Implement COALESCE function

[linmagit]

Here's what we get now:

postgres=# create table a(id int primary key, value varchar);
create table a(id int primary key, value varchar) 0
postgres=# insert into a values(1, 'hi');
insert into a values(1, 'hi') 1
postgres=# select * from a;
 id | value 
----+-------
  1 | hi
(1 row)

postgres=# select coalesce(id, value) as coal from a;
function coalesce not found.

It seems that we already have the function type:
https://github.com/cmu-db/peloton/blob/master/src/include/type/types.h#L227
But there's no implementation for that.

[sumanthrao]

I would like to work on this issue,

[apavlo]

@sumanthrao Thanks for the offer. This blocked on #838 though. Once we get the new built-in function infrastructure implemented, then this will definitely be doable.

[poojanilangekar]

@sumanthrao Are you still interested in working on this? PR #838 has been merged and so it should be pretty straightforward to implement the COALESCE function.

[schedutron]

@poojanilangekar May I work on this?

[poojanilangekar]

@schedutron I was just cleaning up old issues in December. It is best you ask @pervazea if anyone is working on this.

[schedutron]

@pervazea May I work on this issue?

[pervazea]

Go ahead.

[schedutron]

What should be the return type of coalesce? UDT?

[pervazea]

According to the Postgres documentation, coalesce returns the first non-null argument. The type will be whatever the type is of that argument. You should be able to exercise the function in Postgres, and and see what it does.

One of the experts should correct me if I'm reading it wrong.

[schedutron]

Yes, that could be any type, so with what return type should I register it in AddBuiltinFunction in catalog.cpp?

[schedutron]

@poojanilangekar COALESCE() is a function which takes a variable number of arguments. A similar function is CONCAT(); I looked at its implementation for inspiration but that didn't help since the current implementation concatenates just two columns. So how do I register COALESCE() as a function that can take a variable number of arguments, in catalog.cpp? Specifically, what should be the second argument passed to AddBuiltinFunction() in catalog.cpp, which is a vector representing the types of the arguments which the function can take?

As a starting step, should I too should implement COALESCE() for just 2 columns?