Re: is there a way to firmly cap postgres worker memory consumption?

Well, here's the problem:

>         ExprContext: 812638208 total in 108 blocks; 183520 free (171
> chunks); 812454688 used

So something involved in expression evaluation is eating memory.
Looking at the query itself, I'd have to bet on this:

>            ARRAY_TO_STRING(ARRAY_AGG(MM.ID::CHARACTER VARYING), ',')

My guess is that this aggregation is being done across a lot more rows
than you were expecting, and the resultant array/string therefore eats
lots of memory.  You might try replacing that with COUNT(*), or even
better SUM(LENGTH(MM.ID::CHARACTER VARYING)), just to get some definitive
evidence about what the query is asking to compute.

Meanwhile, it seems like ulimit -v would provide the safety valve
you asked for originally.