Add a WINDOW attribute to CREATE FUNCTION, and teach pg_dump about it,

so that user-defined window functions are possible.  For the moment you'll
have to write them in C, for lack of any interface to the WindowObject API
in the available PLs, but it's better than no support at all.

There was some debate about the best syntax for this.  I ended up choosing
the "it's an attribute" position --- the other approach will inevitably be
more work, and the likely market for user-defined window functions is
probably too small to justify it.

doc/src/sgml/ref/create_function.sgml

 <!--
-$PostgreSQL: pgsql/doc/src/sgml/ref/create_function.sgml,v 1.83 2008/12/18 18:20:33 tgl Exp $
+$PostgreSQL: pgsql/doc/src/sgml/ref/create_function.sgml,v 1.84 2008/12/31 02:25:03 tgl Exp $
 -->
 
 <refentry id="SQL-CREATEFUNCTION">
@@ -25,6 +25,7 @@ CREATE [ OR REPLACE ] FUNCTION
     [ RETURNS <replaceable class="parameter">rettype</replaceable>
       | RETURNS TABLE ( <replaceable class="parameter">colname</replaceable> <replaceable class="parameter">coltype</replaceable> [, ...] ) ]
   { LANGUAGE <replaceable class="parameter">langname</replaceable>
+    | WINDOW
     | IMMUTABLE | STABLE | VOLATILE
     | CALLED ON NULL INPUT | RETURNS NULL ON NULL INPUT | STRICT
     | [ EXTERNAL ] SECURITY INVOKER | [ EXTERNAL ] SECURITY DEFINER
@@ -241,6 +242,20 @@ CREATE [ OR REPLACE ] FUNCTION
      </listitem>
     </varlistentry>
 
+    <varlistentry>
+     <term><literal>WINDOW</literal></term>
+
+     <listitem>
+      <para>
+       <literal>WINDOW</literal> indicates that the function is a
+       <firstterm>window function</> rather than a plain function.
+       This is currently only useful for functions written in C.
+       The <literal>WINDOW</> attribute cannot be changed when
+       replacing an existing function definition.
+      </para>
+     </listitem>
+    </varlistentry>
+
     <varlistentry>
      <term><literal>IMMUTABLE</literal></term>
      <term><literal>STABLE</literal></term>

src/backend/catalog/pg_aggregate.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/catalog/pg_aggregate.c,v 1.99 2008/12/18 18:20:33 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/catalog/pg_aggregate.c,v 1.100 2008/12/31 02:25:03 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -217,6 +217,7 @@ AggregateCreate(const char *aggName,
 							  "aggregate_dummy",		/* placeholder proc */
 							  NULL,		/* probin */
 							  true,		/* isAgg */
+							  false,	/* isWindowFunc */
 							  false,	/* security invoker (currently not
 										 * definable for agg) */
 							  false,	/* isStrict (not needed for agg) */

src/backend/catalog/pg_proc.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/catalog/pg_proc.c,v 1.158 2008/12/28 18:53:54 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/catalog/pg_proc.c,v 1.159 2008/12/31 02:25:03 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -53,7 +53,7 @@ static bool match_prosrc_to_literal(const char *prosrc, const char *literal,
  *
  * Note: allParameterTypes, parameterModes, parameterNames, and proconfig
  * are either arrays of the proper types or NULL.  We declare them Datum,
- * not "ArrayType *", to avoid importing array.h into pg_proc.h.
+ * not "ArrayType *", to avoid importing array.h into pg_proc_fn.h.
  * ----------------------------------------------------------------
  */
 Oid
@@ -67,6 +67,7 @@ ProcedureCreate(const char *procedureName,
 				const char *prosrc,
 				const char *probin,
 				bool isAgg,
+				bool isWindowFunc,
 				bool security_definer,
 				bool isStrict,
 				char volatility,
@@ -80,8 +81,6 @@ ProcedureCreate(const char *procedureName,
 				float4 prorows)
 {
 	Oid			retval;
-	/* XXX we don't currently have a way to make new window functions */
-	bool		isWindowFunc = false;
 	int			parameterCount;
 	int			allParamCount;
 	Oid		   *allParams;

