Text-any concatenation volatility acting as optimization barrier

Hi list,

Andrew Dunstan reported an awkward-seeming case on IRC where shifting
around a concatenation expression in a view made the planner choose a
good or a bad execution plan.

Simplified, it boils down to this:

db=# create table foo(i int);
db=# explain verbose select i from (select i, i::text || 'x' as asd
from foo) as subq;
Seq Scan on public.foo (cost=0.00..34.00 rows=2400 width=4)
Output: foo.i

db=# explain verbose select i from (select i, i || 'x'::text as asd
from foo) as subq;
Subquery Scan on subq (cost=0.00..76.00 rows=2400 width=4)
Output: subq.i
-> Seq Scan on public.foo (cost=0.00..52.00 rows=2400 width=4)
Output: foo.i, ((foo.i)::text || 'x'::text)

Case #1 uses the normal textcat(text, text) operator by automatically
coercing 'x' as text.
However, case #2 uses the anytextcat(anynonarray, text), which is
marked as volatile thus acts as an optimization barrier. Later, the
anytextcat SQL function is inlined and the EXPLAIN VERBOSE output has
no trace of what happened.

Is this something we can, or want, to fix?

One way would be doing preprocess_expression() before
pull_up_subqueries() so function inlining happens earlier, but I can't
imagine what unintended consequences that might have.

Another option would be creating explicit immutable text || foo
operators for common types, but that sounds pretty hacky.

Regards,
Marti