Hi, On 2020-06-24 13:12:03 -0400, Bruce Momjian wrote: > Well, my point is that merge join works that way, and no one has needed > a knob to avoid mergejoin if it is going to spill to disk. If they are > adjusting work_mem to prevent spill of merge join, they can do the same > for hash agg. We just need to document this in the release notes.
I don't think this is comparable. For starters, the IO indirectly triggered by mergejoin actually leads to plenty people just straight out disabling it. For lots of workloads there's never a valid reason to use a mergejoin (and often the planner will never choose one). Secondly, the planner has better information about estimating the memory usage for the to-be-sorted data than it has about the size of the transition values. And lastly, there's a difference between a long existing cause for bad IO behaviour and one that's suddenly kicks in after a major version upgrade, to which there's no escape hatch (it's rarely realistic to disable hash aggs, in contrast to merge joins). Greetings, Andres Freund
