From: bgiles at coyotesong.com (Bear Giles)
Date: Sun, 9 Aug 2015 10:13:15 -0600
Subject: [Pljava-dev] I remembered why we might want bytecode scalar types
Message-ID: 
 <CALBNtw7v22n2Vadgz3KFenhL1ePxkEoqV0mQy258ysQhk8K18w@mail.gmail.com>

I mentioned a bytecode scalar type a few weeks ago and I just remember why
I thought they were important.

AFAIK right now we can install a jar and define functions that call methods
in it but we cannot write SP directly in java. That is, I can't write

CREATE FUNCTION  get_colors() RETURNS SETOF varchar AS $$
BEGIN
   List<String> colors = Arrays.asList("red", "green", "blue");
   return colors;
END $$
LANGUAGE JAVA;

and have it automatically translated to

public class GetColorsVoid {
   public Iterator<String> getColors() {
      List<String> colors = Arrays.asList("red", "green", "blue");
      return colors.iterator();
    }
}

then compiled and stored somewhere in the classpath.

The first big win is that developers don't need to know the details of how
things fit together. E.g., they can return a Collection instead of having
to learn to return an Iterator instead. (Not that it's difficult to learn
this, it's just one more thing to deal with.)

The second big win is that devops don't need to create the infrastructure
to produce, maintain, and distribute jars if they only need a handful of
functions. Obviously nobody should write a large application this way but
sometimes you only need a handful of functions.

The compiler itself isn't an issue. Eclipse had an embedded one years ago
and I believe there are several more available now.

The translation shouldn't be a big problem if the first round only supports
primitive types. The SETOF is a little more complex but you can directly
translate the original code, use inspection to determine if it returns a
Collection, and if so add a facade that wraps the original method and
converts it to an iterator.

The problem is where do you put the results. I think other dbs put them
into the database itself as a bytecode object. Hence my earlier question.

Upon reflection it would probably be possible to define a Foreign Data
Wrapper for jar files. Write to the jar file and kick a custom classloader
to tell it that there's new content.

This brings up a second thought....

Bear
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