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Troubleshooting large amount of undo/redo generated due to dblink usage

前不久一客户的数据库出现了较高的enq: US – contention和latch: undo global data等待,从等待事件可以状态出现了较高的undo争用, 当然后来通过SQL的执行量TOP1 发现了SQL中使用了dblink,在墨天轮上记录了这个问题,查看请移步这里, 在几年前我blog 记录过一篇dblink会分配undo段的问题,请看这篇《Lots of Long transaction caused by database link, and undo hdr show DBA for that slot is 0x00000000》, 这次的案例sql的执行次数和v$undostat.TXNCOUNT 匹配98%以上,所以可以定性导致,这里简单的记录如果它的执行量不是TOP 有没有可能通过session statistic去查找一下此类问题。

— 测试环境 19.13, dblink指向自已。

[oracle@oel7db1 ~]$ sqlplus anbob/anbob@cdb1pdb1

SQL*Plus: Release 19.0.0.0.0 - Production on Tue Mar 1 15:49:38 2022
Version 19.3.0.0.0
Copyright (c) 1982, 2019, Oracle.  All rights reserved.

Connected to:
Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production
Version 19.3.0.0.0


USERNAME             INST_NAME            HOST_NAME                  I# SID   SERIAL#  VERSION    STARTED  SPID       OPID  CPID            SADDR            PADDR
-------------------- -------------------- ------------------------- --- ----- -------- ---------- -------- ---------- ----- --------------- ---------------- ----------------
ANBOB                PDB1-anbob19c        oel7db1                     1 72    21418    19.0.0.0.0 20220301 24185      42    24183           0000000078532180 0000000079103FA8


SQL> select sysdate from dual@dl;
SYSDATE
-------------------
2022-03-01 15:49:49

SQL> select addr taddr,UBAFIL,UBABLK,UBASQN,status,START_TIME,XIDUSN,XIDSLOT,XIDSQN,xid from v$transaction where ses_addr='0000000078532180';
TADDR                UBAFIL     UBABLK     UBASQN STATUS           START_TIME               XIDUSN    XIDSLOT     XIDSQN XID
---------------- ---------- ---------- ---------- ---------------- -------------------- ---------- ---------- ---------- ----------------
00000000754D8878          0          0          0 ACTIVE           03/01/22 15:49:48             4         27      17776 04001B0070450000

SQL> select sysdate from dual@dl;
SYSDATE
-------------------
2022-03-01 15:50:51

SQL> select addr taddr,UBAFIL,UBABLK,UBASQN,status,START_TIME,XIDUSN,XIDSLOT,XIDSQN,xid from v$transaction where ses_addr='0000000078532180';
TADDR                UBAFIL     UBABLK     UBASQN STATUS           START_TIME               XIDUSN    XIDSLOT     XIDSQN XID
---------------- ---------- ---------- ---------- ---------------- -------------------- ---------- ---------- ---------- ----------------
00000000754D8878          0          0          0 ACTIVE           03/01/22 15:49:48             4         27      17776 04001B0070450000

SQL> commit;
Commit complete.

SQL> select addr taddr,UBAFIL,UBABLK,UBASQN,status,START_TIME,XIDUSN,XIDSLOT,XIDSQN,xid from v$transaction where ses_addr='0000000078532180';
no rows selected

SQL> select sysdate from dual@dl;
SYSDATE
-------------------
2022-03-01 15:51:10

SQL> select addr taddr,UBAFIL,UBABLK,UBASQN,status,START_TIME,XIDUSN,XIDSLOT,XIDSQN,xid from v$transaction where ses_addr='0000000078532180';

TADDR                UBAFIL     UBABLK     UBASQN STATUS           START_TIME               XIDUSN    XIDSLOT     XIDSQN XID
---------------- ---------- ---------- ---------- ---------------- -------------------- ---------- ---------- ---------- ----------------
00000000754D8878          0          0          0 ACTIVE           03/01/22 15:51:10             1         12      18518 01000C0056480000

SQL>

Note:
同一个会话,如果没有发起commit/rollback 等隐式 结束事务,多次查询dblink时会复用之前的undo段, 如果commit结束事务下次再查询时会生成新undo段, 下面测试如果把量级加大看看统计信息的指标

循环使用dblink不提交,复用undo

SQL> create table s1 as select * from v$sesstat where sid=57;
Table created.

declare
l_now date;
begin
 loop
  select sysdate into l_now from dual@dl;
 end loop;
end;


SQL> create table s2 as select * from v$sesstat where sid=57;
Table created.

