os: centos 7.4
db: oracle 11.2.0.4

默认值为 all_rows

optimizer_mode 有几种枚举值呢？

first_rows_n

The optimizer uses a cost-based approach and optimizes with a goal of best response time to return the first n rows (where n = 1, 10, 100, 1000).

first_rows

The optimizer uses a mix of costs and heuristics to find a best plan for fast delivery of the first few rows.

all_rows

The optimizer uses a cost-based approach for all SQL statements in the session and optimizes with a goal of best throughput (minimum resource use to complete the entire statement).

在 session 级别修改试试看

以上都设置成功，如果那个 n 为别的数字了？可以看到报错了

另外有个 first_rows(n) 的hint，这个 n 是可以为其余数字的,如下：

参考：
https://docs.oracle.com/cd/E11882_01/server.112/e40402/initparams171.htm#REFRN10145
https://docs.oracle.com/cd/E11882_01/server.112/e41573/optimops.htm#i38217