Forum OpenACS Development: dynamic strings in plpsql?

Posted by Dan Wickstrom on
I recently saw this post on the pg-hackers mailing list. If it turns out to be true, it would be a nice feature which might prove to be useful in porting acs 4.0.
Phil Steinke  writes:
> The problem with my code seems to be that the "given_field" variable
> being interpolated in the assignment statement.

plpgsql is not a string-substitution language; you cannot expect the
value of a variable to be used as a table or field name in a query.
This is true because plpgsql precompiles queries into query plans
and saves the plans for repeated execution.  That's a win for speed
but costs flexibility.

You *can* get that sort of result in pltcl or plperl, which don't do
any fancy caching.  You just form the query as a string value using
usual expression evaluation rules of those languages, and that string
gets submitted to the SQL parser and query engine.  Less speed, more

I believe 7.1's plpgsql will have an EXECUTE <string> command
that lets you get the second effect in plpgsql too.

			regards, tom lane

Posted by Don Baccus on
Yes, it would, I don't know if it is in, though ... I'll try to check over the weekend.  Best of both worlds - in general PL/Tcl kind of sucks for doing queries because you have to do any query caching yourself since it uses the SPI interface.

PL/pgSQL hasn't allowed dynamic SQL because it is a compiled language (into bytecode+query plans).  Having a dynamic EXECUTE (which undoubtably uses the same SPI interface as used in PL/Tcl and referential integrity checking, etc) would be a great improvement.