SQL> select s1.statistic#,sn.name,s1.value,s2.value, s2.value-s1.value diff, (s2.value-s1.value)/decode(s1.value,0,-1,s1.value) pct 
from s1,s2,v$statname sn 
where s1.statistic#=sn.statistic# and s1.sid=s2.sid and s1.statistic#=s2.statistic# and s2.value>s1.value order by 5 desc;

STATISTIC# NAME                                                                  VALUE      VALUE       DIFF        PCT
---------- ---------------------------------------------------------------- ---------- ---------- ---------- ----------
      1882 bytes sent via SQL*Net to dblink                                       3779 1095048570 1095044791 289771.048
      1883 bytes received via SQL*Net from dblink                                 4424  844073989  844069565 190793.301
        46 enqueue conversions                                                      14    8963541    8963527 640251.929
        29 non-idle wait count                                                      93    5975839    5975746 64255.3333
      1884 SQL*Net roundtrips to/from dblink                                        10    2987872    2987862   298786.2
         9 recursive calls                                                         478    1494557    1494079 3125.68828
       306 calls to get snapshot scn: kcmgss                                       100    1494066    1493966   14939.66
         4 opened cursors cumulative                                               103    1494058    1493955 14504.4175
      1878 execute count                                                            99    1494052    1493953 15090.4343
      1755 session cursor cache hits                                                61    1494001    1493940 24490.8197
       171 logical read bytes from cache                                       2801664    3948544    1146880 .409356725
       177 physical read bytes                                                  180224     311296     131072 .727272727
        67 cell physical IO interconnect bytes                                  180224     311296     131072 .727272727
        57 physical read total bytes                                            180224     311296     131072 .727272727
        33 session uga memory                                                   956560    1022048      65488 .068461989
        28 non-idle wait time                                                        4      43309      43305   10826.25
        19 CPU used by this session                                                  6      42314      42308 7051.33333
        10 recursive cpu usage                                                       2      40616      40614      20307
       405 file io wait time                                                       653      19197      18544 28.3981623
       313 redo size                                                               964       8760       7796 8.08713693
       610 undo change vector size                                                 272       2400       2128 7.82352941
      1880 bytes received via SQL*Net from client                                 6398       6838        440 .068771491
        32 process last non-idle time                                       1646118238 1646118513        275 1.6706E-07
        14 session logical reads                                                   342        482        140 .409356725
       164 consistent gets from cache                                              337        422         85 .252225519
       163 consistent gets                                                         337        422         85 .252225519
       160 db block gets from cache                                                  5         60         55         11
       159 db block gets                                                             5         60         55         11
      1799 buffer is not pinned count                                              275        322         47 .170909091
       178 db block changes                                                          6         52         46 7.66666667
       165 consistent gets pin                                                     235        281         46 .195744681
       161 db block gets from cache (fastpath)                                       4         44         40         10
       653 no work - consistent read gets                                          219        258         39 .178082192
       168 consistent gets examination (fastpath)                                   97        136         39 .402061856
       167 consistent gets examination                                             102        141         39 .382352941
       166 consistent gets pin (fastpath)                                          218        252         34 .155963303
      1096 index range scans                                                        74        106         32 .432432432
        45 enqueue requests                                                         15         43         28 1.86666667
        47 enqueue releases                                                         13         40         27 2.07692308
       312 redo entries                                                              3         27         24          8
      1873 parse count (total)                                                      52         75         23 .442307692
      1889 sorts (memory)                                                           17         40         23 1.35294118
      1874 parse count (hard)                                                        3         24         21          7
      1872 parse time elapsed                                                       12         30         18        1.5
      1010 table scan rows gotten                                                 2006       2024         18 .008973081
      1013 table scan disk non-IMC rows gotten                                    2006       2024         18 .008973081
       244 commit cleanouts successfully completed                                   1         18         17         17
       243 commit cleanouts                                                          1         18         17         17
       230 free buffer requested                                                    23         39         16 .695652174
       176 physical read IO requests                                                22         38         16 .727272727
        52 physical read total IO requests                                          22         38         16 .727272727
       172 physical reads                                                           22         38         16 .727272727
       173 physical reads cache                                                     22         38         16 .727272727
       665 deferred (CURRENT) block cleanout applications                            1         14         13         13
        27 scheduler wait time                                                       0         11         11        -11
      1798 buffer is pinned count                                                 1019       1028          9 .008832188
       302 calls to kcmgcs                                                          18         25          7 .388888889
      1728 CCursor + sql area evicted                                                0          6          6         -6
      1004 table scans (short tables)                                                6         12          6          1
      1015 table scan blocks gotten                                                 36         42          6 .166666667
      1726 sql area purged                                                           0          6          6         -6
      1875 parse count (failures)                                                    0          6          6         -6
      1727 sql area evicted                                                          0          6          6         -6
      1871 parse time cpu                                                            5         10          5          1
       674 blocks cleaned out using minact                                           0          5          5         -5
      1113 HSC Heap Segment Block Changes                                            1          5          4          4
       304 calls to kcmgas                                                           1          5          4          4
       404 file io service time                                                      0          3          3         -3
       275 shared hash latch upgrades - no wait                                      9         12          3 .333333333
       663 immediate (CURRENT) block cleanout applications                           0          2          2         -2
        11 pinned cursors current                                                    0          2          2         -2
         8 user calls                                                               26         28          2 .076923077
      1783 cursor authentications                                                    0          2          2         -2
         1 Requests to/from client                                                  18         19          1 .055555556
         5 opened cursors current                                                    2          3          1         .5
        26 user I/O wait time                                                        0          1          1         -1
        35 messages sent                                                             1          2          1          1
      1016 table fetch by rowid                                                    590        591          1 .001694915
      1095 index fetch by key                                                       16         17          1      .0625
      1802 workarea executions - optimal                                            13         14          1 .076923077
      1891 sorts (rows)                                                            160        161          1     .00625

