oracle检查scn headroom的正确方法

11.2.0.2及之后的scn每秒最大增长速率是32k，由隐含参数_max_reasonable_scn_rate控制。11.2.0.2之前每秒最大的scn增长速度是16k，所以要分开计算。计算出的“headroom”值的意思是按照每秒最大速度增长，多少天之后就会达到最大值。 现在网上流传的计算方法都是按照之前每秒最大增长16k来计算的，这样计算出来的结果是有误差的，虽然误差不是很大，但是oracle还是要讲究一下准确性的。用下面的sql检查结果是准确的。 SELECT VERSION,        TO_CHAR(CURRENT_SCN),        DATE_TIME,        TO_CHAR(ROUND(HEADROOM, 2))   FROM (SELECT VERSION,                DBMS_FLASHBACK.GET_SYSTEM_CHANGE_NUMBER CURRENT_SCN,                TO_CHAR(SYSDATE, 'YYYY-MM-DD HH24:MI:SS') DATE_TIME,                case                  when replace(version, '.', '') < 112020 then                   ((((((TO_NUMBER(TO_CHAR(SYSDATE, 'YYYY')) - 1988) * 12 * 31 * 24 * 60 * 60) +                   ((TO_NUMBER(TO_CHAR(SYSDATE, 'MM')) - 1) * 31 * 24 * 60 * 60) +                   (((TO_NUMBER(TO_CHAR(SYSDATE, 'DD')) - 1)) * 24 * 60 * 60) +                   (TO_NUMBER(TO_CHAR(SYSDATE, 'HH24')) * 60 * 60) +                   (TO_NUMBER(TO_CHAR(SYSDATE, 'MI')) * 60) +                   (TO_NUMBER(TO_CHAR(SYSDATE, 'SS')))) * (16 * 1024)) -                   DBMS_FLASHBACK.GET_SYSTEM_CHANGE_NUMBER) /                   (16 * 1024 * 60 * 60 * 24))                  else                   ((((((TO_NUMBER(TO_CHAR(SYSDATE, 'YYYY')) - 1988) * 12 * 31 * 24 * 60 * 60) +                   ((TO_NUMBER(TO_CHAR(SYSDATE, 'MM')) - 1) * 31 * 24 * 60 * 60) +                   (((TO_NUMBER(TO_CHAR(SYSDATE, 'DD')) - 1)) * 24 * 60 * 60) +                   (TO_NUMBER(TO_CHAR(SYSDATE, 'HH24')) * 60 * 60) +                   (TO_NUMBER(TO_CHAR(SYSDATE, 'MI')) * 60) +                   (TO_NUMBER(TO_CHAR(SYSDATE, 'SS')))) * (32 * 1024)) -                   DBMS_FLASHBACK.GET_SYSTEM_CHANGE_NUMBER) /                   (32 * 1024 * 60 * 60 * 24))                end HEADROOM           FROM V$INSTANCE);