Dynamic expression evaluation

[Philippe Lang]

Hello,

Imagine we have the following kind of table, with two values (a and b), and a varchar (f) representing an expression.

----------------------------------
CREATE TABLE public.test
(
id serial NOT NULL,
a int4,
b int4,
f varchar(50),
CONSTRAINT id PRIMARY KEY (id)
) WITHOUT OIDS;

INSERT INTO public.test(a,b,f) VALUES(2,3,'a+b');
INSERT INTO public.test(a,b,f) VALUES(12,3,'a*b');
INSERT INTO public.test(a,b,f) VALUES(5,6,'a+2*b');
----------------------------------

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
------------------------------
Has anyone done anything like that already?

Thanks!

Philippe

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[Tom Lane]

"Philippe Lang" <ph***********@attiksystem.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

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