81 rows selected.

SQL> @uds
Show undo statistics from V$UNDOSTAT....

                                                                  MAX MAX            ORA-    SPC    UNEXP      EXP
BEGIN_TIME          END_TIME   TXNCOUNT         MB       MB/s  QRYLEN QRY_ID         1555    ERR    STEAL    STEAL
------------------- -------- ---------- ---------- ---------- ------- ------------- ----- ------ -------- --------
2022-03-01 15:12:05 15:16:10          2        .00        .00       0                   0      0        0        0
2022-03-01 15:02:05 15:12:05        116        .05        .00       0                   0      0        0        0
2022-03-01 14:52:05 15:02:05        101        .06        .00       0                   0      0        0        0
2022-03-01 14:42:05 14:52:05          2        .00        .00       0                   0      0        0        0
2022-03-01 14:32:05 14:42:05         15        .13        .00       0                   0      0        0        0
2022-03-01 14:22:05 14:32:05          0        .00        .00       0                   0      0        0        0

Note:

循环调用dblink近150万次,产生了极小的undo和redo。 Top 指标为*via SQL*Net from/to dblink

循环使用dblink提交,undo频繁创建

SQL> alter database datafile '/u01/app/oracle/oradata/ANBOB19C/pdb1/undotbs01.dbf' AUTOEXTEND off;
Database altered.

create table s4 as select * from v$sesstat where sid=65;

declare
l_now date;
begin
 loop
  select sysdate into l_now from dual@dl;
  -- close tx
    commit;
 end loop;
end;


create table s5 as select * from v$sesstat where sid=65;

select s4.statistic#,sn.name,s4.value,s5.value, s5.value-s4.value diff, (s5.value-s4.value)/decode(s4.value,0,-1,s4.value) pct
from s4,s5,v$statname sn
where s4.statistic#=sn.statistic# and s4.sid=s5.sid and s4.statistic#=s5.statistic# and s5.value>s4.value
 order by 5 desc;

