ALTER TABLE could throw an exception "object already exists" in some cases.

h2/src/main/org/h2/command/ddl/AlterTableAlterColumn.java

@@ -11,6 +11,7 @@ import java.sql.SQLException;
 import org.h2.command.Parser;
 import org.h2.command.Prepared;
 import org.h2.constant.ErrorCode;
+import org.h2.constraint.Constraint;
 import org.h2.constraint.ConstraintReferential;
 import org.h2.engine.Database;
 import org.h2.engine.DbObject;
@@ -326,7 +327,17 @@ public class AlterTableAlterColumn extends SchemaCommand {
             }
             if (name.startsWith(tempName + "_")) {
                 name = name.substring(tempName.length() + 1);
-                db.renameSchemaObject(session, (SchemaObject) child, name);
+                SchemaObject so = (SchemaObject) child;
+                if (so instanceof Constraint) {
+                    if (so.getSchema().findConstraint(session, name) != null) {
+                        name = so.getSchema().getUniqueConstraintName(session, newTable);
+                    }
+                } else if (so instanceof Index) {
+                    if (so.getSchema().findIndex(session, name) != null) {
+                        name = so.getSchema().getUniqueIndexName(session, newTable, name);
+                    }
+                }
+                db.renameSchemaObject(session, so, name);
             }
         }
     }

h2/src/test/org/h2/test/db/TestCases.java

@@ -36,6 +36,7 @@ public class TestCases extends TestBase {
     }
 
     public void test() throws Exception {
+        testConstraintAlterTable();
         testJoinWithView();
         testLobDecrypt();
         testInvalidDatabaseName();
@@ -71,6 +72,18 @@ public class TestCases extends TestBase {
         deleteDb("cases");
     }
 
+    private void testConstraintAlterTable() throws SQLException {
+        deleteDb("cases");
+        Connection conn = getConnection("cases");
+        Statement stat = conn.createStatement();
+        stat.execute("create table parent (pid int)");
+        stat.execute("create table child (cid int primary key, pid int)");
+        stat.execute("alter table child add foreign key (pid) references parent(pid)");
+        stat.execute("alter table child add column c2 int");
+        stat.execute("alter table parent add column p2 varchar");
+        conn.close();
+    }
+
     private void testEmptyBtreeIndex() throws SQLException {
         deleteDb("cases");
         Connection conn;