@Override
  public String getDisplayString(String[] children) {
    assert (children.length >= 2 && children.length <= 4);
    return getStandardDisplayString("instr", children);
  }
Generic UDF for string function <code>INSTR(str,substr[,pos[,occurrence]])</code>.
This extends the function from MySQL
http://dev.mysql.com/doc/refman/5.1/en/string-functions.html#function_instr
and mimics the function from Oracle
https://docs.oracle.com/database/121/SQLRF/functions089.htm#SQLRF00651

<pre>
usage:
INSTR(str, substr[, pos[, occurrence]])
</pre>
<p>
ql/src/java/org/apache/hadoop/hive/ql/udf/generic/GenericUDFInstr.java