STATISTIC# NAME                                                                  VALUE      VALUE       DIFF        PCT
---------- ---------------------------------------------------------------- ---------- ---------- ---------- ----------
       897 KTFB alloc space (block)                                         1.9574E+10 3.8997E+10 1.9423E+10 .992292652
       171 logical read bytes from cache                                    1.5734E+10 2.7931E+10 1.2197E+10 .775161102
      1882 bytes sent via SQL*Net to dblink                                  254715226  366200315  111485089 .437685217
      1883 bytes received via SQL*Net from dblink                            136773596  196636514   59862918 .437678907
       313 redo size                                                          81686552  124219108   42532556 .520680026
       610 undo change vector size                                            19993072   31701152   11708080 .585606854
       900 KTFB free space (block)                                             2324280    4695280    2371000 1.02010085
        14 session logical reads                                               1920673    3409504    1488831 .775161102
       160 db block gets from cache                                            1599499    2849606    1250107 .781561601
       159 db block gets                                                       1599499    2849606    1250107 .781561601
       161 db block gets from cache (fastpath)                                 1372932    2518083    1145151 .834091565
        45 enqueue requests                                                    1451144    2490390    1039246 .716156357
        47 enqueue releases                                                    1451141    2490387    1039246 .716157837
      1013 table scan disk non-IMC rows gotten                                  594567    1463631     869064 1.46167547
      1010 table scan rows gotten                                               594567    1463631     869064 1.46167547
        46 enqueue conversions                                                 1639593    2506793     867200 .528911748
        29 non-idle wait count                                                 2444144    3225246     781102 .319581007
         9 recursive calls                                                      930465    1630012     699547 .751825163
      1798 buffer is pinned count                                               699381    1367117     667736 .954752846
       903 KTFB apply time (ms)                                                 676874    1313069     636195 .939901666
       178 db block changes                                                     773223    1185137     411914 .532723419
      1884 SQL*Net roundtrips to/from dblink                                    869768    1250454     380686 .437686831
       898 KTFB alloc time (ms)                                               31125201   31458593     333392  .01071132
       302 calls to kcmgcs                                                      519496     821805     302309 .581927484
       312 redo entries                                                         537190     834531     297341 .553511793
       306 calls to get snapshot scn: kcmgss                                    472295     713628     241333 .510979367
       164 consistent gets from cache                                           321174     559898     238724 .743285571
       163 consistent gets                                                      321174     559898     238724 .743285571
         4 opened cursors cumulative                                            463213     694941     231728 .500262298
      1755 session cursor cache hits                                            463170     694895     231725 .500302265
       166 consistent gets pin (fastpath)                                       103730     247282     143552 1.38390051
       165 consistent gets pin                                                  103731     247283     143552 1.38388717
      1878 execute count                                                        245776     382330     136554 .555603476
       304 calls to kcmgas                                                      226552     331413     104861  .46285621
       234 free buffer inspected                                                217872     313289      95417 .437949805
       230 free buffer requested                                                217604     312953      95349 .438176688
       167 consistent gets examination                                          217443     312615      95172 .437687118
       168 consistent gets examination (fastpath)                               217443     312615      95172 .437687118
       687 commit batch/immediate performed                                     217439     312610      95171 .437690571
       685 commit batch requested                                               217439     312610      95171 .437690571
       662 transaction rollbacks                                                217439     312610      95171 .437690571
       684 commit batch/immediate requested                                     217439     312610      95171 .437690571
       688 commit batch performed                                               217439     312610      95171 .437690571
       231 dirty buffers inspected                                              214755     309301      94546 .440250518
        57 physical read total bytes                                        5938921472 5939003392      81920 .000013794
       177 physical read bytes                                                   16384      98304      81920          5
        67 cell physical IO interconnect bytes                              5.1178E+10 5.1178E+10      81920 1.6007E-06
       653 no work - consistent read gets                                        28506      69890      41384 1.45176454
      1004 table scans (short tables)                                            28323      69707      41384 1.46114465
      1015 table scan blocks gotten                                              28440      69824      41384 1.45513361
      1873 parse count (total)                                                   28341      69724      41383 1.46018136
       677 drop segment calls in space pressure                                  13935      45773      31838 2.28475063
       682 space was found by tune down                                          11512      38569      27057 2.35033009
       681 steps of tune down ret. in space pressure                             11512      38569      27057 2.35033009
       680 tune down retentions in space pressure                                11512      38569      27057 2.35033009
       902 KTFB apply req                                                        18865      38458      19593 1.03858998
       901 KTFB free time (ms)                                                   15431      27838      12407 .804030847
       899 KTFB free req                                                          9334      19131       9797  1.0496036
       896 KTFB alloc req                                                         9531      19327       9796 1.02780401
        19 CPU used by this session                                              19324      28129       8805 .455651004
        10 recursive cpu usage                                                   18734      27242       8508 .454147539
        28 non-idle wait time                                                    43042      50203       7161  .16637238
        35 messages sent                                                          2819       4439       1620 .574671869
       228 change write time                                                       884       1366        482 .545248869
       405 file io wait time                                                        98        375        277 2.82653061
        43 enqueue waits                                                           158        265        107  .67721519
      1872 parse time elapsed                                                       78        173         95 1.21794872
       261 write clones created in foreground                                        8         92         84       10.5
      1871 parse time cpu                                                           67        131         64 .955223881
       698 local undo segment hints helped                                          81        140         59 .728395062
       232 pinned buffers inspected                                                  0         24         24        -24
       172 physical reads                                                            2         12         10          5
       173 physical reads cache                                                      2         12         10          5
       176 physical read IO requests                                                 2         12         10          5
        52 physical read total IO requests                                      492200     492210         10 .000020317
       676 auto extends on undo tablespace                                       10699      10706          7 .000654267
       233 hot buffers moved to head of LRU                                        410        416          6 .014634146
       275 shared hash latch upgrades - no wait                                      2          6          4          2

