Re: Polymorphic function calls

On Sun, Dec 29, 2013 at 8:44 AM, knizhnik <knizhnik@garret.ru> wrote:
> create function volume(r base_table) returns integer as $$ begin return
> r.x*r.y; end; $$ language plpgsql strict stable;
> create function volume(r derived_table) returns integer as $$ begin return
> r.x*r.y*r.z; end; $$ language plpgsql strict stable;

[...]

> Execution plans of first and second queries are very similar.
> The difference is that type of r_1 in first query is "base_table".
> It is obvious that query should return fixed set of columns, so
>
>     select * from base_table;
>
> can not return "z" column.
> But passing direved_table type instead of base_table type to volume()
> function for record belonging to derived_table seems to be possible and not
> contradicting something, isn't it?

Correct.

Postgres chooses a function based on the passed signature. When you
specify base_table it will choose volume(base_table) and for
base_table it will be volume(derived_table) as well.

FYI, there is a common practice to follow the DRY principle with
inheritance and polymorphic functions in Postgres. On your example it
might be shown like this:

create function volume(r base_table) returns integer as $$ begin
return r.x*r.y;
end; $$ language plpgsql strict stable;

create function volume(r derived_table) returns integer as $$ begin
return volume(r::base_table) *r.z;
end; $$ language plpgsql strict stable;

-- 
Kind regards,
Sergey Konoplev
PostgreSQL Consultant and DBA

http://www.linkedin.com/in/grayhemp
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