public String vectorExpressionParameters() {
    return getColumnParamString(0, inputColumnNum[0]);
  }
This is a superclass for unary string functions that operate directly on the
input and set the output. It is suitable for direct, in-place operations on
strings, such as for fast implementations of TRIM(), LTRIM(), and RTRIM().
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/expressions/StringUnaryUDFDirect.java