PL/pgsql EXECUTE 'SELECT INTO ...'

I have looked a little bit at what it'd take to make SELECT INTO inside
an EXECUTE work the same as it does in plain plpgsql --- that is, the
INTO should reference plpgsql variables, not a destination table.
It looks to me like this is possible but would require some nontrivial
re-engineering inside plpgsql.  What I'm visualizing is that EXECUTE
should read its string argument not just as an SPI_exec() string, but
as an arbitrary plpgsql proc_stmt.  This would offer some interesting
capabilities, like building a whole FOR-loop for dynamic execution.
But there are a number of problems to be surmounted, notably arranging
for the parsetree built by the plpgsql compiler not to be irretrievably
memory-leaked.  (That ties into something I'd wanted to do anyway,
which is to have the plpgsql compiler build its trees in a memory
context associated with the function, not via malloc().)

This does not look like something to be tackling when we're already
in late beta, unfortunately.  So we have to decide what to do for 7.1.
If we do nothing now, and then implement this feature in 7.2, we will
have a backwards compatibility problem: EXECUTE 'SELECT INTO ...'
will completely change in meaning.

I am inclined to keep our options open by forbidding EXECUTE 'SELECT
INTO ...' for now.  That's more than a tad annoying, because that leaves
no useful way to do a dynamically-built SELECT, but if we don't forbid
it I think we'll regret it later.

Comments?
        regards, tom lane