78 rows selected.

SQL> l
  1  select * from (
  2      select
  3          begin_time,
  4          to_char(end_time, 'HH24:MI:SS') end_time,
  5          txncount,
  6          undoblks * (select block_size from dba_tablespaces where upper(tablespace_name) =
  7                          (select upper(value) from v$parameter where name = 'undo_tablespace')
  8                     ) / 1048576 uds_MB ,
  9          undoblks * (select block_size from dba_tablespaces where upper(tablespace_name) =
 10                          (select upper(value) from v$parameter where name = 'undo_tablespace')
 11                     ) / ((end_time-begin_time) * 86400) / 1048576 uds_MB_sec ,
 12          maxquerylen uds_maxquerylen,
 13          maxqueryid  uds_maxqueryid,
 14          ssolderrcnt uds_ssolderrcnt,
 15          nospaceerrcnt uds_nospaceerrcnt,
 16     unxpstealcnt uds_unxpstealcnt,
 17     expstealcnt uds_expstealcnt
 18      from
 19          v$undostat
 20      order by
 21          begin_time desc
 22* ) where rownum <= 30


                                                                  MAX MAX            ORA-    SPC    UNEXP      EXP
BEGIN_TIME          END_TIME   TXNCOUNT         MB       MB/s  QRYLEN QRY_ID         1555    ERR    STEAL    STEAL
------------------- -------- ---------- ---------- ---------- ------- ------------- ----- ------ -------- --------
2022-03-01 17:12:05 17:15:28          3        .00        .00       0                   0      0        0        0
2022-03-01 17:02:05 17:12:05          0        .00        .00       0                   0      0        0        0
2022-03-01 16:52:05 17:02:05      53784     293.07        .49       0                   0      0     3172    11591
2022-03-01 16:42:05 16:52:05     292426    1737.87       2.90       0                   0      0     4954    47402
2022-03-01 16:32:05 16:42:05     122635     952.98       1.59       0                   0      0        0      281
2022-03-01 16:22:05 16:32:05     112743     678.39       1.13       0                   0      0     1167    14951
2022-03-01 16:12:05 16:22:05     194688    1507.20       2.51       0                   0      0       20     1087
2022-03-01 16:02:05 16:12:05          0        .00        .00       0                   0      0        0        0
2022-03-01 15:52:05 16:02:05         15        .02        .00       0                   0      0        0        0
2022-03-01 15:42:05 15:52:05          6        .00        .00       0                   0      0        0        0
2022-03-01 15:32:05 15:42:05         17        .01        .00       0                   0      0        0        0
2022-03-01 15:22:05 15:32:05         24        .01        .00       0                   0      0        0        0
2022-03-01 15:12:05 15:22:05          9        .02        .00       0                   0      0        0        0
2022-03-01 15:02:05 15:12:05        116        .05        .00       0                   0      0        0        0
2022-03-01 14:52:05 15:02:05        101        .06        .00       0                   0      0        0        0
2022-03-01 14:42:05 14:52:05          2        .00        .00       0                   0      0        0        0
2022-03-01 14:32:05 14:42:05         15        .13        .00       0                   0      0        0        0
2022-03-01 14:22:05 14:32:05          0        .00        .00       0                   0      0        0        0
2022-03-01 14:12:05 14:22:05          1        .00        .00       0                   0      0        0        0
2022-03-01 14:02:05 14:12:05          0        .00        .00       0                   0      0        0        0
2022-03-01 13:52:05 14:02:05          2        .00        .00       0                   0      0        0        0
2022-03-01 13:42:05 13:52:05          0        .00        .00       0                   0      0        0        0
2022-03-01 13:32:05 13:42:05          4        .01        .00       0                   0      0        0        0
2022-03-01 13:22:05 13:32:05          0        .00        .00       0                   0      0        0        0
2022-03-01 13:12:05 13:22:05          1        .00        .00       0                   0      0        0        0
2022-03-01 13:02:05 13:12:05          0        .00        .00       0                   0      0        0        0
2022-03-01 12:52:05 13:02:05          5        .00        .00       0                   0      0        0        0
2022-03-01 12:42:05 12:52:05          0        .00        .00       0                   0      0        0        0
2022-03-01 12:32:05 12:42:05          4        .01        .00       0                   0      0        0        0
2022-03-01 12:22:05 12:32:05          0        .00        .00       0                   0      0        0        0