src/backend/commands/functioncmds.c

@@ -10,7 +10,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/functioncmds.c,v 1.104 2008/12/28 18:53:55 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/functioncmds.c,v 1.105 2008/12/31 02:25:03 tgl Exp $
  *
  * DESCRIPTION
  *	  These routines take the parse tree and pick out the
@@ -503,6 +503,7 @@ static void
 compute_attributes_sql_style(List *options,
 							 List **as,
 							 char **language,
+							 bool *windowfunc_p,
 							 char *volatility_p,
 							 bool *strict_p,
 							 bool *security_definer,
@@ -513,6 +514,7 @@ compute_attributes_sql_style(List *options,
 	ListCell   *option;
 	DefElem    *as_item = NULL;
 	DefElem    *language_item = NULL;
+	DefElem    *windowfunc_item = NULL;
 	DefElem    *volatility_item = NULL;
 	DefElem    *strict_item = NULL;
 	DefElem    *security_item = NULL;
@@ -540,6 +542,14 @@ compute_attributes_sql_style(List *options,
 						 errmsg("conflicting or redundant options")));
 			language_item = defel;
 		}
+		else if (strcmp(defel->defname, "window") == 0)
+		{
+			if (windowfunc_item)
+				ereport(ERROR,
+						(errcode(ERRCODE_SYNTAX_ERROR),
+						 errmsg("conflicting or redundant options")));
+			windowfunc_item = defel;
+		}
 		else if (compute_common_attribute(defel,
 										  &volatility_item,
 										  &strict_item,
@@ -578,6 +588,8 @@ compute_attributes_sql_style(List *options,
 	}
 
 	/* process optional items */
+	if (windowfunc_item)
+		*windowfunc_p = intVal(windowfunc_item->arg);
 	if (volatility_item)
 		*volatility_p = interpret_func_volatility(volatility_item);
 	if (strict_item)
@@ -735,7 +747,8 @@ CreateFunction(CreateFunctionStmt *stmt, const char *queryString)
 	ArrayType  *parameterNames;
 	List	   *parameterDefaults;
 	Oid			requiredResultType;
-	bool		isStrict,
+	bool		isWindowFunc,
+				isStrict,
 				security;
 	char		volatility;
 	ArrayType  *proconfig;
@@ -756,6 +769,7 @@ CreateFunction(CreateFunctionStmt *stmt, const char *queryString)
 					   get_namespace_name(namespaceId));
 
 	/* default attributes */
+	isWindowFunc = false;
 	isStrict = false;
 	security = false;
 	volatility = PROVOLATILE_VOLATILE;
@@ -766,7 +780,8 @@ CreateFunction(CreateFunctionStmt *stmt, const char *queryString)
 	/* override attributes from explicit list */
 	compute_attributes_sql_style(stmt->options,
 								 &as_clause, &language,
-								 &volatility, &isStrict, &security,
+								 &isWindowFunc, &volatility,
+								 &isStrict, &security,
 								 &proconfig, &procost, &prorows);
 
 	/* Convert language name to canonical case */
@@ -892,6 +907,7 @@ CreateFunction(CreateFunctionStmt *stmt, const char *queryString)
 					prosrc_str, /* converted to text later */
 					probin_str, /* converted to text later */
 					false,		/* not an aggregate */
+					isWindowFunc,
 					security,
 					isStrict,
 					volatility,

src/backend/commands/proclang.c

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/proclang.c,v 1.82 2008/12/18 18:20:33 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/proclang.c,v 1.83 2008/12/31 02:25:03 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -140,6 +140,7 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 										 pltemplate->tmplhandler,
 										 pltemplate->tmpllibrary,
 										 false, /* isAgg */
+										 false,	/* isWindowFunc */
 										 false, /* security_definer */
 										 false, /* isStrict */
 										 PROVOLATILE_VOLATILE,
@@ -174,6 +175,7 @@ CreateProceduralLanguage(CreatePLangStmt *stmt)
 										 pltemplate->tmplvalidator,
 										 pltemplate->tmpllibrary,
 										 false, /* isAgg */
+										 false,	/* isWindowFunc */
 										 false, /* security_definer */
 										 false, /* isStrict */
 										 PROVOLATILE_VOLATILE,

