Re: Limiting memory allocation

On Tue, May 24, 2022 at 07:40:45PM -0400, Tom Lane wrote:
> Bruce Momjian  writes:
> > If the plan output is independent of work_mem,
> 
> ... it isn't ...

Good.

> > I always wondered why we
> > didn't just determine the number of simultaneous memory requests in the
> > plan and just allocate accordingly, e.g. if there are four simultaneous
> > memory requests in the plan, each gets work_mem/4.
> 
> (1) There are not a predetermined number of allocations.  For example,
> if we do a given join as nestloop+inner index scan, that doesn't require
> any large amount of memory; but if we do it as merge or hash join then
> it will consume memory.

Uh, we know from the plan whether we are doing a nestloop+inner or merge
or hash join, right?  I was suggesting we look at the plan before
execution and set the proper percentage of work_mem for each node.

> (2) They may not all need the same amount of memory, eg joins might
> be working on different amounts of data.

True. but we could cap it like we do now for work_mem, but as a
percentage of a GUC work_mem total.

-- 
  Bruce Momjian          https://momjian.us
  EDB                                      https://enterprisedb.com

  Indecision is a decision.  Inaction is an action.  Mark Batterson