Re: varchar vs. text + constraint/triggers was: Help request to improve function performance

On Thu, Apr 23, 2009 at 11:33:34AM +0200, Ivan Sergio Borgonovo wrote:

> Karsten Hilbert <Karsten(dot)Hilbert(at)gmx(dot)net> wrote:
>
> > On Thu, Apr 23, 2009 at 12:02:13AM +0100, Seref Arikan wrote:
> >
> > > I have a set of dynamically composed objects represented in
> > > Java, with string values for various attributes, which have
> > > variable length. In case you have suggestions for a better type
> > > for this case, it would be my pleasure to hear about them.
> >
> > Seref, he's suggesting you use TEXT instead of
> > VARCHAR(something). In PG it's actually usually *less*
> > overhead to use the unbounded text datatype (no length check
> > required).
> >
> > Length checks mandated by business logic can be added by
> > more dynamic means -- check constraints, triggers, etc which
> > allow for less invasive change if needed.
>
> Could you point us to some example of a constraint/trigger (etc...)
> that is going to provide the same checking of varchar and explain
> (if the code/example... doesn't make it clear) why it should be
> faster or less invasive?

check constraint based:

create table foo (
txt text
check (char_length(txt) < 1001)
);

trigger based:

create function trf_check_length_1000() ... returns trigger ... $$...$$;

create table foo (
txt text
);

create trigger check_txt_length before INSERT or UPDATE ... execute trf_check_length_1000();

faster:

- TEXT is (judging by previous comments on this list)
marginally faster than VARCHAR(1000) because a) it runs
the same code but b) doesn't have to check for the 1000
length

- other options (VARCHAR, constraint, trigger) incur
additional overhead and are thus slower

less invasive:

Well, poor wording on my part, perhaps. What I meant is that
changing a check constraint or trigger appears to be a less
costly operation on a table than changing the datatype of a
column (although I seem to remember there being some
optimizations in place for the case of changing the *length*
of a varchar).

I may be wrong in the above and if so it better be brought
to our collective attention for the benefit of readers.

Karsten
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