dual )
  6     loop
  7          dbms_output.put_line( x.x );
  8     end loop;
  9 
10 
11     execute immediate 'alter session set current_schema=ops$tkyte';
12     for x in ( select sys_context('userenv','current_schema') x from dual )
13     loop
14          dbms_output.put_line( x.x );
15     end loop;
16  end;
17  /

Procedure created.

ops$tkyte@ORA817DEV.US.ORACLE.COM> exec p
OPS$TKYTE
OPS$TKYTE

PL/SQL procedure successfully completed.

you cannot change your identity in a definers rights procedure, it is fixed AT COMPILE time by definition (if you have my book "expert one on one", read the chapter on invoker vs definer rights). Now, try this:

ops$tkyte@ORA817DEV.US.ORACLE.COM> create or replace procedure p 2 AUTHID CURRENT_USER
  3  as
  4  begin
  5     execute immediate 'alter session set current_schema=scott';
  6     for x in ( select sys_context('userenv','current_schema') x from dual )
  7     loop
  8          dbms_output.put_line( x.x );
  9     end loop;
10 
11 
12     execute immediate 'alter session set current_schema=ops$tkyte';
13     for x in ( select sys_context('userenv','current_schema') x from dual )
14     loop
15          dbms_output.put_line( x.x );
16     end loop;
17  end;
18  /

Procedure created.

ops$tkyte@ORA817DEV.US.ORACLE.COM> exec p
SCOTT
OPS$TKYTE

PL/SQL procedure successfully completed.

可见对于DEFINERS RIGHTS procedure ，alter session set current_schema是无影响的；