Ranier Vilela <ranier.vf@gmail.com> writes:
> Em qua., 15 de set. de 2021 às 12:00, Tom Lane <tgl@sss.pgh.pa.us> escreveu:
>> We could, in fact, not bother with removing the no-longer-referenced
>> subplans, and it probably wouldn't be all that awful. But the intent
>> of the original patch was to save the executor startup time for such
>> subplans, so I wanted to preserve that goal if I could.
> I'm sorry if I'm being persistent with this issue, but I'd like to give it
> one last try before I let it go
> I modified the way the subplane deletion is done and it seems to me that
> this really happens.
It looks like what this fragment is doing is clobbering the List
substructure of the AlternativeSubPlan node itself. That's not
going to make any difference, since the whole point of the exercise
is that the AlternativeSubPlan gets cut out of the finished tree.
But the list that we want to modify, in order to save the
executor time, is the root->glob->subplans list (which ends
up being PlannedStmt.subplans). And that's global to the
query, so we can't fix it correctly on the basis of a single
AlternativeSubPlan.
Ok, I can see now.
But this leads me to the conclusion that AlternativeSubPlan *asplan
does not seem to me to be a good approach for this function, better to deal with it directly:
"root->glob->subplans" which, it seems, works too.
i = 0;
foreach(lc, root->glob->subplans)
{
SubPlan *curplan = (SubPlan *) lfirst(lc);
Cost curcost;
curcost = curplan->startup_cost + num_exec * curplan->per_call_cost;
if (bestplan == NULL || curcost <= bestcost)
{
bestplan = curplan;
bestcost = curcost;
}
i++;
}
if (bestplan != NULL)
{
foreach(lc, root->glob->subplans)
{
SubPlan *curplan = (SubPlan *) lfirst(lc);
if (curplan != bestplan)
lfirst(lc) = NULL;
}
j = 0;
foreach(lc, root->glob->subplans)
{
SubPlan *curplan = (SubPlan *) lfirst(lc);
if (curplan != NULL)
j++;
}
if (j != i)
{
ereport(ERROR,
(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
errmsg("too many subplans: total_plans=%d, remain_plans=%d", i, j)));
}
}
postgres=# explain (costs off)
postgres-# select * from exists_tbl t1
postgres-# where (exists(select 1 from exists_tbl t2 where t1.c1 = t2.c2) or c3 < 0);
ERROR: too many subplans: total_plans=2, remain_plans=1
postgres=# select * from exists_tbl t1
postgres-# where (exists(select 1 from exists_tbl t2 where t1.c1 = t2.c2) or c3 < 0);
ERROR: too many subplans: total_plans=2, remain_plans=1
Anyway, thank you for the explanations.