30 rows selected.

SQL> select begin_time,(END_TIME-begin_time)*3600*24 secs,undoblks,txncount,MAXCONCURRENCY, EXPSTEALCNT,EXPBLKRELCNT,EXPBLKREUCNT,EXPIREDBLKS from v$undostat where begin_time>sysdate-1 order by begin_time;

                                                                       # Expired
BEGIN_TIME                SECS   UNDOBLKS   TXNCOUNT MAXCONCURRENCY       Reused EXPBLKRELCNT EXPBLKREUCNT EXPIREDBLKS
------------------- ---------- ---------- ---------- -------------- ------------ ------------ ------------ -----------
...
2022-03-01 15:32:05        600          1         17              1            0            0            0           0
2022-03-01 15:42:05        600          0          6              1            0            0            0           0
2022-03-01 15:52:05        600          2         15              2            0            0            0           0
2022-03-01 16:02:05        600          0          0              1            0            0            0           0
2022-03-01 16:12:05        600     192921     194688              3        1,087        61296            0           0
2022-03-01 16:22:05        600      86834     112743              2       14,951      2319648            0           0
2022-03-01 16:32:05        600     121981     122635              4          281        73672            0           0
2022-03-01 16:42:05        600     222447     292426              3       47,402      3629736            0        8592
2022-03-01 16:52:05        600      37513      53784              4       11,591      1052896            0        8592
2022-03-01 17:02:05        600          0          0              0            0            0            0        8592
2022-03-01 17:12:05        180          0          3              0            0            0            0        8464

SQL> show parameter undo

PARAMETER_NAME                                               TYPE        VALUE
------------------------------------------------------------ ----------- ----------------------------------------------------------------------------------------------------
_undo_autotune                                               boolean     FALSE
temp_undo_enabled                                            boolean     FALSE
undo_management                                              string      AUTO
undo_retention                                               integer     900
undo_tablespace                                              string      UNDOTBS1


Note:
可见undo出现了undo segment STEAL,从未过期的undo段偷窃, 这里才执行了14万次,会话指标出现了较高的KTFB alloc space,等几个上次调用未出现的wait event, 且产生了大量的redo和UNDO, 我这仅仅只是select sysdate from dual@dblink 。

Summary:
因为使用dblink需要分配undo段来标示分布式事务,如果在循环中使用dblink并commit,每次会分配新的undo段,同时undo retention如果保留时间较常,那可能会导致undo自动扩展很大(autoextent on ), 或者会出现undo段争用,从未过期的undo段偷窃, 就会影响正常的DML事务。 记的前年的一次线上技术分享中我也描述过,分析性能问题常用的手段如top event, 统计信息指标,dump event,进程call stack, 如果以后有这类问题时通过上面的测试可以判断即使只有dblink 查询也可以找top undo占用多的会话,同时结合如dblink 和ktfb类指标去检查是否大量使用了dblink。

— enjoy —

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