执行表空间使用率SQL突然变慢问题分析

一、问题现象

   客户反馈，通过监控软件发现DB 在今天凌晨2-3点期间，DB 负载比较高，并且有一个显著异常 一个执行表空间使用率SQL执行了1个多小时！

二、问题分析

根据Oracle  ash定位问题 sql or event 
select to_char(SAMPLE_TIME,'yyyy-mm-dd hh24') as "date",
 event,sql_id,count(*) 
  from gv$active_session_history 
  where 
SAMPLE_TIME between to_date('2021-01-07 00','yyyy-mm-dd hh24')
 and to_date('2021-01-07 04','yyyy-mm-dd hh24')
  group by to_char(SAMPLE_TIME,'yyyy-mm-dd hh24') ,event,sql_id
   order by 4,1,2,3;
date          EVENT                                                            SQL_ID          COUNT(*)
------------- ---------------------------------------------------------------- ------------- ----------
2021-01-07 03 db file sequential read                                          1tu4z2y5hywu4         39
2021-01-07 03 enq: TX - contention                                             gjm43un5cy843         39
2021-01-07 03                                                                  3nkd3g3ju5ph1         63
2021-01-07 03 enq: TX - contention                                             12bxyygja63sp       3120

33 rows selected.
可以发现造成本次DB TIME 上涨的原因是行锁

进一步分析行锁的情况
select count(*),to_char(SAMPLE_TIME,'yyyy-mm-dd hh24') as "date",inst_id,SESSION_ID,SESSION_SERIAL#,
 user_id,SQL_ID,BLOCKING_SESSION,BLOCKING_SESSION_SERIAL#,BLOCKING_INST_ID
  from gv$active_session_history 
  where 
SAMPLE_TIME between to_date('2021-01-07 00','yyyy-mm-dd hh24')
 and to_date('2021-01-07 04','yyyy-mm-dd hh24') and event='enq: TX - contention'
  group by to_char(SAMPLE_TIME,'yyyy-mm-dd hh24'),inst_id,SESSION_ID,SESSION_SERIAL#,
 user_id,SQL_ID,BLOCKING_SESSION,BLOCKING_SESSION_SERIAL#,BLOCKING_INST_ID
  order by 1;
  
  COUNT(*) date             INST_ID SESSION_ID SESSION_SERIAL#    USER_ID SQL_ID        BLOCKING_SESSION BLOCKING_SESSION_SERIAL# BLOCKING_INST_ID
---------- ------------- ---------- ---------- --------------- ---------- ------------- ---------------- ------------------------ ----------------
        24 2021-01-07 03          1         75           19617        144 12bxyygja63sp              698                    42689                1
        24 2021-01-07 03          1        257           25783        144 12bxyygja63sp              698                    42689                1
        24 2021-01-07 03          1        823           33555        144 12bxyygja63sp              698                    42689                1
        24 2021-01-07 03          1        892           36567        144 12bxyygja63sp              698                    42689                1
        24 2021-01-07 03          1       1017           47891        144 12bxyygja63sp              698                    42689                
·····
        24 2021-01-07 03          1       2968            4809        144 12bxyygja63sp              698                    42689                1
        24 2021-01-07 03          1        512           39289        144 12bxyygja63sp              698                    42689                1
        24 2021-01-07 03          1        759           28393          0 gjm43un5cy843              698                    42689                1
        24 2021-01-07 03          1        830           52047        144 12bxyygja63sp              698                    42689                1
        24 2021-01-07 03          1       1263           48807        144 12bxyygja63sp              698                    42689                1
        24 2021-01-07 03          1       1390            4301        144 12bxyygja63sp              698                    42689                1
        24 2021-01-07 03          1       1579           58223        144 12bxyygja63sp              698                    42689                1
        24 2021-01-07 03          1       1708           22485        144 12bxyygja63sp              698                    42689                1
        24 2021-01-07 03          1       2090           46765        144 12bxyygja63sp              698                    42689                1

81 rows selected.

行锁阻塞源头都是同一个session 定位该阻塞源头
select count(*),to_char(SAMPLE_TIME,'yyyy-mm-dd hh24') as "date",inst_id,SESSION_ID,SESSION_SERIAL#,
 user_id,SQL_ID,BLOCKING_SESSION,BLOCKING_SESSION_SERIAL#,BLOCKING_INST_ID
  from gv$active_session_history 
  where 
SAMPLE_TIME between to_date('2021-01-07 00','yyyy-mm-dd hh24')
 and to_date('2021-01-07 04','yyyy-mm-dd hh24') and inst_id=1 and SESSION_ID=698 and SESSION_SERIAL#=42689
 group by to_char(SAMPLE_TIME,'yyyy-mm-dd hh24'),inst_id,SESSION_ID,SESSION_SERIAL#,
 user_id,SQL_ID,BLOCKING_SESSION,BLOCKING_SESSION_SERIAL#,BLOCKING_INST_ID
  order by 1;
  COUNT(*) date             INST_ID SESSION_ID SESSION_SERIAL#    USER_ID SQL_ID        BLOCKING_SESSION BLOCKING_SESSION_SERIAL# BLOCKING_INST_ID
---------- ------------- ---------- ---------- --------------- ---------- ------------- ---------------- ------------------------ ----------------
        14 2021-01-07 03          1        698           42689          0 1t68u12gc1mzh

      检查相关SQL信息！  
select sql_id,sql_fulltext from v$sql where sql_id in('12bxyygja63sp','gjm43un5cy843','1t68u12gc1mzh');
SQL_ID        SQL_FULLTEXT
------------- --------------------------------------------------------------------------------
gjm43un5cy843 SELECT SUM(USED), SUM(TOTAL) FROM (SELECT /*+ ORDERED */ SUM(D.BYTES)/(1024*1024
12bxyygja63sp select a.tablespace_name,round((1-a.use_byte/b.max_byte)*100,2) freepercent,

1t68u12gc1mzh  BEGIN
                 SYS.KUPW$WORKER.MAIN('SYS_IMPORT_TABLE_01', 'SYS', 0);
               END;

三、问题总结

  经过与客户沟通确认SQL文本，可以得到如下信息：

阻塞源头, sys用户执行impdp导入某个表！

被阻塞的会话，执行dba_segments,dba_data_files视图！现象是行锁！

基本上可以理解为，数据泵在执行某个对象表的导入过程中， SQL查询dba_segments 涉及了这个表段的统计，由于导入过程中段大小是存在变换,因此存在行锁现象！

正常情况下，select 与insert并不冲突，因此selct dba_segments时，有内置操作，这个我们不太清楚，mos并未搜到这种情况，无法认为Bug.

   建议查询与导入错开，或者手工kill select 查询语句！