Implement IF NOT EXISTS for CREATE SEQUENCE.

Fabrízio de Royes Mello

doc/src/sgml/ref/create_sequence.sgml

@@ -21,7 +21,7 @@ PostgreSQL documentation
 
  <refsynopsisdiv>
 <synopsis>
-CREATE [ TEMPORARY | TEMP ] SEQUENCE <replaceable class="parameter">name</replaceable> [ INCREMENT [ BY ] <replaceable class="parameter">increment</replaceable> ]
+CREATE [ TEMPORARY | TEMP ] [ IF NOT EXISTS ] SEQUENCE <replaceable class="parameter">name</replaceable> [ INCREMENT [ BY ] <replaceable class="parameter">increment</replaceable> ]
     [ MINVALUE <replaceable class="parameter">minvalue</replaceable> | NO MINVALUE ] [ MAXVALUE <replaceable class="parameter">maxvalue</replaceable> | NO MAXVALUE ]
     [ START [ WITH ] <replaceable class="parameter">start</replaceable> ] [ CACHE <replaceable class="parameter">cache</replaceable> ] [ [ NO ] CYCLE ]
     [ OWNED BY { <replaceable class="parameter">table_name</replaceable>.<replaceable class="parameter">column_name</replaceable> | NONE } ]
@@ -89,6 +89,18 @@ SELECT * FROM <replaceable>name</replaceable>;
     </listitem>
    </varlistentry>
 
+   <varlistentry>
+    <term><literal>IF NOT EXISTS</literal></term>
+    <listitem>
+     <para>
+      Do not throw an error if a relation with the same name already exists.
+      A notice is issued in this case. Note that there is no guarantee that
+      the existing relation is anything like the sequence that would have
+      been created - it might not even be a sequence.
+     </para>
+    </listitem>
+   </varlistentry>
+
    <varlistentry>
     <term><replaceable class="parameter">name</replaceable></term>
     <listitem>

src/backend/commands/sequence.c

@@ -122,6 +122,24 @@ DefineSequence(CreateSeqStmt *seq)
 				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
 				 errmsg("unlogged sequences are not supported")));
 
+	/*
+	 * If if_not_exists was given and a relation with the same name already
+	 * exists, bail out. (Note: we needn't check this when not if_not_exists,
+	 * because DefineRelation will complain anyway.)
+	 */
+	if (seq->if_not_exists)
+	{
+		RangeVarGetAndCheckCreationNamespace(seq->sequence, NoLock, &seqoid);
+		if (OidIsValid(seqoid))
+		{
+			ereport(NOTICE,
+					(errcode(ERRCODE_DUPLICATE_TABLE),
+					 errmsg("relation \"%s\" already exists, skipping",
+							seq->sequence->relname)));
+			return InvalidOid;
+		}
+	}
+
 	/* Check and set all option values */
 	init_params(seq->options, true, &new, &owned_by);
 
@@ -210,7 +228,7 @@ DefineSequence(CreateSeqStmt *seq)
 	stmt->options = NIL;
 	stmt->oncommit = ONCOMMIT_NOOP;
 	stmt->tablespacename = NULL;
-	stmt->if_not_exists = false;
+	stmt->if_not_exists = seq->if_not_exists;
 
 	seqoid = DefineRelation(stmt, RELKIND_SEQUENCE, seq->ownerId);
 	Assert(seqoid != InvalidOid);

src/backend/nodes/copyfuncs.c

@@ -3330,6 +3330,7 @@ _copyCreateSeqStmt(const CreateSeqStmt *from)
 	COPY_NODE_FIELD(sequence);
 	COPY_NODE_FIELD(options);
 	COPY_SCALAR_FIELD(ownerId);
+	COPY_SCALAR_FIELD(if_not_exists);
 
 	return newnode;
 }

src/backend/nodes/equalfuncs.c

@@ -1566,6 +1566,7 @@ _equalCreateSeqStmt(const CreateSeqStmt *a, const CreateSeqStmt *b)
 	COMPARE_NODE_FIELD(sequence);
 	COMPARE_NODE_FIELD(options);
 	COMPARE_SCALAR_FIELD(ownerId);
+	COMPARE_SCALAR_FIELD(if_not_exists);
 
 	return true;
 }

src/backend/parser/gram.y

@@ -3486,6 +3486,17 @@ CreateSeqStmt:
 					n->sequence = $4;
 					n->options = $5;
 					n->ownerId = InvalidOid;
+					n->if_not_exists = false;
+					$$ = (Node *)n;
+				}
+			| CREATE OptTemp SEQUENCE IF_P NOT EXISTS qualified_name OptSeqOptList
+				{
+					CreateSeqStmt *n = makeNode(CreateSeqStmt);
+					$7->relpersistence = $2;
+					n->sequence = $7;
+					n->options = $8;
+					n->ownerId = InvalidOid;
+					n->if_not_exists = true;
 					$$ = (Node *)n;
 				}
 		;

src/include/nodes/parsenodes.h

@@ -1979,6 +1979,7 @@ typedef struct CreateSeqStmt
 	RangeVar   *sequence;		/* the sequence to create */
 	List	   *options;
 	Oid			ownerId;		/* ID of owner, or InvalidOid for default */
+	bool		if_not_exists;	/* just do nothing if it already exists? */
 } CreateSeqStmt;
 
 typedef struct AlterSeqStmt

src/test/regress/expected/sequence.out

@@ -91,6 +91,8 @@ SELECT nextval('serialTest2_f6_seq');
 
 -- basic sequence operations using both text and oid references
 CREATE SEQUENCE sequence_test;
+CREATE SEQUENCE IF NOT EXISTS sequence_test;
+NOTICE:  relation "sequence_test" already exists, skipping
 SELECT nextval('sequence_test'::text);
  nextval 
 ---------

src/test/regress/sql/sequence.sql

@@ -59,6 +59,7 @@ SELECT nextval('serialTest2_f6_seq');
 
 -- basic sequence operations using both text and oid references
 CREATE SEQUENCE sequence_test;
+CREATE SEQUENCE IF NOT EXISTS sequence_test;
 
 SELECT nextval('sequence_test'::text);
 SELECT nextval('sequence_test'::regclass);