Flip the default typispreferred setting from true to false. This affects

only type categories in which the previous coding made *every* type
preferred; so there is no change in effective behavior, because the function
resolution rules only do something different when faced with a choice
between preferred and non-preferred types in the same category.  It just
seems safer and less surprising to have CREATE TYPE default to non-preferred
status ...

doc/src/sgml/ref/create_type.sgml

 <!--
-$PostgreSQL: pgsql/doc/src/sgml/ref/create_type.sgml,v 1.75 2008/07/30 17:05:04 tgl Exp $
+$PostgreSQL: pgsql/doc/src/sgml/ref/create_type.sgml,v 1.76 2008/07/30 19:35:12 tgl Exp $
 PostgreSQL documentation
 -->
 
@@ -535,9 +535,9 @@ CREATE TYPE <replaceable class="parameter">name</replaceable>
     <listitem>
      <para>
       True if this type is a preferred type within its type category,
-      else false.  The default is true (which is appropriate for
-      all entries in category <literal>U</>, but is usually not
-      appropriate for new types in other categories &mdash; beware!).
+      else false.  The default is false.  Be very careful about creating
+      a new preferred type within an existing type category, as this
+      could cause surprising changes in behavior.
      </para>
     </listitem>
    </varlistentry>

doc/src/sgml/typeconv.sgml

-<!-- $PostgreSQL: pgsql/doc/src/sgml/typeconv.sgml,v 1.56 2008/07/30 17:05:04 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/typeconv.sgml,v 1.57 2008/07/30 19:35:12 tgl Exp $ -->
 
 <chapter Id="typeconv">
 <title>Type Conversion</title>
@@ -160,25 +160,13 @@ categories</firstterm>, including <type>boolean</type>, <type>numeric</type>,
 <type>timespan</type>, <type>geometric</type>, <type>network</type>, and
 user-defined.  (For a list see <xref linkend="catalog-typcategory-table">;
 but note it is also possible to create custom type categories.)  Within each
-category there are one or more <firstterm>preferred types</firstterm>, which
+category there can be one or more <firstterm>preferred types</firstterm>, which
 are preferentially selected when there is ambiguity.  With careful selection
 of preferred types and available implicit casts, it is possible to ensure that
 ambiguous expressions (those with multiple candidate parsing solutions) can be
 resolved in a useful way.
 </para>
 
-<note>
- <para>
-  For what are now historical reasons, types in the <quote>user-defined</>
-  category are normally always marked as <quote>preferred</>.  Since all types
-  in this category are preferred, the heuristic that favors preferred types
-  accomplishes nothing, and thus this situation is equivalent to treating them
-  all as non-preferred.  The <quote>preferred</> marking is useful within the
-  system-defined type categories, and can be useful within custom type
-  categories.
- </para>
-</note>
-
 <para>
 All type conversion rules are designed with several principles in mind:
 

src/backend/catalog/heap.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/catalog/heap.c,v 1.335 2008/07/30 17:05:04 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/catalog/heap.c,v 1.336 2008/07/30 19:35:13 tgl Exp $
  *
  *
  * INTERFACE ROUTINES
@@ -762,7 +762,7 @@ AddNewRelationType(const char *typeName,
 				   -1,			/* internal size (varlena) */
 				   TYPTYPE_COMPOSITE,	/* type-type (composite) */
 				   TYPCATEGORY_COMPOSITE, /* type-category (ditto) */
-				   true,		/* all composite types are preferred */
+				   false,		/* composite types are never preferred */
 				   DEFAULT_TYPDELIM,	/* default array delimiter */
 				   F_RECORD_IN, /* input procedure */
 				   F_RECORD_OUT,	/* output procedure */
@@ -941,7 +941,7 @@ heap_create_with_catalog(const char *relname,
 				   -1,			/* Internal size (varlena) */
 				   TYPTYPE_BASE,	/* Not composite - typelem is */
 				   TYPCATEGORY_ARRAY, /* type-category (array) */
-				   true,		/* all array types are preferred */
+				   false,		/* array types are never preferred */
 				   DEFAULT_TYPDELIM,	/* default array delimiter */
 				   F_ARRAY_IN,	/* array input proc */
 				   F_ARRAY_OUT, /* array output proc */

