Oracle 21C 新特性：设置参数时可以使用表达式

Oracle之前的版本在设置参数时，必须给定绝对值，从21C开始设置的时候可以使用表达式，如加减乘除，比如：

ALTER SYSTEM SET sga_target = 'sga_max_size*80/100';
ALTER SYSTEM SET job_queue_processes='processes*10/100' SCOPE=BOTH;
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表达式会一直保留，随着表达式中变量的变化而变化。

ALTER SYSTEM SET AQ_TM_PROCESSES = 'MIN(40, PROCESSES * .1)' SCOPE=BOTH;
SQL> SHOW PARAMETER processes

NAME                                 TYPE        VALUE
------------------------------------ ----------- ----------------------------
aq_tm_processes                      integer     40
db_writer_processes                  integer     1
gcs_server_processes                 integer     0
global_txn_processes                 integer     1
job_queue_processes                  integer     40
log_archive_max_processes            integer     4
processes                            integer     400

SQL> ALTER SYSTEM SET PROCESSES = 500 SCOPE=SPFILE;

System altered.
SQL> CONNECT / AS SYSDBA
Connected.
SQL> SHUTDOWN IMMEDIATE
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> STARTUP
ORACLE instance started.

Total System Global Area 1140848912 bytes
Fixed Size                  9566480 bytes
Variable Size             352321536 bytes
Database Buffers          771751936 bytes
Redo Buffers                7208960 bytes
Database mounted.
Database opened.
SQL> ALTER PLUGGABLE DATABASE pdb21 OPEN;

Pluggable database altered.

SQL> SHOW PARAMETER processes

NAME                                 TYPE        VALUE
------------------------------------ ----------- ----------------------------
aq_tm_processes                      integer     40
db_writer_processes                  integer     1
gcs_server_processes                 integer     0
global_txn_processes                 integer     1
job_queue_processes                  integer     50
log_archive_max_processes            integer     4
processes                            integer     500
SQL>
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从上面可以看出，重启后aq_tm_processes取了最小值40。

还可以使用操作系统的环境变量：

SQL> ALTER SYSTEM SET db_recovery_file_dest='$HOME' SCOPE=BOTH;

System altered.

SQL> SHOW PARAMETER db_recovery_file_dest

NAME                                 TYPE        VALUE
------------------------------------ ----------- ----------------------------
db_recovery_file_dest                string      $HOME
db_recovery_file_dest_size           big integer 15000M
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