IT16177: RUNNING UPDATE STATISTICS LOW WITH USTLOW_SAMPLE ENABLED WILL CRASH SERVER IF DATA SKEW FOUND WITH "THREAD EXITED WITH # BUFFERS

Error description

• Running Update Statistics Low with USTLOW_SAMPLE enabled will
  crash the engine when exiting with "Thread exited with # buffers
  held", if on a big index data skew is found.
  
  You'll get a warning in the online.log about the inaccurate
  index statistics.
  
  13:30:51  Warning: update statistics low using sampling may
  generate inaccurate index statistics for index informix.xxxxxx
  due to data skew
  
  14:54:40
  14:54:40  IBM Informix Dynamic Server Version 12.10.FC6X5WE
  Software Serial Number AAA#B000000
  
  14:54:40  Assert Failed: Thread exited with 2 buffers held.
  
  14:54:40   Who: Session(193, informix@xxxxxxx, 19844,
  0x4db7a550)
  		Thread(227, sqlexec, 4db3dde8, 1)
  		File: rsshmem.c Line: 2083
  14:54:40   Results: Dynamic Server must abort
  14:54:40  SHM Globals and Master Pool/Master Block Adresses:
  
  14:54:40  shmcb =           0x000000004419dc28
  14:54:40  rhead =           0x000000004420a800
  14:54:40  pool list =       0x000000004419dd00
  14:54:40  block pool list = 0x0000000044204908
  14:54:40  TRANSP =          0x000000004db7a550
  14:54:40  PARTP =           0x0000000000000000
  14:54:40  PARTNP =          0x0000000000000000
  14:54:40  OPENP =           0x000000004f1d9828
  14:54:40  FILEP =           0x000000004f1dd4e0
  14:54:40  Raw hex dump of stack located in
  /opt/informix/tmp/af.4cbedff.rawstk
  14:54:40  Stack for thread: 227 sqlexec
  
   base: 0x000000004f1f4000
    len:   69632
     pc: 0x00000000013f50e7
    tos: 0x000000004f202b50
  state: running
     vp: 1
  
  (/opt/informix/bin/oninit) afstack
  (/opt/informix/bin/oninit) afhandler
  (/opt/informix/bin/oninit) afcrash_interface
  (/opt/informix/bin/oninit) rstcb_cleanup
  (/opt/informix/bin/oninit) destroy_session
  (/opt/informix/bin/oninit) sq_exit
  (/opt/informix/bin/oninit) sqmain
  (/opt/informix/bin/oninit) spawn_thread
  (/opt/informix/bin/oninit) th_init_initgls
  0x000000000144e858 (/opt/informix/bin/oninit) startup
  
  Onstat -b shows:
  
  ===========------------- - - - - - -
  /opt/informix/bin/onstat -b:
  
  IBM Informix Dynamic Server Version 12.10.FC6X5WE -- On-Line --
  Up 03:18:23 -- 3573872 Kbytes
  
  Buffers
  address           userthread        flgs pagenum
  memaddr           nslots pgflgs xflgs owner             waitlist
  
  
  Buffer pool page size: 2048
  71a972d8         0                802  5:3432386        7e954000
  0      2890   10    0                0
  7ade6598         0                802  5:6639819        f46e3000
  22     890    10    0                0
   45 modified, 1310720 total, 2097152 hash buckets, 2048 buffer
  size
  
  and those pagenum belongs to index(es) for table reported,
  including the index reported on the warning.