PATCH: make plpgsql IN args mutable (v1)

[Steve Prentice <prentice(at)cisco(dot)com>]

Since I didn't get completely shot out of the water and a couple  
people seemed to think it was helpful, I'm submitting this patch for  
consideration in the next commitfest.

This patch changes plpgsql IN parameters so they are mutable.  
Previously, they were being forced constant. This patch modifies the  
plpgsql.sql regression test and corresponding .out file. The  
regression test also makes sure the passed in parameter does not get  
changed in the calling function.

I decided not to update the docs for this change because the docs  
don't currently indicate that an IN parameter is constant and I didn't  
want to encourage it because it isn't universally considered good  
programming practice to assign to an IN parameter. If others think we  
need a doc change for this, I'll update the patch.

The following function will compile with this patch:

  create or replace function param_assign_test(a int, val int)  
returns void as $$
  begin
    a := val;
  end
  $$ language plpgsql;

This function would have failed to compile previously.

-Steve

Attachment: plpgsql_in_args_mutable-v1.diff
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