src/backend/commands/typecmds.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/typecmds.c,v 1.120 2008/07/30 17:05:04 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/typecmds.c,v 1.121 2008/07/30 19:35:13 tgl Exp $
  *
  * DESCRIPTION
  *	  The "DefineFoo" routines take the parse tree and pick out the
@@ -112,7 +112,7 @@ DefineType(List *names, List *parameters)
 	char	   *defaultValue = NULL;
 	bool		byValue = false;
 	char		category = TYPCATEGORY_USER;
-	bool		preferred = true;
+	bool		preferred = false;
 	char		delimiter = DEFAULT_TYPDELIM;
 	char		alignment = 'i';	/* default alignment */
 	char		storage = 'p';	/* default TOAST storage method */
@@ -475,7 +475,7 @@ DefineType(List *names, List *parameters)
 			   -1,				/* internal size (always varlena) */
 			   TYPTYPE_BASE,	/* type-type (base type) */
 			   TYPCATEGORY_ARRAY, /* type-category (array) */
-			   true,			/* all array types are preferred */
+			   false,			/* array types are never preferred */
 			   DEFAULT_TYPDELIM,	/* array element delimiter */
 			   F_ARRAY_IN,		/* input procedure */
 			   F_ARRAY_OUT,		/* output procedure */
@@ -919,7 +919,7 @@ DefineDomain(CreateDomainStmt *stmt)
 				   internalLength,		/* internal size */
 				   TYPTYPE_DOMAIN,		/* type-type (domain type) */
 				   category,	/* type-category */
-				   false,		/* domains are never preferred types */
+				   false,		/* domain types are never preferred */
 				   delimiter,	/* array element delimiter */
 				   inputProcedure,		/* input procedure */
 				   outputProcedure,		/* output procedure */
@@ -1032,7 +1032,7 @@ DefineEnum(CreateEnumStmt *stmt)
 				   sizeof(Oid), /* internal size */
 				   TYPTYPE_ENUM,	/* type-type (enum type) */
 				   TYPCATEGORY_ENUM,	/* type-category (enum type) */
-				   true,		/* all enum types are preferred */
+				   false,		/* enum types are never preferred */
 				   DEFAULT_TYPDELIM,	/* array element delimiter */
 				   F_ENUM_IN,	/* input procedure */
 				   F_ENUM_OUT,	/* output procedure */
@@ -1070,7 +1070,7 @@ DefineEnum(CreateEnumStmt *stmt)
 			   -1,				/* internal size (always varlena) */
 			   TYPTYPE_BASE,	/* type-type (base type) */
 			   TYPCATEGORY_ARRAY, /* type-category (array) */
-			   true,			/* all array types are preferred */
+			   false,			/* array types are never preferred */
 			   DEFAULT_TYPDELIM,	/* array element delimiter */
 			   F_ARRAY_IN,		/* input procedure */
 			   F_ARRAY_OUT,		/* output procedure */

