//Java Code 
public class FileExists 
{ 
     public static boolean checkFile (String input) throws Exception 
  { 
  String errorMessage; 

  try 
  { 
   // Check file exists and pass the value back thru the OUT paramenter 

   File f = new File(input); 

   return (f.exists()); 

  } 
  catch (SecurityException ioe) 
  { 
   errorMessage = ioe.getMessage(); 
   throw new Exception( errorMessage + " FAILED" ); 
  } 
 } 
} 


-- PL/Java Wrapper Fuction 
CREATE FUNCTION fileexists(character varying) RETURNS boolean 
    LANGUAGE java 
    AS $$FileExists.checkFile(java.lang.String)$$; 

--Results 
mydb=# select fileexists('/home/postgres/myfile.txt'); 
ERROR:  java.lang.Exception: read on /home/postgres/myfile.txt FAILED

mydb=# CREATE OR REPLACE FUNCTION fileexists(character varying) RETURNS boolean
mydb-#    LANGUAGE javaU
mydb-#    AS $$FileExists.checkFile(java.lang.String)$$;
CREATE FUNCTION
mydb=# select fileexists('/home/postgres/myfile.txt');
 fileexists 
------------
 t
(1 row)

mydb=# select fileexists('/home/postgres/myxxyfile.txt');
 fileexists 
------------
 f
(1 row)