Downgrade implicit casts to text to be assignment-only, except for the ones

from the other string-category types; this eliminates a lot of surprising
interpretations that the parser could formerly make when there was no directly
applicable operator.

Create a general mechanism that supports casts to and from the standard string
types (text,varchar,bpchar) for *every* datatype, by invoking the datatype's
I/O functions.  These new casts are assignment-only in the to-string direction,
explicit-only in the other, and therefore should create no surprising behavior.
Remove a bunch of thereby-obsoleted datatype-specific casting functions.

The "general mechanism" is a new expression node type CoerceViaIO that can
actually convert between *any* two datatypes if their external text
representations are compatible.  This is more general than needed for the
immediate feature, but might be useful in plpgsql or other places in future.

This commit does nothing about the issue that applying the concatenation
operator || to non-text types will now fail, often with strange error messages
due to misinterpreting the operator as array concatenation.  Since it often
(not always) worked before, we should either make it succeed or at least give
a more user-friendly error; but details are still under debate.

Peter Eisentraut and Tom Lane

1 files changed, 14 insertions, 1 deletions

diff --git a/src/include/nodes/execnodes.h b/src/include/nodes/execnodes.h
index b6b7ddcde6..ccc243cfd0 100644
--- a/src/include/nodes/execnodes.h
+++ b/src/include/nodes/execnodes.h
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/nodes/execnodes.h,v 1.175 2007/05/21 17:57:34 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/nodes/execnodes.h,v 1.176 2007/06/05 21:31:08 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -638,6 +638,19 @@ typedef struct FieldStoreState
 } FieldStoreState;
 
 /* ----------------
+ *		CoerceViaIOState node
+ * ----------------
+ */
+typedef struct CoerceViaIOState
+{
+	ExprState	xprstate;
+	ExprState  *arg;			/* input expression */
+	FmgrInfo	outfunc;		/* lookup info for source output function */
+	FmgrInfo	infunc;			/* lookup info for result input function */
+	Oid			intypioparam;	/* argument needed for input function */
+} CoerceViaIOState;
+
+/* ----------------
  *		ArrayCoerceExprState node
  * ----------------
  */