src/bin/pg_dump/pg_dump.c

@@ -12,7 +12,7 @@
  *	by PostgreSQL
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/bin/pg_dump/pg_dump.c,v 1.498 2008/07/30 17:05:04 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/bin/pg_dump/pg_dump.c,v 1.499 2008/07/30 19:35:13 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -5780,7 +5780,7 @@ dumpBaseType(Archive *fout, TypeInfo *tinfo)
 						  "typmodin::pg_catalog.oid as typmodinoid, "
 						  "typmodout::pg_catalog.oid as typmodoutoid, "
 						  "typanalyze::pg_catalog.oid as typanalyzeoid, "
-						  "'U' as typcategory, true as typispreferred, "
+						  "'U' as typcategory, false as typispreferred, "
 						  "typdelim, typbyval, typalign, typstorage, "
 						  "pg_catalog.pg_get_expr(typdefaultbin, 'pg_catalog.pg_type'::pg_catalog.regclass) as typdefaultbin, typdefault "
 						  "FROM pg_catalog.pg_type "
@@ -5799,7 +5799,7 @@ dumpBaseType(Archive *fout, TypeInfo *tinfo)
 						  "typsend::pg_catalog.oid as typsendoid, "
 						  "0 as typmodinoid, 0 as typmodoutoid, "
 						  "typanalyze::pg_catalog.oid as typanalyzeoid, "
-						  "'U' as typcategory, true as typispreferred, "
+						  "'U' as typcategory, false as typispreferred, "
 						  "typdelim, typbyval, typalign, typstorage, "
 						  "pg_catalog.pg_get_expr(typdefaultbin, 'pg_catalog.pg_type'::pg_catalog.regclass) as typdefaultbin, typdefault "
 						  "FROM pg_catalog.pg_type "
@@ -5818,7 +5818,7 @@ dumpBaseType(Archive *fout, TypeInfo *tinfo)
 						  "typsend::pg_catalog.oid as typsendoid, "
 						  "0 as typmodinoid, 0 as typmodoutoid, "
 						  "0 as typanalyzeoid, "
-						  "'U' as typcategory, true as typispreferred, "
+						  "'U' as typcategory, false as typispreferred, "
 						  "typdelim, typbyval, typalign, typstorage, "
 						  "pg_catalog.pg_get_expr(typdefaultbin, 'pg_catalog.pg_type'::pg_catalog.regclass) as typdefaultbin, typdefault "
 						  "FROM pg_catalog.pg_type "
@@ -5837,7 +5837,7 @@ dumpBaseType(Archive *fout, TypeInfo *tinfo)
 						  "0 as typreceiveoid, 0 as typsendoid, "
 						  "0 as typmodinoid, 0 as typmodoutoid, "
 						  "0 as typanalyzeoid, "
-						  "'U' as typcategory, true as typispreferred, "
+						  "'U' as typcategory, false as typispreferred, "
 						  "typdelim, typbyval, typalign, typstorage, "
 						  "pg_catalog.pg_get_expr(typdefaultbin, 'pg_catalog.pg_type'::pg_catalog.regclass) as typdefaultbin, typdefault "
 						  "FROM pg_catalog.pg_type "
@@ -5860,7 +5860,7 @@ dumpBaseType(Archive *fout, TypeInfo *tinfo)
 						  "0 as typreceiveoid, 0 as typsendoid, "
 						  "0 as typmodinoid, 0 as typmodoutoid, "
 						  "0 as typanalyzeoid, "
-						  "'U' as typcategory, true as typispreferred, "
+						  "'U' as typcategory, false as typispreferred, "
 						  "typdelim, typbyval, typalign, typstorage, "
 						  "NULL as typdefaultbin, typdefault "
 						  "FROM pg_type "
@@ -5883,7 +5883,7 @@ dumpBaseType(Archive *fout, TypeInfo *tinfo)
 						  "0 as typreceiveoid, 0 as typsendoid, "
 						  "0 as typmodinoid, 0 as typmodoutoid, "
 						  "0 as typanalyzeoid, "
-						  "'U' as typcategory, true as typispreferred, "
+						  "'U' as typcategory, false as typispreferred, "
 						  "typdelim, typbyval, typalign, typstorage, "
 						  "NULL as typdefaultbin, NULL as typdefault "
 						  "FROM pg_type "
@@ -5902,7 +5902,7 @@ dumpBaseType(Archive *fout, TypeInfo *tinfo)
 						  "0 as typreceiveoid, 0 as typsendoid, "
 						  "0 as typmodinoid, 0 as typmodoutoid, "
 						  "0 as typanalyzeoid, "
-						  "'U' as typcategory, true as typispreferred, "
+						  "'U' as typcategory, false as typispreferred, "
 						  "typdelim, typbyval, typalign, "
 						  "'p'::char as typstorage, "
 						  "NULL as typdefaultbin, NULL as typdefault "
@@ -6022,8 +6022,8 @@ dumpBaseType(Archive *fout, TypeInfo *tinfo)
 		appendStringLiteralAH(q, typcategory, fout);
 	}
 
-	if (strcmp(typispreferred, "f") == 0)
-		appendPQExpBuffer(q, ",\n    PREFERRED = false");
+	if (strcmp(typispreferred, "t") == 0)
+		appendPQExpBuffer(q, ",\n    PREFERRED = true");
 
 	if (typdelim && strcmp(typdelim, ",") != 0)
 	{

src/include/catalog/catversion.h

@@ -37,7 +37,7 @@
  * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/catalog/catversion.h,v 1.472 2008/07/30 17:05:05 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/catalog/catversion.h,v 1.473 2008/07/30 19:35:13 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -53,6 +53,6 @@
  */
 
 /*							yyyymmddN */
-#define CATALOG_VERSION_NO	200807291
+#define CATALOG_VERSION_NO	200807301
 
 #endif

src/test/regress/expected/create_type.out

@@ -20,7 +20,7 @@ CREATE TYPE city_budget (
    output = int44out, 
    element = int4,
    category = 'x',   -- just to verify the system will take it
-   preferred = false -- ditto
+   preferred = true  -- ditto
 );
 -- Test creation and destruction of shell types
 CREATE TYPE shell;

src/test/regress/sql/create_type.sql

@@ -22,7 +22,7 @@ CREATE TYPE city_budget (
    output = int44out, 
    element = int4,
    category = 'x',   -- just to verify the system will take it
-   preferred = false -- ditto
+   preferred = true  -- ditto
 );
 
 -- Test creation and destruction of shell types