调高session_cached_cursors大小，性能下降

目前环境中主要问题就是调整session_cached_cursors>2，io就变高，性能下降明显，且awr报告中Execute to Parse %：一直0 ；

同等环境下，session_cached_cursors=1时，tops5等待时间为

Event Waits Total Wait Time (sec) Wait Avg(ms) % DB time Wait Class

library cache: mutex X 27,612,313 208.3K 8 42.7 Concurrency

DB CPU 162.6K 33.4

latch: cache buffers chains 992,268 24.2K 24 5.0 Concurrency

cursor: pin S 2,352,880 19.1K 8 3.9 Concurrency

cursor: mutex S 1,244,010 10.8K 9 2.2 Concurrency

session_cached_cursors=2500时，tops5等待事件为

DB CPU 147.3K 50.1

db file sequential read 3,838,745 38.1K 10 13.0 User I/O

db file scattered read 2,436,293 32.8K 13 11.2 User I/O

latch: cache buffers chains 259,040 23.5K 91 8.0 Concurrency

SQL*Net more data from client 18,272,934 1153.3 0 .4 Network