Robert Haas <> 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

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