create_schema_if_not_exists_v4.patch

diff --git a/doc/src/sgml/ref/create_schema.sgml b/doc/src/sgml/ref/create_schema.sgml
index 930d876..70aeec3 100644
--- a/doc/src/sgml/ref/create_schema.sgml
+++ b/doc/src/sgml/ref/create_schema.sgml
@@ -21,8 +21,8 @@ PostgreSQL documentation
 
  <refsynopsisdiv>
 <synopsis>
-CREATE SCHEMA <replaceable class="parameter">schema_name</replaceable> [ AUTHORIZATION <replaceable class="parameter">user_name</replaceable> ] [ <replaceable class="parameter">schema_element</replaceable> [ ... ] ]
-CREATE SCHEMA AUTHORIZATION <replaceable class="parameter">user_name</replaceable> [ <replaceable class="parameter">schema_element</replaceable> [ ... ] ]
+CREATE SCHEMA [ IF NOT EXISTS ] <replaceable class="parameter">schema_name</replaceable> [ AUTHORIZATION <replaceable class="parameter">user_name</replaceable> ] [ <replaceable class="parameter">schema_element</replaceable> [ ... ] ]
+CREATE SCHEMA [ IF NOT EXISTS ] AUTHORIZATION <replaceable class="parameter">user_name</replaceable> [ <replaceable class="parameter">schema_element</replaceable> [ ... ] ]
 </synopsis>
  </refsynopsisdiv>
 
@@ -62,6 +62,16 @@ CREATE SCHEMA AUTHORIZATION <replaceable class="parameter">user_name</replaceabl
 
     <variablelist>
      <varlistentry>
+      <term><literal>IF NOT EXISTS</literal></term>
+      <listitem>
+       <para>
+        Do not throw an error if a schema with the same name already exists.
+        A notice is issued in this case.
+       </para>
+      </listitem>
+     </varlistentry>
+
+     <varlistentry>
       <term><replaceable class="parameter">schema_name</replaceable></term>
       <listitem>
        <para>
diff --git a/src/backend/commands/schemacmds.c b/src/backend/commands/schemacmds.c
index 4974025..f8a6c15 100644
--- a/src/backend/commands/schemacmds.c
+++ b/src/backend/commands/schemacmds.c
@@ -49,6 +49,7 @@ CreateSchemaCommand(CreateSchemaStmt *stmt, const char *queryString)
 	Oid			saved_uid;
 	int			save_sec_context;
 	AclResult	aclresult;
+    bool        if_not_exists = stmt->if_not_exists;
 
 	GetUserIdAndSecContext(&saved_uid, &save_sec_context);
 
@@ -93,6 +94,15 @@ CreateSchemaCommand(CreateSchemaStmt *stmt, const char *queryString)
 		SetUserIdAndSecContext(owner_uid,
 							save_sec_context | SECURITY_LOCAL_USERID_CHANGE);
 
+	/* If schema already exists, skip */
+	if (OidIsValid(get_namespace_oid(schemaName, true)) && if_not_exists) {
+        ereport(NOTICE,
+                (errcode(ERRCODE_DUPLICATE_SCHEMA),
+                 errmsg("schema \"%s\" already exists, skipping",
+                        schemaName)));
+		return;
+	}
+
 	/* Create the schema's namespace */
 	namespaceId = NamespaceCreate(schemaName, owner_uid, false);
 
diff --git a/src/backend/nodes/copyfuncs.c b/src/backend/nodes/copyfuncs.c
index 71d5323..3c51c96 100644
--- a/src/backend/nodes/copyfuncs.c
+++ b/src/backend/nodes/copyfuncs.c
@@ -3600,6 +3600,7 @@ _copyCreateSchemaStmt(const CreateSchemaStmt *from)
 	COPY_STRING_FIELD(schemaname);
 	COPY_STRING_FIELD(authid);
 	COPY_NODE_FIELD(schemaElts);
+	COPY_SCALAR_FIELD(if_not_exists);
 
 	return newnode;
 }
diff --git a/src/backend/nodes/equalfuncs.c b/src/backend/nodes/equalfuncs.c
index d690ca7..12c4538 100644
--- a/src/backend/nodes/equalfuncs.c
+++ b/src/backend/nodes/equalfuncs.c
@@ -1901,6 +1901,7 @@ _equalCreateSchemaStmt(const CreateSchemaStmt *a, const CreateSchemaStmt *b)
 	COMPARE_STRING_FIELD(schemaname);
 	COMPARE_STRING_FIELD(authid);
 	COMPARE_NODE_FIELD(schemaElts);
+	COMPARE_SCALAR_FIELD(if_not_exists);
 
