Department of second thoughts: make checks for replacing a view slightly

more flexible, and improve the error reporting.  Also, add documentation
for REPLACE RULE/VIEW.

doc/src/sgml/ref/create_rule.sgml

 <!--
-$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_rule.sgml,v 1.36 2002/05/18 15:44:47 petere Exp $
+$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_rule.sgml,v 1.37 2002/09/02 20:04:39 tgl Exp $
 PostgreSQL documentation
 -->
 
@@ -21,17 +21,15 @@ PostgreSQL documentation
    <date>2001-01-05</date>
   </refsynopsisdivinfo>
   <synopsis>
-CREATE RULE <replaceable class="parameter">name</replaceable> AS ON <replaceable class="parameter">event</replaceable>
+CREATE [ OR REPLACE ] RULE <replaceable class="parameter">name</replaceable> AS ON <replaceable class="parameter">event</replaceable>
     TO <replaceable class="parameter">table</replaceable> [ WHERE <replaceable class="parameter">condition</replaceable> ]
     DO [ INSTEAD ] <replaceable class="parameter">action</replaceable>
 
 where <replaceable class="PARAMETER">action</replaceable> can be:
 
 NOTHING
-|
-<replaceable class="parameter">query</replaceable>
-|
-( <replaceable class="parameter">query</replaceable> ; <replaceable class="parameter">query</replaceable> ... )
+| <replaceable class="parameter">query</replaceable>
+| ( <replaceable class="parameter">query</replaceable> ; <replaceable class="parameter">query</replaceable> ... )
   </synopsis>
 
   <refsect2 id="R2-SQL-CREATERULE-1">
@@ -76,9 +74,10 @@ NOTHING
       <term><replaceable class="parameter">condition</replaceable></term>
       <listitem>
        <para>
-	Any SQL conditional expression (returning <type>boolean</type>).  The condition expression may not
+	Any SQL conditional expression (returning <type>boolean</type>).
+	The condition expression may not
 	refer to any tables except <literal>new</literal> and
-	<literal>old</literal>.
+	<literal>old</literal>, and may not contain aggregate functions.
        </para>
       </listitem>
      </varlistentry>
@@ -142,6 +141,14 @@ CREATE RULE
    Description
   </title>
 
+  <para>
+   <command>CREATE RULE</command> defines a new rule applying to a specified
+   table or view.
+   <command>CREATE OR REPLACE RULE</command> will either create a
+   new rule, or replace an existing rule of the same name for the same
+   table.
+  </para>
+
   <para>
    The <productname>PostgreSQL</productname> 
    <firstterm>rule system</firstterm> allows one to define an
@@ -318,7 +325,7 @@ UPDATE mytable SET name = 'foo' WHERE id = 42;
    </title>
 
    <para>
-    <command>CREATE RULE</command> statement is a <productname>PostgreSQL</productname>
+    <command>CREATE RULE</command> is a <productname>PostgreSQL</productname>
     language extension.
     There is no <command>CREATE RULE</command> statement in <acronym>SQL92</acronym>.
    </para>

doc/src/sgml/ref/create_view.sgml

 <!--
-$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_view.sgml,v 1.18 2002/05/18 15:44:47 petere Exp $
+$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_view.sgml,v 1.19 2002/09/02 20:04:39 tgl Exp $
 PostgreSQL documentation
 -->
 
@@ -21,7 +21,7 @@ PostgreSQL documentation
    <date>2000-03-25</date>
   </refsynopsisdivinfo>
   <synopsis>
-CREATE VIEW <replaceable class="PARAMETER">view</replaceable> [ ( <replaceable
+CREATE [ OR REPLACE ] VIEW <replaceable class="PARAMETER">view</replaceable> [ ( <replaceable
 class="PARAMETER">column name list</replaceable> ) ] AS SELECT <replaceable class="PARAMETER">query</replaceable>
   </synopsis>
 
@@ -132,13 +132,21 @@ CREATE VIEW vista AS SELECT text 'Hello World'
   <title>
    Description
   </title>
+
   <para>
-   <command>CREATE VIEW</command> will define a view of a query.
+   <command>CREATE VIEW</command> defines a view of a query.
    The view is not physically materialized. Instead, a query
    rewrite rule (an <literal>ON SELECT</> rule) is automatically generated to
    support SELECT operations on views.
   </para>
 
+  <para>
+   <command>CREATE OR REPLACE VIEW</command> is similar, but if a view
+   of the same name already exists, it is replaced.  You can only replace
+   a view with a new query that generates the identical set of columns
+   (i.e., same column names and data types).
+  </para>
+
   <para>
    If a schema name is given (for example, <literal>CREATE VIEW
    myschema.myview ...</>) then the view is created in the
@@ -206,6 +214,7 @@ SELECT * FROM kinds;
    <title>
     SQL92
    </title>
+
    <para>
     SQL92 specifies some additional capabilities for the
     <command>CREATE VIEW</command> statement:
@@ -253,6 +262,12 @@ CREATE VIEW <replaceable class="parameter">view</replaceable> [ <replaceable cla
      </varlistentry>
     </variablelist>
    </para>
+
+   <para>
+    <command>CREATE OR REPLACE VIEW</command> is a
+    <productname>PostgreSQL</productname> language extension.
+   </para>
+
   </refsect2>
  </refsect1>
 </refentry>

doc/src/sgml/release.sgml

 <!--
-$Header: /cvsroot/pgsql/doc/src/sgml/release.sgml,v 1.156 2002/08/30 22:18:05 tgl Exp $
+$Header: /cvsroot/pgsql/doc/src/sgml/release.sgml,v 1.157 2002/09/02 20:04:39 tgl Exp $
 -->
 
