Add type conversion discussion.

Author: bmomjian

doc/TODO.detail/typeconv

@@ -122,7 +122,7 @@ From tgl@sss.pgh.pa.us Sun May 14 17:30:56 2000
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@@ -757,3 +757,160 @@ Comments?
 
 			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>
+Precedence: bulk
+Sender: pgsql-general-owner@postgresql.org
+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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