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Re: Implicit casts with generic arrays

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Gregory Stark <stark(at)enterprisedb(dot)com>
Cc: "Peter Eisentraut" <peter_e(at)gmx(dot)net>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Implicit casts with generic arrays
Date: 2007-06-05 01:25:41
Message-ID: 2486.1181006741@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
Gregory Stark <stark(at)enterprisedb(dot)com> writes:
> "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us> writes:
>> we should do is make oper() specifically test for the case of operator
>> 349 with UNKNOWN left input, or operator 374 with UNKNOWN right input,
>> and throw a custom error message hinting that the other operand
>> needs to be cast to text.

> Wouldn't that mean that 'foo'||'bar' would *still* fail?

No, because that would preferentially match to text || text, it being
a preferred-type case.  The current behavior with the implicit casts
removed is

template1=# select 'abc' || '34';
 ?column? 
----------
 abc34
(1 row)

ie, this was matched to the text || text operator;

template1=# select 'abc' || 34;
ERROR:  array value must start with "{" or dimension information

ie, this was matched to the anyarray || anyelement operator --- because
it clearly can't match text || text.

> It really seems to me that at some point down the line we're going to
> cave and admit that users do expect 'foo' to be a string first and
> cast to other types only if the context requires it.

We already do that to some extent, as shown above; and it's got
approximately nothing to do with this problem anyway.  The cases where
we have got a problem are where the other argument is clearly *not*
text.

But having said that, I'm currently leaning to the other solution of
generalizing the || operator (and only that operator) instead of
fooling with the type resolution rules.

			regards, tom lane

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