Question

First I am really new to pl/pgsql. Need it for a project.

I am stuck with this (simplified) problem.

My db schema has a n to m relationship (author, books, author_books)

Now I want to have a pl/psgsql function insert_book. (I do know that all authors are definitely already in the author table, so I just want to pass their primary keys).

This function outline is what I have in mind.

 create or replace function insert_book(book_to_insert book, authors integer[])
  returns void as $$
begin
    -- insert book into table books
    -- for each author add an entry to author_books table
end;
 $$ language plpgsql;

As arguments I thought to pass a record of type book and the authors that wrote it. But how exactly would this work? I googled quite a bit and can’t seem to figure this out…

Question 1: Is the function outline “correct”/does it make sense?

Question 2: How to insert record book into table book? Do I have to go over all fields of book (title, isbn, publisher,…) and add them to an INSERT INTO statement or is there a “smarter” way?

Question 3: How would I call my function insert_book? I found this example here (http://dbaspot.com/postgresql/206142-passing-record-function-argument-pl-pgsql.html), but that doesn’t really help me. For testing purposes I am using the shell, but later on we will use Java with JDBC.

Thank you very much for your help.

Answer 1

Using unnest() and a data-modifying CTE (requires Postgres 9.1 or later), this can be a simple SQL query:

WITH x AS (SELECT '(1,foo_book)'::book AS _book
                , '{1,2,3}'::int[]     AS _authors)
   , y AS (
   INSERT INTO book  -- no column list, correct due to composite type
   SELECT (x._book).*
   FROM   x
   RETURNING book_id
   )
INSERT INTO author_book (book_id, author_id)
SELECT y.book_id, unnest(x._authors)
FROM   x,y;  -- CROSS JOIN ok, only 1 row for x and y

The first CTE x is just for simplified data input and not strictly needed.

SQL Fiddle.

As to your questions:

Question 1: Is the function outline "correct"/does it make sense?

Might be easier to pass base types instead of the composite type book, but it is a perfectly valid approach. You have to know your way around the syntax for complex types, though. For instance, note the parenthesis around the name in my example: (x._book).*.

A plpgsql function could look like this:

CREATE OR REPLACE FUNCTION f_insert_book(_book book, _authors integer[])
   RETURNS void AS 
$func$
BEGIN
    WITH y AS (
        INSERT INTO book b
        SELECT (_book).*
        RETURNING b.book_id
        )
    INSERT INTO author_book (book_id, author_id)
    SELECT y.book_id, unnest(_authors)
    FROM   y;
END
$func$ LANGUAGE plpgsql;

Question 2: How to insert record book into table book? (…) or is there a "smarter" way?

The smarter way is to decompose the composite type with (variable_name).*.

As the type is guaranteed to match the table (being derived from it), this is one of the rare cases, where it is perfectly ok, not to provide a column list for the INSERT command in persisted code.

Question 3: How would I call my function insert_book? …

SELECT f_insert_book('(1,foo_book)'::book, '{1,2,3}'::int[]);

Within other plpgsql functions, use PERFORM instead of SELECT if you don’t provide a target (INTO foo) for the (non-existing) results.