 <appendix id="release">
@@ -24,6 +24,7 @@ CDATA means the content is "SGML-free", so you can write without
 worries about funny characters.
 -->
 <literallayout><![CDATA[
+CREATE OR REPLACE VIEW, CREATE OR REPLACE RULE are available
 No-autocommit mode is available (set autocommit to off)
 Substantial improvements in functionality for functions returning sets
 Client libraries older than 6.3 no longer supported (version 0 protocol removed)

src/backend/commands/view.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $Header: /cvsroot/pgsql/src/backend/commands/view.c,v 1.69 2002/09/02 02:13:01 tgl Exp $
+ *	  $Header: /cvsroot/pgsql/src/backend/commands/view.c,v 1.70 2002/09/02 20:04:40 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -29,6 +29,9 @@
 #include "utils/lsyscache.h"
 
 
+static void checkViewTupleDesc(TupleDesc newdesc, TupleDesc olddesc);
+
+
 /*---------------------------------------------------------------------
  * DefineVirtualRelation
  *
@@ -111,15 +114,9 @@ DefineVirtualRelation(const RangeVar *relation, List *tlist, bool replace)
 		/*
 		 * Create a tuple descriptor to compare against the existing view,
 		 * and verify it matches.
-		 *
-		 * XXX the error message is a bit cheesy here: would be useful to
-		 * give a more specific complaint about the difference in the
-		 * descriptors.  No time for it at the moment though.
 		 */
 	    descriptor = BuildDescForRelation(attrList);
-		if (!equalTupleDescs(descriptor, rel->rd_att))
-			elog(ERROR, "Cannot change column set of existing view %s",
-				 RelationGetRelationName(rel));
+		checkViewTupleDesc(descriptor, rel->rd_att);
 
 		/*
 		 * Seems okay, so return the OID of the pre-existing view.
@@ -149,6 +146,46 @@ DefineVirtualRelation(const RangeVar *relation, List *tlist, bool replace)
 	}
 }
 
+/*
+ * Verify that tupledesc associated with proposed new view definition
+ * matches tupledesc of old view.  This is basically a cut-down version
+ * of equalTupleDescs(), with code added to generate specific complaints.
+ */
+static void
+checkViewTupleDesc(TupleDesc newdesc, TupleDesc olddesc)
+{
+	int			i;
+
+	if (newdesc->natts != olddesc->natts)
+		elog(ERROR, "Cannot change number of columns in view");
+	/* we can ignore tdhasoid */
+
+	for (i = 0; i < newdesc->natts; i++)
+	{
+		Form_pg_attribute newattr = newdesc->attrs[i];
+		Form_pg_attribute oldattr = olddesc->attrs[i];
+
+		/* XXX not right, but we don't support DROP COL on view anyway */
+		if (newattr->attisdropped != oldattr->attisdropped)
+			elog(ERROR, "Cannot change number of columns in view");
+
+		if (strcmp(NameStr(newattr->attname), NameStr(oldattr->attname)) != 0)
+			elog(ERROR, "Cannot change name of view column \"%s\"",
+				 NameStr(oldattr->attname));
+		/* XXX would it be safe to allow atttypmod to change?  Not sure */
+		if (newattr->atttypid != oldattr->atttypid ||
+			newattr->atttypmod != oldattr->atttypmod)
+			elog(ERROR, "Cannot change datatype of view column \"%s\"",
+				 NameStr(oldattr->attname));
+		/* We can ignore the remaining attributes of an attribute... */
+	}
+	/*
+	 * We ignore the constraint fields.  The new view desc can't have any
+	 * constraints, and the only ones that could be on the old view are
+	 * defaults, which we are happy to leave in place.
+	 */
+}
+
 static RuleStmt *
 FormViewRetrieveRule(const RangeVar *view, Query *viewParse, bool replace)
 {

src/test/regress/expected/create_view.out

@@ -44,14 +44,14 @@ SELECT * FROM viewtest;
 -- should fail
 CREATE OR REPLACE VIEW viewtest AS
 	SELECT a FROM viewtest_tbl WHERE a <> 20;
-ERROR:  Cannot change column set of existing view viewtest
+ERROR:  Cannot change number of columns in view
 -- should fail
 CREATE OR REPLACE VIEW viewtest AS
 	SELECT 1, * FROM viewtest_tbl;
-ERROR:  Cannot change column set of existing view viewtest
+ERROR:  Cannot change number of columns in view
 -- should fail
 CREATE OR REPLACE VIEW viewtest AS
 	SELECT a, b::numeric FROM viewtest_tbl;
-ERROR:  Cannot change column set of existing view viewtest
+ERROR:  Cannot change datatype of view column "b"
 DROP VIEW viewtest;
 DROP TABLE viewtest_tbl;