Support CREATE SCHEMA IF NOT EXISTS.

Per discussion, schema-element subcommands are not allowed together with
this option, since it's not very obvious what should happen to the element
objects.

Fabrízio de Royes Mello

doc/src/sgml/ref/create_schema.sgml

@@ -23,6 +23,8 @@ PostgreSQL documentation
 <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> ]
+CREATE SCHEMA IF NOT EXISTS AUTHORIZATION <replaceable class="parameter">user_name</replaceable>
 </synopsis>
  </refsynopsisdiv>
 
@@ -98,6 +100,17 @@ CREATE SCHEMA AUTHORIZATION <replaceable class="parameter">user_name</replaceabl
        </para>
       </listitem>
      </varlistentry>
+
+     <varlistentry>
+      <term><literal>IF NOT EXISTS</literal></term>
+      <listitem>
+       <para>
+        Do nothing (except issuing a notice) if a schema with the same name
+        already exists.  <replaceable class="parameter">schema_element</>
+        subcommands cannot be included when this option is used.
+       </para>
+      </listitem>
+     </varlistentry>
     </variablelist>
  </refsect1>
 
@@ -129,6 +142,15 @@ CREATE SCHEMA AUTHORIZATION joe;
 </programlisting>
   </para>
 
+  <para>
+   Create a schema named <literal>test</> that will be owned by user
+   <literal>joe</>, unless there already is a schema named <literal>test</>.
+   (It does not matter whether <literal>joe</> owns the pre-existing schema.)
+<programlisting>
+CREATE SCHEMA IF NOT EXISTS test AUTHORIZATION joe;
+</programlisting>
+  </para>
+
   <para>
    Create a schema and create a table and view within it:
 <programlisting>
@@ -177,6 +199,11 @@ CREATE VIEW hollywood.winners AS
    schema owner.  This can happen only if the schema owner grants the
    <literal>CREATE</> privilege on his schema to someone else.
   </para>
+
+  <para>
+   The <literal>IF NOT EXISTS</literal> option is a
+   <productname>PostgreSQL</productname> extension.
+  </para>
  </refsect1>
 
  <refsect1>

src/backend/commands/extension.c

@@ -1376,6 +1376,7 @@ CreateExtension(CreateExtensionStmt *stmt)
 			csstmt->schemaname = schemaName;
 			csstmt->authid = NULL;		/* will be created by current user */
 			csstmt->schemaElts = NIL;
+			csstmt->if_not_exists = false;
 			CreateSchemaCommand(csstmt, NULL);
 
 			/*

src/backend/commands/schemacmds.c

@@ -83,6 +83,23 @@ CreateSchemaCommand(CreateSchemaStmt *stmt, const char *queryString)
 				 errmsg("unacceptable schema name \"%s\"", schemaName),
 		   errdetail("The prefix \"pg_\" is reserved for system schemas.")));
 
+	/*
+	 * If if_not_exists was given and the schema already exists, bail out.
+	 * (Note: we needn't check this when not if_not_exists, because
+	 * NamespaceCreate will complain anyway.)  We could do this before making
+	 * the permissions checks, but since CREATE TABLE IF NOT EXISTS makes its
+	 * creation-permission check first, we do likewise.
+	 */
+	if (stmt->if_not_exists &&
+		SearchSysCacheExists1(NAMESPACENAME, PointerGetDatum(schemaName)))
+	{
+		ereport(NOTICE,
+				(errcode(ERRCODE_DUPLICATE_SCHEMA),
+				 errmsg("schema \"%s\" already exists, skipping",
+						schemaName)));
+		return;
+	}
+
 	/*
 	 * If the requested authorization is different from the current user,
 	 * temporarily set the current user so that the object(s) will be created

src/backend/nodes/copyfuncs.c

@@ -3613,6 +3613,7 @@ _copyCreateSchemaStmt(const CreateSchemaStmt *from)
 	COPY_STRING_FIELD(schemaname);
 	COPY_STRING_FIELD(authid);
 	COPY_NODE_FIELD(schemaElts);
+	COPY_SCALAR_FIELD(if_not_exists);
 
 	return newnode;
 }

src/backend/nodes/equalfuncs.c

@@ -1909,6 +1909,7 @@ _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;
 }

src/backend/parser/gram.y

@@ -1169,6 +1169,7 @@ CreateSchemaStmt:
 						n->schemaname = $5;
 					n->authid = $5;
 					n->schemaElts = $6;
+					n->if_not_exists = false;
 					$$ = (Node *)n;
 				}
 			| CREATE SCHEMA ColId OptSchemaEltList
@@ -1178,6 +1179,40 @@ CreateSchemaStmt:
 					n->schemaname = $3;
 					n->authid = NULL;
 					n->schemaElts = $4;
+					n->if_not_exists = false;
+					$$ = (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;
+					if ($9 != NIL)
+						ereport(ERROR,
+								(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+								 errmsg("CREATE SCHEMA IF NOT EXISTS cannot include schema elements"),
+								 parser_errposition(@9)));
+					n->schemaElts = $9;
+					n->if_not_exists = true;
+					$$ = (Node *)n;
+				}
+			| CREATE SCHEMA IF_P NOT EXISTS ColId OptSchemaEltList
+				{
+					CreateSchemaStmt *n = makeNode(CreateSchemaStmt);
+					/* ...but not both */
+					n->schemaname = $6;
+					n->authid = NULL;
+					if ($7 != NIL)
+						ereport(ERROR,
+								(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+								 errmsg("CREATE SCHEMA IF NOT EXISTS cannot include schema elements"),
+								 parser_errposition(@7)));
+					n->schemaElts = $7;
+					n->if_not_exists = true;
 					$$ = (Node *)n;
 				}
 		;

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

src/test/regress/expected/namespace.out

@@ -36,6 +36,19 @@ SELECT * FROM test_schema_1.abc_view;
  4 |  
 (3 rows)
 
+-- test IF NOT EXISTS cases
+CREATE SCHEMA test_schema_1; -- fail, already exists
+ERROR:  schema "test_schema_1" already exists
+CREATE SCHEMA IF NOT EXISTS test_schema_1; -- ok with notice
+NOTICE:  schema "test_schema_1" already exists, skipping
+CREATE SCHEMA IF NOT EXISTS test_schema_1 -- fail, disallowed
+       CREATE TABLE abc (
+              a serial,
+              b int UNIQUE
+       );
+ERROR:  CREATE SCHEMA IF NOT EXISTS cannot include schema elements
+LINE 1: CREATE SCHEMA IF NOT EXISTS test_schema_1 
+                                    ^
 DROP SCHEMA test_schema_1 CASCADE;
 NOTICE:  drop cascades to 2 other objects
 DETAIL:  drop cascades to table test_schema_1.abc

src/test/regress/sql/namespace.sql

@@ -24,6 +24,15 @@ INSERT INTO test_schema_1.abc DEFAULT VALUES;
 SELECT * FROM test_schema_1.abc;
 SELECT * FROM test_schema_1.abc_view;
 
+-- test IF NOT EXISTS cases
+CREATE SCHEMA test_schema_1; -- fail, already exists
+CREATE SCHEMA IF NOT EXISTS test_schema_1; -- ok with notice
+CREATE SCHEMA IF NOT EXISTS test_schema_1 -- fail, disallowed
+       CREATE TABLE abc (
+              a serial,
+              b int UNIQUE
+       );
+
 DROP SCHEMA test_schema_1 CASCADE;
 
 -- verify that the objects were dropped