Re: Dynamic expression evaluation

"Philippe Lang" <philippe(dot)lang(at)attiksystem(dot)ch> writes:
> Is there a simple way of doing "kind of" a
> SELECT *, EVAL(f) FROM public.test;

> ... and having f evaluated as an expression, so that we get back:

> ------------------------------
> id a b f eval
> ------------------------------
> 1 2 3 a+b 5
> 2 12 3 a*b 36
> 3 5 6 a+2*b 17
> ------------------------------

Not really. You can sort of approximate eval() with plpgsql's EXECUTE:

regression=# create or replace function eval(text) returns int as '
regression'# declare res record;
regression'# begin
regression'# for res in execute ''select '' || $1 || '' as result'' loop
regression'# return res.result;
regression'# end loop;
regression'# end' language plpgsql;
CREATE FUNCTION
regression=# select eval ('23+34');
eval
------
57
(1 row)

regression=#

but this has a problem with supporting more than one result type (hmm,
maybe you could fake that with 7.4's polymorphism?). And I don't see
any way at all for the function to have access to the other values in
the row, as your example presumes it would do.

regards, tom lane