presto-hive/src/main/java/io/prestosql/plugin/hive/metastore/file/FileHiveMetastore.java/FileHiveMetastore/addColumn
class FileHiveMetastore:    @Override
    public synchronized void addColumn(HiveIdentity identity, String databaseName, String tableName, String columnName, HiveType columnType, String columnComment)
    {
        alterTable(databaseName, tableName, oldTable -> {
            if (oldTable.getColumn(columnName).isPresent()) {
                throw new PrestoException(ALREADY_EXISTS, "Column already exists: " + columnName);
            }

            return oldTable.withDataColumns(ImmutableList.<Column>builder()
                    .addAll(oldTable.getDataColumns())
                    .add(new Column(columnName, columnType, Optional.ofNullable(columnComment)))
                    .build());
        });
    }