create_user_mapping_if_not_exists_3.0.patch

diff --git a/doc/src/sgml/ref/create_user_mapping.sgml b/doc/src/sgml/ref/create_user_mapping.sgml
index 44fe302..9bde687 100644
--- a/doc/src/sgml/ref/create_user_mapping.sgml
+++ b/doc/src/sgml/ref/create_user_mapping.sgml
@@ -21,7 +21,7 @@ PostgreSQL documentation
 
  <refsynopsisdiv>
 <synopsis>
-CREATE USER MAPPING FOR { <replaceable class="parameter">user_name</replaceable> | USER | CURRENT_USER | PUBLIC }
+CREATE USER MAPPING [IF NOT EXISTS] FOR { <replaceable class="parameter">user_name</replaceable> | USER | CURRENT_USER | PUBLIC }
     SERVER <replaceable class="parameter">server_name</replaceable>
     [ OPTIONS ( <replaceable class="PARAMETER">option</replaceable> '<replaceable class="PARAMETER">value</replaceable>' [ , ... ] ) ]
 </synopsis>
@@ -50,6 +50,18 @@ CREATE USER MAPPING FOR { <replaceable class="parameter">user_name</replaceable>
   <title>Parameters</title>
 
   <variablelist>
+  <varlistentry>
+    <term><literal>IF NOT EXISTS</></term>
+    <listitem>
+     <para>
+      Do not throw an error if a mapping of given user to the given foreign
+      server already exists. A notice is issued in this case.  Note that there
+      is no guarantee that   the existing user mapping is anything like the one
+      that would have been created.
+     </para>
+    </listitem>
+   </varlistentry>
+
    <varlistentry>
     <term><replaceable class="parameter">user_name</replaceable></term>
     <listitem>
diff --git a/src/backend/commands/foreigncmds.c b/src/backend/commands/foreigncmds.c
index d5d40e6..8bd6ac4 100644
--- a/src/backend/commands/foreigncmds.c
+++ b/src/backend/commands/foreigncmds.c
@@ -1152,12 +1152,27 @@ CreateUserMapping(CreateUserMappingStmt *stmt)
 	umId = GetSysCacheOid2(USERMAPPINGUSERSERVER,
 						   ObjectIdGetDatum(useId),
 						   ObjectIdGetDatum(srv->serverid));
+
 	if (OidIsValid(umId))
-		ereport(ERROR,
+	{
+		if (stmt->if_not_exists)
+		{
+			ereport(NOTICE,
+				(errcode(ERRCODE_DUPLICATE_OBJECT),
+				 errmsg("user mapping \"%s\" already exists for server %s, skipping",
+						MappingUserName(useId),
+						stmt->servername)));
+
+			heap_close(rel, RowExclusiveLock);
+			return InvalidObjectAddress;
+		}
+		else
+			ereport(ERROR,
 				(errcode(ERRCODE_DUPLICATE_OBJECT),
 				 errmsg("user mapping \"%s\" already exists for server %s",
 						MappingUserName(useId),
 						stmt->servername)));
+	}
 
 	fdw = GetForeignDataWrapper(srv->fdwid);
 
diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index e7acc2d..c7cf5d7 100644
--- a/src/backend/parser/gram.y
+++ b/src/backend/parser/gram.y
@@ -4853,6 +4853,16 @@ CreateUserMappingStmt: CREATE USER MAPPING FOR auth_ident SERVER name create_gen
 					n->user = $5;
 					n->servername = $7;
 					n->options = $8;
+					n->if_not_exists = false;
+					$$ = (Node *) n;
+				}
+				| CREATE USER MAPPING IF_P NOT EXISTS FOR auth_ident SERVER name create_generic_options
+				{
+					CreateUserMappingStmt *n = makeNode(CreateUserMappingStmt);
+					n->user = $8;
+					n->servername = $10;
+					n->options = $11;
+					n->if_not_exists = true;
 					$$ = (Node *) n;
 				}
 		;
diff --git a/src/include/nodes/parsenodes.h b/src/include/nodes/parsenodes.h
index a44d217..7954bc8 100644
--- a/src/include/nodes/parsenodes.h
+++ b/src/include/nodes/parsenodes.h
@@ -2185,6 +2185,7 @@ typedef struct CreateUserMappingStmt
 	NodeTag		type;
 	RoleSpec   *user;			/* user role */
 	char	   *servername;		/* server name */
+	bool		if_not_exists;	/* just do nothing if it already exists? */
 	List	   *options;		/* generic options to server */
 } CreateUserMappingStmt;
 
diff --git a/src/test/regress/expected/foreign_data.out b/src/test/regress/expected/foreign_data.out
index a0f969f..29fec07 100644
--- a/src/test/regress/expected/foreign_data.out
+++ b/src/test/regress/expected/foreign_data.out
@@ -221,6 +221,10 @@ CREATE FOREIGN DATA WRAPPER foo;
 CREATE SERVER s1 FOREIGN DATA WRAPPER foo;
 COMMENT ON SERVER s1 IS 'foreign server';
 CREATE USER MAPPING FOR current_user SERVER s1;
+CREATE USER MAPPING FOR current_user SERVER s1;				-- ERROR
+ERROR:  user mapping "regress_foreign_data_user" already exists for server s1
+CREATE USER MAPPING IF NOT EXISTS FOR current_user SERVER s1; -- NOTICE
+NOTICE:  user mapping "regress_foreign_data_user" already exists for server s1, skipping
 \dew+
                                                 List of foreign-data wrappers
     Name    |           Owner           | Handler |        Validator         | Access privileges | FDW Options | Description 
diff --git a/src/test/regress/sql/foreign_data.sql b/src/test/regress/sql/foreign_data.sql
index c13d5ff..abea0f9 100644
--- a/src/test/regress/sql/foreign_data.sql
+++ b/src/test/regress/sql/foreign_data.sql
@@ -104,6 +104,8 @@ CREATE FOREIGN DATA WRAPPER foo;
 CREATE SERVER s1 FOREIGN DATA WRAPPER foo;
 COMMENT ON SERVER s1 IS 'foreign server';
 CREATE USER MAPPING FOR current_user SERVER s1;
+CREATE USER MAPPING FOR current_user SERVER s1;				-- ERROR
+CREATE USER MAPPING IF NOT EXISTS FOR current_user SERVER s1; -- NOTICE
 \dew+
 \des+
 \deu+