开启Oracle跟踪的语句set autotrace on要在sqlplus环境中运行，在PL/SQL Developer中执行始终弹出cannot set autotrace。加了索引的SQL，查询速度会很快！

Oracle统计信息，还不明白是什么意思

9 recursive calls

0 db block gets

61 consistent gets

4 physical reads

0 redo size

420 bytes sent via SQL*NET to client

400 bytes received via SQL*NET from client

2 SQL*NET roundtrips to/from client

0 sorts(memory)

0 sorts(disk)

1 rows processed