private Index createIndex(Table t, IndexColumn[] cols, boolean unique) {
   int indexId = getObjectId();
   IndexType indexType;
   if (unique) {
     // for unique constraints
     indexType = IndexType.createUnique(t.isPersistIndexes(), false);
   } else {
     // constraints
     indexType = IndexType.createNonUnique(t.isPersistIndexes());
   }
   indexType.setBelongsToConstraint(true);
   String prefix = constraintName == null ? "CONSTRAINT" : constraintName;
   String indexName = t.getSchema().getUniqueIndexName(session, t, prefix + "_INDEX_");
   try {
     return t.addIndex(session, indexName, indexId, cols, indexType, true, null);
   } finally {
     getSchema().freeUniqueName(indexName);
   }
 }
 /**
  * Try to execute the statement.
  *
  * @return the update count
  */
 private int tryUpdate() {
   if (!transactional) {
     session.commit(true);
   }
   Database db = session.getDatabase();
   Table table = getSchema().getTableOrView(session, tableName);
   if (getSchema().findConstraint(session, constraintName) != null) {
     if (ifNotExists) {
       return 0;
     }
     throw DbException.get(ErrorCode.CONSTRAINT_ALREADY_EXISTS_1, constraintName);
   }
   session.getUser().checkRight(table, Right.ALL);
   db.lockMeta(session);
   table.lock(session, true, true);
   Constraint constraint;
   switch (type) {
     case CommandInterface.ALTER_TABLE_ADD_CONSTRAINT_PRIMARY_KEY:
       {
         IndexColumn.mapColumns(indexColumns, table);
         index = table.findPrimaryKey();
         ArrayList<Constraint> constraints = table.getConstraints();
         for (int i = 0; constraints != null && i < constraints.size(); i++) {
           Constraint c = constraints.get(i);
           if (Constraint.PRIMARY_KEY.equals(c.getConstraintType())) {
             throw DbException.get(ErrorCode.SECOND_PRIMARY_KEY);
           }
         }
         if (index != null) {
           // if there is an index, it must match with the one declared
           // we don't test ascending / descending
           IndexColumn[] pkCols = index.getIndexColumns();
           if (pkCols.length != indexColumns.length) {
             throw DbException.get(ErrorCode.SECOND_PRIMARY_KEY);
           }
           for (int i = 0; i < pkCols.length; i++) {
             if (pkCols[i].column != indexColumns[i].column) {
               throw DbException.get(ErrorCode.SECOND_PRIMARY_KEY);
             }
           }
         }
         if (index == null) {
           IndexType indexType =
               IndexType.createPrimaryKey(table.isPersistIndexes(), primaryKeyHash);
           String indexName =
               table.getSchema().getUniqueIndexName(session, table, Constants.PREFIX_PRIMARY_KEY);
           int id = getObjectId();
           try {
             index = table.addIndex(session, indexName, id, indexColumns, indexType, true, null);
           } finally {
             getSchema().freeUniqueName(indexName);
           }
         }
         index.getIndexType().setBelongsToConstraint(true);
         int constraintId = getObjectId();
         String name = generateConstraintName(table);
         ConstraintUnique pk = new ConstraintUnique(getSchema(), constraintId, name, table, true);
         pk.setColumns(indexColumns);
         pk.setIndex(index, true);
         constraint = pk;
         break;
       }
     case CommandInterface.ALTER_TABLE_ADD_CONSTRAINT_UNIQUE:
       {
         IndexColumn.mapColumns(indexColumns, table);
         boolean isOwner = false;
         if (index != null && canUseUniqueIndex(index, table, indexColumns)) {
           isOwner = true;
           index.getIndexType().setBelongsToConstraint(true);
         } else {
           index = getUniqueIndex(table, indexColumns);
           if (index == null) {
             index = createIndex(table, indexColumns, true);
             isOwner = true;
           }
         }
         int id = getObjectId();
         String name = generateConstraintName(table);
         ConstraintUnique unique = new ConstraintUnique(getSchema(), id, name, table, false);
         unique.setColumns(indexColumns);
         unique.setIndex(index, isOwner);
         constraint = unique;
         break;
       }
     case CommandInterface.ALTER_TABLE_ADD_CONSTRAINT_CHECK:
       {
         int id = getObjectId();
         String name = generateConstraintName(table);
         ConstraintCheck check = new ConstraintCheck(getSchema(), id, name, table);
         TableFilter filter = new TableFilter(session, table, null, false, null);
         checkExpression.mapColumns(filter, 0);
         checkExpression = checkExpression.optimize(session);
         check.setExpression(checkExpression);
         check.setTableFilter(filter);
         constraint = check;
         if (checkExisting) {
           check.checkExistingData(session);
         }
         break;
       }
     case CommandInterface.ALTER_TABLE_ADD_CONSTRAINT_REFERENTIAL:
       {
         Table refTable = refSchema.getTableOrView(session, refTableName);
         session.getUser().checkRight(refTable, Right.ALL);
         if (!refTable.canReference()) {
           throw DbException.get(
               ErrorCode.FEATURE_NOT_SUPPORTED_1, "Reference " + refTable.getSQL());
         }
         boolean isOwner = false;
         IndexColumn.mapColumns(indexColumns, table);
         if (index != null && canUseIndex(index, table, indexColumns)) {
           isOwner = true;
           index.getIndexType().setBelongsToConstraint(true);
         } else {
           index = getIndex(table, indexColumns);
           if (index == null) {
             index = createIndex(table, indexColumns, false);
             isOwner = true;
           }
         }
         if (refIndexColumns == null) {
           Index refIdx = refTable.getPrimaryKey();
           refIndexColumns = refIdx.getIndexColumns();
         } else {
           IndexColumn.mapColumns(refIndexColumns, refTable);
         }
         if (refIndexColumns.length != indexColumns.length) {
           throw DbException.get(ErrorCode.COLUMN_COUNT_DOES_NOT_MATCH);
         }
         boolean isRefOwner = false;
         if (refIndex != null
             && refIndex.getTable() == refTable
             && canUseIndex(refIndex, refTable, refIndexColumns)) {
           isRefOwner = true;
           refIndex.getIndexType().setBelongsToConstraint(true);
         } else {
           refIndex = null;
         }
         if (refIndex == null) {
           refIndex = getIndex(refTable, refIndexColumns);
           if (refIndex == null) {
             refIndex = createIndex(refTable, refIndexColumns, true);
             isRefOwner = true;
           }
         }
         int id = getObjectId();
         String name = generateConstraintName(table);
         ConstraintReferential ref = new ConstraintReferential(getSchema(), id, name, table);
         ref.setColumns(indexColumns);
         ref.setIndex(index, isOwner);
         ref.setRefTable(refTable);
         ref.setRefColumns(refIndexColumns);
         ref.setRefIndex(refIndex, isRefOwner);
         if (checkExisting) {
           ref.checkExistingData(session);
         }
         constraint = ref;
         refTable.addConstraint(constraint);
         ref.setDeleteAction(deleteAction);
         ref.setUpdateAction(updateAction);
         break;
       }
     default:
       throw DbException.throwInternalError("type=" + type);
   }
   // parent relationship is already set with addConstraint
   constraint.setComment(comment);
   if (table.isTemporary() && !table.isGlobalTemporary()) {
     session.addLocalTempTableConstraint(constraint);
   } else {
     db.addSchemaObject(session, constraint);
   }
   table.addConstraint(constraint);
   return 0;
 }