commit 4237a23d6674da774b0772ffc953e5f499c04803
Author: Anastasia <a.lubennikova@postgrespro.ru>
Date:   Fri Jan 13 16:23:53 2017 +0300

    Add [IF NOT EXISTS] option to CREATE USER MAPPING statement

diff --git a/doc/src/sgml/ref/create_user_mapping.sgml b/doc/src/sgml/ref/create_user_mapping.sgml
index 44fe302..680906b 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>
@@ -49,6 +49,18 @@ CREATE USER MAPPING FOR { <replaceable class="parameter">user_name</replaceable>
  <refsect1>
   <title>Parameters</title>
 
+  <varlistentry>
+    <term><literal>IF NOT EXISTS</></term>
+    <listitem>
+     <para>
+      Do not throw an error if a user mapping with the same name 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>
+
   <variablelist>
    <varlistentry>
     <term><replaceable class="parameter">user_name</replaceable></term>
diff --git a/src/backend/commands/foreigncmds.c b/src/backend/commands/foreigncmds.c
index 0b0114c..32fa6a5 100644
--- a/src/backend/commands/foreigncmds.c
+++ b/src/backend/commands/foreigncmds.c
@@ -1172,12 +1172,27 @@ 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 ab4b3b1..2fb713f 100644
--- a/src/backend/parser/gram.y
+++ b/src/backend/parser/gram.y
@@ -4897,6 +4897,16 @@ 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 8a79ec0..bb7c02f 100644
--- a/src/include/nodes/parsenodes.h
+++ b/src/include/nodes/parsenodes.h
@@ -2110,6 +2110,7 @@ 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;