 	return true;
 }
diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index 90ea1f9..487cf9e 100644
--- a/src/backend/parser/gram.y
+++ b/src/backend/parser/gram.y
@@ -1169,8 +1169,23 @@ CreateSchemaStmt:
 						n->schemaname = $5;
 					n->authid = $5;
 					n->schemaElts = $6;
+					n->if_not_exists = false;
 					$$ = (Node *)n;
 				}
+			| CREATE SCHEMA IF_P NOT EXISTS OptSchemaName AUTHORIZATION RoleId OptSchemaEltList
+				{
+					CreateSchemaStmt *n = makeNode(CreateSchemaStmt);
+					/* One can omit the schema name or the authorization id. */
+					if ($6 != NULL)
+						n->schemaname = $6;
+					else
+						n->schemaname = $8;
+					n->authid = $8;
+					n->schemaElts = $9;
+					n->if_not_exists = true;
+					$$ = (Node *)n;
+				}
+
 			| CREATE SCHEMA ColId OptSchemaEltList
 				{
 					CreateSchemaStmt *n = makeNode(CreateSchemaStmt);
@@ -1178,8 +1193,20 @@ CreateSchemaStmt:
 					n->schemaname = $3;
 					n->authid = NULL;
 					n->schemaElts = $4;
+					n->if_not_exists = false;
 					$$ = (Node *)n;
 				}
+			| CREATE SCHEMA IF_P NOT EXISTS ColId OptSchemaEltList
+				{
+					CreateSchemaStmt *n = makeNode(CreateSchemaStmt);
+					/* ...but not both */
+					n->schemaname = $6;
+					n->authid = NULL;
+					n->schemaElts = $7;
+					n->if_not_exists = true;
+					$$ = (Node *)n;
+				}
+
 		;
 
 OptSchemaName:
diff --git a/src/include/nodes/parsenodes.h b/src/include/nodes/parsenodes.h
index f433166..b2abdb1 100644
--- a/src/include/nodes/parsenodes.h
+++ b/src/include/nodes/parsenodes.h
@@ -1157,6 +1157,7 @@ typedef struct CreateSchemaStmt
 	char	   *schemaname;		/* the name of the schema to create */
 	char	   *authid;			/* the owner of the created schema */
 	List	   *schemaElts;		/* schema components (list of parsenodes) */
+	bool		if_not_exists;	/* just do nothing if schema already exists? */
 } CreateSchemaStmt;
 
 typedef enum DropBehavior
diff --git a/src/test/regress/expected/namespace.out b/src/test/regress/expected/namespace.out
index 7c26da5..9ec5b4f 100644
--- a/src/test/regress/expected/namespace.out
+++ b/src/test/regress/expected/namespace.out
@@ -9,6 +9,11 @@ CREATE SCHEMA test_schema_1
               a serial,
               b int UNIQUE
        );
+-- verify if schema already exists
+CREATE SCHEMA test_schema_1;
+ERROR:  schema "test_schema_1" already exists
+CREATE SCHEMA IF NOT EXISTS test_schema_1;
+NOTICE:  schema "test_schema_1" already exists, skipping
 -- verify that the objects were created
 SELECT COUNT(*) FROM pg_class WHERE relnamespace =
     (SELECT oid FROM pg_namespace WHERE nspname = 'test_schema_1');
diff --git a/src/test/regress/sql/namespace.sql b/src/test/regress/sql/namespace.sql
index 919f72a..b776b3e 100644
--- a/src/test/regress/sql/namespace.sql
+++ b/src/test/regress/sql/namespace.sql
@@ -13,6 +13,9 @@ CREATE SCHEMA test_schema_1
               b int UNIQUE
        );
 
+-- verify if schema already exists
+CREATE SCHEMA test_schema_1;
+CREATE SCHEMA IF NOT EXISTS test_schema_1;
 -- verify that the objects were created
 SELECT COUNT(*) FROM pg_class WHERE relnamespace =
     (SELECT oid FROM pg_namespace WHERE nspname = 'test_schema_1');