Re: Parallel Index Scan vs BTP_DELETED and BTP_HALF_DEAD

Hi,

Here's a reproducer which enabled me to reach this stuck state:

  pid  |  wait_event   |                                    query
-------+---------------+-----------------------------------------------------------------------------
 64617 |               | select pid, wait_event, query from
pg_stat_activity where state = 'active';
 64619 | BufferPin     | VACUUM jobs
 64620 | ExecuteGather | SELECT COUNT(*) FROM jobs
 64621 | ExecuteGather | SELECT COUNT(*) FROM jobs
 64622 | ExecuteGather | SELECT COUNT(*) FROM jobs
 64623 | ExecuteGather | SELECT COUNT(*) FROM jobs
 84167 | BtreePage     | SELECT COUNT(*) FROM jobs
 84168 | BtreePage     | SELECT COUNT(*) FROM jobs
 96440 |               | SELECT COUNT(*) FROM jobs
 96438 |               | SELECT COUNT(*) FROM jobs
 96439 |               | SELECT COUNT(*) FROM jobs
(11 rows)

The main thread deletes stuff in the middle of the key range (not sure
if this is important) and vacuum in a loop, and meanwhile 4 threads
(probably not important, might as well be 1) run Parallel Index Scans
over the whole range, in the hope of hitting the interesting case.  In
the locked-up case I just saw now opaque->btpo_flags had the
BTP_DELETED bit set, not BTP_HALF_DEAD (I could tell because I added
logging).  Clearly pages are periodically being marked half-dead but I
haven't yet managed to get an index scan to hit one of those.

-- 
Thomas Munro
http://www.enterprisedb.com