src/backend/parser/gram.y

@@ -11,7 +11,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/parser/gram.y,v 2.649 2008/12/31 00:08:36 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/parser/gram.y,v 2.650 2008/12/31 02:25:04 tgl Exp $
  *
  * HISTORY
  *	  AUTHOR			DATE			MAJOR EVENT
@@ -4801,6 +4801,10 @@ createfunc_opt_item:
 				{
 					$$ = makeDefElem("language", (Node *)makeString($2));
 				}
+			| WINDOW
+				{
+					$$ = makeDefElem("window", (Node *)makeInteger(TRUE));
+				}
 			| common_func_opt_item
 				{
 					$$ = $1;

src/backend/utils/adt/ruleutils.c

@@ -9,7 +9,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/adt/ruleutils.c,v 1.292 2008/12/31 00:08:37 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/adt/ruleutils.c,v 1.293 2008/12/31 02:25:04 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1466,6 +1466,8 @@ pg_get_functiondef(PG_FUNCTION_ARGS)
 	/* Emit some miscellaneous options on one line */
 	oldlen = buf.len;
 
+	if (proc->proiswindow)
+		appendStringInfoString(&buf, " WINDOW");
 	switch (proc->provolatile)
 	{
 		case PROVOLATILE_IMMUTABLE:

src/bin/pg_dump/pg_dump.c

@@ -12,7 +12,7 @@
  *	by PostgreSQL
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/bin/pg_dump/pg_dump.c,v 1.509 2008/12/19 16:25:18 petere Exp $
+ *	  $PostgreSQL: pgsql/src/bin/pg_dump/pg_dump.c,v 1.510 2008/12/31 02:25:05 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -6916,6 +6916,7 @@ dumpFunc(Archive *fout, FuncInfo *finfo)
 	char	   *proallargtypes;
 	char	   *proargmodes;
 	char	   *proargnames;
+	char	   *proiswindow;
 	char	   *provolatile;
 	char	   *proisstrict;
 	char	   *prosecdef;
@@ -6956,7 +6957,7 @@ dumpFunc(Archive *fout, FuncInfo *finfo)
 						  "pg_catalog.pg_get_function_arguments(oid) as funcargs, "
 						  "pg_catalog.pg_get_function_identity_arguments(oid) as funciargs, "
 						  "pg_catalog.pg_get_function_result(oid) as funcresult, "
-						  "provolatile, proisstrict, prosecdef, "
+						  "proiswindow, provolatile, proisstrict, prosecdef, "
 						  "proconfig, procost, prorows, "
 						  "(SELECT lanname FROM pg_catalog.pg_language WHERE oid = prolang) as lanname "
 						  "FROM pg_catalog.pg_proc "
@@ -6968,6 +6969,7 @@ dumpFunc(Archive *fout, FuncInfo *finfo)
 		appendPQExpBuffer(query,
 						  "SELECT proretset, prosrc, probin, "
 						  "proallargtypes, proargmodes, proargnames, "
+						  "false as proiswindow, "
 						  "provolatile, proisstrict, prosecdef, "
 						  "proconfig, procost, prorows, "
 						  "(SELECT lanname FROM pg_catalog.pg_language WHERE oid = prolang) as lanname "
@@ -6980,6 +6982,7 @@ dumpFunc(Archive *fout, FuncInfo *finfo)
 		appendPQExpBuffer(query,
 						  "SELECT proretset, prosrc, probin, "
 						  "proallargtypes, proargmodes, proargnames, "
+						  "false as proiswindow, "
 						  "provolatile, proisstrict, prosecdef, "
 						  "null as proconfig, 0 as procost, 0 as prorows, "
 						  "(SELECT lanname FROM pg_catalog.pg_language WHERE oid = prolang) as lanname "
@@ -6994,6 +6997,7 @@ dumpFunc(Archive *fout, FuncInfo *finfo)
 						  "null as proallargtypes, "
 						  "null as proargmodes, "
 						  "proargnames, "
+						  "false as proiswindow, "
 						  "provolatile, proisstrict, prosecdef, "
 						  "null as proconfig, 0 as procost, 0 as prorows, "
 						  "(SELECT lanname FROM pg_catalog.pg_language WHERE oid = prolang) as lanname "
@@ -7008,6 +7012,7 @@ dumpFunc(Archive *fout, FuncInfo *finfo)
 						  "null as proallargtypes, "
 						  "null as proargmodes, "
 						  "null as proargnames, "
+						  "false as proiswindow, "
 						  "provolatile, proisstrict, prosecdef, "
 						  "null as proconfig, 0 as procost, 0 as prorows, "
 						  "(SELECT lanname FROM pg_catalog.pg_language WHERE oid = prolang) as lanname "
@@ -7022,9 +7027,10 @@ dumpFunc(Archive *fout, FuncInfo *finfo)
 						  "null as proallargtypes, "
 						  "null as proargmodes, "
 						  "null as proargnames, "
+						  "false as proiswindow, "
 			 "case when proiscachable then 'i' else 'v' end as provolatile, "
 						  "proisstrict, "
-						  "'f'::boolean as prosecdef, "
+						  "false as prosecdef, "
 						  "null as proconfig, 0 as procost, 0 as prorows, "
 		  "(SELECT lanname FROM pg_language WHERE oid = prolang) as lanname "
 						  "FROM pg_proc "
@@ -7038,9 +7044,10 @@ dumpFunc(Archive *fout, FuncInfo *finfo)
 						  "null as proallargtypes, "
 						  "null as proargmodes, "
 						  "null as proargnames, "
+						  "false as proiswindow, "
 			 "case when proiscachable then 'i' else 'v' end as provolatile, "
-						  "'f'::boolean as proisstrict, "
-						  "'f'::boolean as prosecdef, "
+						  "false as proisstrict, "
+						  "false as prosecdef, "
 						  "null as proconfig, 0 as procost, 0 as prorows, "
 		  "(SELECT lanname FROM pg_language WHERE oid = prolang) as lanname "
 						  "FROM pg_proc "
@@ -7077,6 +7084,7 @@ dumpFunc(Archive *fout, FuncInfo *finfo)
 		proargnames = PQgetvalue(res, 0, PQfnumber(res, "proargnames"));
 		funcargs = funciargs = funcresult = NULL;
 	}
+	proiswindow = PQgetvalue(res, 0, PQfnumber(res, "proiswindow"));
 	provolatile = PQgetvalue(res, 0, PQfnumber(res, "provolatile"));
 	proisstrict = PQgetvalue(res, 0, PQfnumber(res, "proisstrict"));
 	prosecdef = PQgetvalue(res, 0, PQfnumber(res, "prosecdef"));
@@ -7217,6 +7225,10 @@ dumpFunc(Archive *fout, FuncInfo *finfo)
 	}
 
 	appendPQExpBuffer(q, "\n    LANGUAGE %s", fmtId(lanname));
+
+	if (proiswindow[0] == 't')
+		appendPQExpBuffer(q, " WINDOW");
+
 	if (provolatile[0] != PROVOLATILE_VOLATILE)
 	{
 		if (provolatile[0] == PROVOLATILE_IMMUTABLE)

src/include/catalog/pg_proc_fn.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/catalog/pg_proc_fn.h,v 1.4 2008/12/18 18:20:35 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/catalog/pg_proc_fn.h,v 1.5 2008/12/31 02:25:06 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -26,6 +26,7 @@ extern Oid ProcedureCreate(const char *procedureName,
 				const char *prosrc,
 				const char *probin,
 				bool isAgg,
+				bool isWindowFunc,
 				bool security_definer,
 				bool isStrict,
 				char volatility,