Implement IF NOT EXIST for CREATE INDEX.

Fabrízio de Royes Mello, reviewed by Marti Raudsepp, Adam Brightwell and me.

src/backend/catalog/index.c

@@ -674,6 +674,8 @@ UpdateIndexRelation(Oid indexoid,
  *		will be marked "invalid" and the caller must take additional steps
  *		to fix it up.
  * is_internal: if true, post creation hook for new index
+ * if_not_exists: if true, do not throw an error if a relation with
+ * 	the same name already exists.
  *
  * Returns the OID of the created index.
  */
@@ -697,7 +699,8 @@ index_create(Relation heapRelation,
 			 bool allow_system_table_mods,
 			 bool skip_build,
 			 bool concurrent,
-			 bool is_internal)
+			 bool is_internal,
+			 bool if_not_exists)
 {
 	Oid			heapRelationId = RelationGetRelid(heapRelation);
 	Relation	pg_class;
@@ -773,10 +776,22 @@ index_create(Relation heapRelation,
 		elog(ERROR, "shared relations must be placed in pg_global tablespace");
 
 	if (get_relname_relid(indexRelationName, namespaceId))
+	{
+		if (if_not_exists)
+		{
+			ereport(NOTICE,
+					(errcode(ERRCODE_DUPLICATE_TABLE),
+					 errmsg("relation \"%s\" already exists, skipping",
+							indexRelationName)));
+			heap_close(pg_class, RowExclusiveLock);
+			return InvalidOid;
+		}
+
 		ereport(ERROR,
 				(errcode(ERRCODE_DUPLICATE_TABLE),
 				 errmsg("relation \"%s\" already exists",
 						indexRelationName)));
+	}
 
 	/*
 	 * construct tuple descriptor for index tuples

src/backend/catalog/toasting.c

@@ -342,7 +342,7 @@ create_toast_table(Relation rel, Oid toastOid, Oid toastIndexOid,
 				 rel->rd_rel->reltablespace,
 				 collationObjectId, classObjectId, coloptions, (Datum) 0,
 				 true, false, false, false,
-				 true, false, false, true);
+				 true, false, false, true, false);
 
 	heap_close(toast_rel, NoLock);
 

src/backend/commands/indexcmds.c

@@ -610,7 +610,14 @@ DefineIndex(Oid relationId,
 					 stmt->isconstraint, stmt->deferrable, stmt->initdeferred,
 					 allowSystemTableMods,
 					 skip_build || stmt->concurrent,
-					 stmt->concurrent, !check_rights);
+					 stmt->concurrent, !check_rights,
+					 stmt->if_not_exists);
+
+	if (!OidIsValid(indexRelationId))
+	{
+		heap_close(rel, NoLock);
+		return indexRelationId;
+	}
 
 	/* Add any requested comment */
 	if (stmt->idxcomment != NULL)

src/backend/nodes/copyfuncs.c

@@ -2907,6 +2907,7 @@ _copyIndexStmt(const IndexStmt *from)
 	COPY_SCALAR_FIELD(deferrable);
 	COPY_SCALAR_FIELD(initdeferred);
 	COPY_SCALAR_FIELD(concurrent);
+	COPY_SCALAR_FIELD(if_not_exists);
 
 	return newnode;
 }

src/backend/nodes/equalfuncs.c

@@ -1210,6 +1210,7 @@ _equalIndexStmt(const IndexStmt *a, const IndexStmt *b)
 	COMPARE_SCALAR_FIELD(deferrable);
 	COMPARE_SCALAR_FIELD(initdeferred);
 	COMPARE_SCALAR_FIELD(concurrent);
+	COMPARE_SCALAR_FIELD(if_not_exists);
 
 	return true;
 }

src/backend/parser/gram.y

@@ -6434,6 +6434,32 @@ IndexStmt:	CREATE opt_unique INDEX opt_concurrently opt_index_name
 					n->isconstraint = false;
 					n->deferrable = false;
 					n->initdeferred = false;
+					n->if_not_exists = false;
+					$$ = (Node *)n;
+				}
+			| CREATE opt_unique INDEX opt_concurrently IF_P NOT EXISTS index_name
+			ON qualified_name access_method_clause '(' index_params ')'
+			opt_reloptions OptTableSpace where_clause
+				{
+					IndexStmt *n = makeNode(IndexStmt);
+					n->unique = $2;
+					n->concurrent = $4;
+					n->idxname = $8;
+					n->relation = $10;
+					n->accessMethod = $11;
+					n->indexParams = $13;
+					n->options = $15;
+					n->tableSpace = $16;
+					n->whereClause = $17;
+					n->excludeOpNames = NIL;
+					n->idxcomment = NULL;
+					n->indexOid = InvalidOid;
+					n->oldNode = InvalidOid;
+					n->primary = false;
+					n->isconstraint = false;
+					n->deferrable = false;
+					n->initdeferred = false;
+					n->if_not_exists = true;
 					$$ = (Node *)n;
 				}
 		;