physical reads：OS blocks read数。包括插入到SGA缓存区的物理读以及PGA中的直读这项统计并非i/o请求数。

select ss.snap_id as snap_id,       to_char(sn.end_interval_time, 'yyyy-mm-dd hh24:mi:ss') as "snap_date",       (sum(decode(stat_name, 'physical reads', value, 0)) -       lag(sum(decode(stat_name, 'physical reads', value, 0)), 1)        over(order by ss.snap_id)) /       ((to_date(to_char(sn.end_interval_time, 'yyyymmddhh24miss'),                 'yyyymmddhh24miss') -       to_date(to_char(sn.begin_interval_time, 'yyyymmddhh24miss'),                 'yyyymmddhh24miss')) * 24 * 60 * 60) "physical reads"  from SYS.DBA_HIST_sysstat ss, SYS.DBA_HIST_snapshot sn where ss.snap_id = sn.snap_id   and ss.stat_name in ('physical reads')   and ss.dbid = sn.dbid   and ss.instance_number = sn.instance_number   and sn.instance_number = (select instance_number from v$instance)   and sn.dbid = (select dbid from v$database)   group by ss.snap_id,          sn.begin_interval_time,          sn.end_interval_time