Re: [sqlsmith] Failed assertion in parallel worker (ExecInitSubPlan)

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> Target list for a relation, you mean? See relation.h:
>>
>> * reltarget - Default Path output tlist for this rel; normally contains
>> * Var and PlaceHolderVar nodes for the values we need to
>> * output from this relation.
>> * List is in no particular order, but all rels of an
>> * appendrel set must use corresponding orders.
>> * NOTE: in an appendrel child relation, may contain
>> * arbitrary expressions pulled up from a subquery!

> Err, wow. That makes my head hurt. Can you explain why this case
> only arises for appendrel children, and not for plain rels?

Well, plain rels only output Vars ;-)

But consider an appendrel representing

(SELECT x+1 FROM t1 UNION ALL SELECT y+2 FROM t2) ss(a)

The RTE for ss will have a reltarget list containing just "a".
Once we pull up the subqueries, the reltarget lists for the two child
appendrel members will need to contain "x+1" and "y+2" in order to be
equivalent to the parent's reltarget list. See set_append_rel_size(),
which does that transformation.

This doesn't happen with ordinary subquery flattening because there
isn't a RelOptInfo corresponding to an ordinary subquery that's been
pulled up into the parent query.

regards, tom lane