Re: BUG #15577: Query returns different results when executed multiple times

On Wed, Jan 9, 2019 at 2:52 PM Thomas Munro
<thomas(dot)munro(at)enterprisedb(dot)com> wrote:
> Now the number of results from the query in the script is not stable,
> varying between 235 or 246 results for me. Not sure what's going on
> yet.

When there are multiple matches found by the Index Scan, sometimes
only the first one makes it through to the Nested Loop, in the leader.
It's not because of single_match being inappropriately set as I
speculated earlier (though that'd behave similarly I guess) but
because the quals are somehow broken in between invocations so that
ExecScan() filters the rest of the matches out, and then fixed again
on the next rescan. This happens whenever the leader takes a break to
read tuple from parallel workers in between executing the Nested Loop
node, which it often does after reading the first of several tuples
resulting from a single rescan.

What exactly is being corrupted and how, I don't yet know, and I need
to leave this here for today, but that's what I've got so far.

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