Virtualize columns in SQL function - postgresql

Question

I'm trying to figure out if it's possible to create an SQL function that treats an argument row as if it were "duck-typed". That is, I would like to be able to pass rows from different tables or views that have certain common column names and operate on those columns within the function.
Here's a very trivial example to try to describe the issue:
=> CREATE TABLE tab1 (
id SERIAL PRIMARY KEY,
has_desc BOOLEAN,
x1 TEXT,
description TEXT
);
CREATE TABLE
=> CREATE FUNCTION get_desc(tab tab1) RETURNS TEXT AS $$
SELECT CASE tab.has_desc
WHEN True THEN
tab.description
ELSE
'Default Description'
END;
$$ LANGUAGE SQL;
=> INSERT INTO tab1 (has_desc, x1, description) VALUES (True, 'Foo', 'FooDesc');
INSERT 0 1
=> INSERT INTO tab1 (has_desc, x1, description) VALUES (True, 'Bar', 'BarDesc');
INSERT 0 1
=> SELECT get_desc(tab1) FROM tab1;
get_desc
----------
BarDesc
FooDesc
(2 rows)
This is of course very artificial. In reality, my table has many more fields, and the function is way more complicated that that.
Now I want to add other tables/views and pass them to the same function. The new tables/views have columns that differ, but the columns the function will care about are common to all of them. To add to the trivial example, I add these two tables:
CREATE TABLE tab2 (
id SERIAL PRIMARY KEY,
has_desc BOOLEAN,
x2 TEXT,
description TEXT
);
CREATE TABLE tab3 (
id SERIAL PRIMARY KEY,
has_desc BOOLEAN,
x3 TEXT,
description TEXT
);
Note all three have the has_desc and description fields that are the only ones actually used in get_desc. But of course if I try to use the existing function with tab2, I get:
=> select get_desc(tab2) FROM tab2;
ERROR: function get_desc(tab2) does not exist
LINE 1: select get_desc(tab2) FROM tab2;
^
HINT: No function matches the given name and argument types. You might need to add explicit type casts.
I would like to be able to define a common function that does the same thing as get_desc but takes as argument a row from any of the three tables. Is there any way to do that?
Or alternatively is there some way to cast entire rows to a common row type that includes only a defined set of fields?
(I realize I could change the function arguments to just take XX.has_desc and XX.description but I'm trying to isolate which fields are used inside the function without needing to expand those in every place the function is called.)

Answer 1

You can create a cast:
CREATE CAST (tab2 AS tab1) WITH INOUT;
INSERT INTO tab2 (has_desc, x2, description) VALUES (True, 'Bar', 'From Tab2');
SELECT get_desc(tab2::tab1) FROM tab2;
get_desc
-----------
From Tab2
(1 row)

Answer 2

I'm adding an answer to show the complete way I solved this for posterity. But thanks to #klin for getting me pointed in the right direction. (One problem with #klin's bare CAST is that it doesn't produce the right row type when the two tables' common columns don't appear in the same relative position within their respective column lists.)
My solution adds a new custom TYPE (gdtab) containing the common fields, then a function that can convert from each source table's row type to the gdtab type, then adding a CAST to make each conversion implicit.
-- Common type for get_desc function
CREATE TYPE gdtab AS (
id INTEGER,
has_desc BOOLEAN,
description TEXT
);
CREATE FUNCTION get_desc(tab gdtab) RETURNS TEXT AS $$
SELECT CASE tab.has_desc
WHEN True THEN
tab.description
ELSE
'Default Description'
END;
$$ LANGUAGE SQL;
CREATE TABLE tab1 (
id SERIAL PRIMARY KEY,
has_desc BOOLEAN,
x1 TEXT,
description TEXT
);
-- Convert tab1 rowtype to gdtab type
CREATE FUNCTION tab1_as_gdtab(t tab1) RETURNS gdtab AS $$
SELECT CAST(ROW(t.id, t.has_desc, t.description) AS gdtab);
$$ LANGUAGE SQL;
-- Implicitly cast from tab1 to gdtab as needed for get_desc
CREATE CAST (tab1 AS gdtab) WITH FUNCTION tab1_as_gdtab(tab1) AS IMPLICIT;
CREATE TABLE tab2 (
id SERIAL PRIMARY KEY,
x2 TEXT,
x2x TEXT,
has_desc BOOLEAN,
description TEXT
);
CREATE FUNCTION tab2_as_gdtab(t tab2) RETURNS gdtab AS $$
SELECT CAST(ROW(t.id, t.has_desc, t.description) AS gdtab);
$$ LANGUAGE SQL;
CREATE CAST (tab2 AS gdtab) WITH FUNCTION tab2_as_gdtab(tab2) AS IMPLICIT;
Test usage:
INSERT INTO tab1 (has_desc, x1, description) VALUES (True, 'FooBlah', 'FooDesc'),
(False, 'BazBlah', 'BazDesc'),
(True, 'BarBlah', 'BarDesc');
INSERT INTO tab2 (has_desc, x2, x2x, description) VALUES (True, 'FooBlah', 'x2x', 'FooDesc'),
(False, 'BazBlah', 'x2x', 'BazDesc'),
(True, 'BarBlah', 'x2x', 'BarDesc');
SELECT get_desc(tab1) FROM tab1;
SELECT get_desc(tab2) FROM tab2;

Answer 3

Postgresql functions depend on the schema of the argument. If the different tables have different schemas, then you can always project them into the common sub-schema needed by get_desc. This can be done in a quick and temporary fashion with a WITH clause before the get_desc use.
If this answer is too thin on details, just add a comment and I'll flesh out some example.
More details:
CREATE TABLE subschema_table ( has_desc boolean, description text ) ;
CREATE FUNCTION get_desc1(tab subschema_table) RETURNS TEXT AS $$
SELECT CASE tab.has_desc
WHEN True THEN
tab.description
ELSE
'Default Description'
END; $$ LANGUAGE SQL;
Now, the following will work (with other tables also):
WITH subschema AS (SELECT has_desc, description FROM tab1)
SELECT get_desc1(subschema) FROM subschema;
The VIEW method didn't work in my test (VIEWs don't seem to have the appropriate schema.
Maybe the other answer gives a better way.