Re: Precedence of Postfix operators

Hi,
Is there any reason why we have given lesser precedence for postfix
operator compared to multiplication/division? Usually postfix operators have
more precedence than the binary operations. Is this some kind of work around
to provide user-defined operators? Can someone help me understand this?

Thanks,
Gokul.

Gokulakannan Somasundaram <gokul007(at)gmail(dot)com> writes:
> Is there any reason why we have given lesser precedence for postfix
> operator compared to multiplication/division? Usually postfix operators have
> more precedence than the binary operations. Is this some kind of work around
> to provide user-defined operators? Can someone help me understand this?

A bit of poking in the CVS logs for gram.y reveals

2001-01-23 17:39 tgl

* src/backend/parser/gram.y: Give 'a_expr ::= a_expr Op' production
a slightly lower precedence than Op, so that the sequence 'a_expr
Op Op a_expr' will be parsed as a_expr Op (Op a_expr) not (a_expr
Op) Op a_expr as formerly. In other words, prefer treating
user-defined operators as prefix operators to treating them as
postfix operators, when there is an ambiguity. Also clean up a
couple of other infelicities in production priority assignment ---
for example, BETWEEN wasn't being given the intended priority, but
that of AND.

There are several other nasty things that we've had to do in order to
keep supporting postfix operators at all. I thibk most people view them
as a legacy feature best avoided.

regards, tom lane

Thanks Tom, for the explanation.

Gokul.

On Sun, Feb 7, 2010 at 10:14 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Gokulakannan Somasundaram <gokul007(at)gmail(dot)com> writes:
> > Is there any reason why we have given lesser precedence for postfix
> > operator compared to multiplication/division? Usually postfix operators
> have
> > more precedence than the binary operations. Is this some kind of work
> around
> > to provide user-defined operators? Can someone help me understand this?
>
> A bit of poking in the CVS logs for gram.y reveals
>
> 2001-01-23 17:39 tgl
>
> * src/backend/parser/gram.y: Give 'a_expr ::= a_expr Op' production
> a slightly lower precedence than Op, so that the sequence 'a_expr
> Op Op a_expr' will be parsed as a_expr Op (Op a_expr) not (a_expr
> Op) Op a_expr as formerly. In other words, prefer treating
> user-defined operators as prefix operators to treating them as
> postfix operators, when there is an ambiguity. Also clean up a
> couple of other infelicities in production priority assignment ---
> for example, BETWEEN wasn't being given the intended priority, but
> that of AND.
>
> There are several other nasty things that we've had to do in order to
> keep supporting postfix operators at all. I thibk most people view them
> as a legacy feature best avoided.
>
> regards, tom lane
>