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regards, tom lane

From pgsql-general-owner+M18949=candle.pha.pa.us=pgman@postgresql.org Sat Dec 29 15:47:47 2001

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From: Bruce Momjian <pgman@candle.pha.pa.us>

Message-ID: <200112292036.fBTKaTg04256@candle.pha.pa.us>

Subject: Re: [GENERAL] Casting Varchar to Numeric

In-Reply-To: <20011206150158.O28880-100000@megazone23.bigpanda.com>

To: Stephan Szabo <sszabo@megazone23.bigpanda.com>

Date: Sat, 29 Dec 2001 15:36:29 -0500 (EST)

cc: Andy Marden <amarden@usa.net>, pgsql-general@postgresql.org

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Status: OR

> On Mon, 3 Dec 2001, Andy Marden wrote:

>

> > Martijn,

> >

> > It does work (believe it or not). I've now tried the method you mention

> > below - that also works and is much nicer. I can't believe that PostgreSQL

> > can't work this out. Surely implementing an algorithm that understands that

> > if you can go from a ->b and b->c then you can certainly go from a->c. If

>

> It's more complicated than that (and postgres does some of this but not

> all), for example the cast text->float8->numeric potentially loses

> precision and should probably not be an automatic cast for that reason.

>

> > this is viewed as too complex a task for the internals - at least a diagram

> > or some way of understanding how you should go from a->c would be immensely

> > helpful wouldn't it! Daunting for anyone picking up the database and trying

> > to do something simple(!)

>

> There may be a need for documentation on this. Would you like to write

> some ;)

OK, I ran some tests:

test=> create table test (x text);

CREATE

test=> insert into test values ('323');

INSERT 5122745 1

test=> select cast (x as numeric) from test;

ERROR: Cannot cast type 'text' to 'numeric'

I can see problems with automatically casting numeric to text because

you have to guess the desired format, but going from text to numeric

seems quite easy to do. Is there a reason we don't do it?

I can cast to integer and float8 fine:

test=> select cast ( x as integer) from test;

?column?

----------

323

(1 row)

test=> select cast ( x as float8) from test;

?column?

----------

323

(1 row)

--

Bruce Momjian | http://candle.pha.pa.us

pgman@candle.pha.pa.us | (610) 853-3000

+ If your life is a hard drive, | 830 Blythe Avenue

+ Christ can be your backup. | Drexel Hill, Pennsylvania 19026

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From pgsql-general-owner+M18951=candle.pha.pa.us=pgman@postgresql.org Sat Dec 29 19:10:38 2001

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To: Bruce Momjian <pgman@candle.pha.pa.us>

cc: Stephan Szabo <sszabo@megazone23.bigpanda.com>,

Andy Marden <amarden@usa.net>, pgsql-general@postgresql.org

Subject: Re: [GENERAL] Casting Varchar to Numeric

In-Reply-To: <200112292036.fBTKaTg04256@candle.pha.pa.us>

References: <200112292036.fBTKaTg04256@candle.pha.pa.us>

Comments: In-reply-to Bruce Momjian <pgman@candle.pha.pa.us>

message dated "Sat, 29 Dec 2001 15:36:29 -0500"

Date: Sat, 29 Dec 2001 18:07:57 -0500

Message-ID: <20242.1009667277@sss.pgh.pa.us>

From: Tom Lane <tgl@sss.pgh.pa.us>

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Status: OR

Bruce Momjian <pgman@candle.pha.pa.us> writes:

> I can see problems with automatically casting numeric to text because

> you have to guess the desired format, but going from text to numeric

> seems quite easy to do. Is there a reason we don't do it?

I do not think it's a good idea to have implicit casts between text and

everything under the sun, because that essentially destroys the type

checking system. What we need (see previous discussion) is a flag in

pg_proc that says whether a type conversion function may be invoked

implicitly or not. I've got no problem with offering text(numeric) and

numeric(text) functions that are invoked by explicit function calls or

casts --- I just don't want the system trying to use them to make

sense of a bogus query.

> I can cast to integer and float8 fine:

I don't believe that those should be available as implicit casts either.

They are, at the moment:

regression=# select 33 || 44.0;

?column?

----------

3344

(1 row)

Ugh.

regards, tom lane

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