Re: a plpgsql bug

"David G. Johnston" <david(dot)g(dot)johnston(at)gmail(dot)com> writes:
> On Mon, Sep 18, 2023 at 11:46 PM daidewei(at)highgo(dot)com <daidewei(at)highgo(dot)com>
> wrote:
>> I found a problem in plpgsql. When there is a large loop in plpgsql,
>> it is found that the change of search_path will cause memory exhaustion and
>> thus disconnect the connection.

> It is impossible to prevent queries from exhausting memory so the fact that
> this one does isn't an indication of a bug on its own. I'm having trouble
> imagining a reasonable use case for this pattern of changing search_path
> frequently in a loop within a single execution of a function. That said,
> I'm not in a position to judge how easy or difficult an improvement in this
> area may be.

I poked into this a bit with valgrind. It seems that the problem
is that the changes to search_path thrash the "simple expression"
mechanism in plpgsql, such that it has to re-plan the various
expressions in the called function each time through. It's good about
tracking the actual cached plans and not leaking those, but what is
getting leaked into transaction-lifespan memory is the data structures
made by

expr->expr_simple_state =
ExecInitExprWithParams(expr->expr_simple_expr,
econtext->ecxt_param_list_info);

We could conceivably reclaim that data if we were willing to set up
yet another per-expression memory context to hold it. That seems
like rather a high overhead though.

The given test case is obviously a bit artificial, but I think it
may be a simplification of fairly plausible use-cases. The triggering
condition is that the same textual expression in a plpgsql function
gets executed repeatedly with different search_path settings, which
doesn't seem that unreasonable.

Perhaps another approach could be to assume that only a small number
of distinct search_path settings will be used in any one transaction,
and cache a separate plan and estate for each one. That would have
the nice side-effect of avoiding the replanning overhead, but then
we'd have to figure out how to manage the cache and keep it from
blowing out.

regards, tom lane