Re: accounting for memory used for BufFile during hash joins

On Tue, May 07, 2019 at 04:28:36PM +1200, Thomas Munro wrote:
>On Tue, May 7, 2019 at 3:15 PM Tomas Vondra
><tomas.vondra@2ndquadrant.com> wrote:
>> On Tue, May 07, 2019 at 01:48:40PM +1200, Thomas Munro wrote:
>> Switching to some other algorithm during execution moves the goal posts
>> to the next galaxy, I'm afraid.
>
>The main problem I'm aware of with sort-merge join is: not all that is
>hashable is sortable.  So BNL is actually the only solution I'm aware
>of for problem B that doesn't involve changing a fundamental thing
>about PostgreSQL's data type requirements.
>

Sure, each of those algorithms has limitations. But I think that's mostly
irrelevant to the main issue - switching between algorithms mid-execution.
At that point some of the tuples might have been already sent sent to the
other nodes, and I have no idea how to "resume" the tuple stream short of
buffering everything locally until the join completes. And that would be
rather terrible, I guess.