The PostgreSQL 9.0 Reference Manual - Volume 2

The PostgreSQL 9.0 Reference Manual - Volume 2 - Programming Guide
by The PostgreSQL Global Development Group
Paperback (6"x9"), 478 pages
ISBN 9781906966065
RRP £14.95 ($19.95)

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PostgreSQL 9.0 Reference Manual
Volume 2

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9.12.1 Porting Examples

section 9.12.1 Porting Examples shows how to port a simple function from PL/SQL to PL/pgSQL.

Porting a Simple Function from PL/SQL to PL/pgSQL:

Here is an Oracle PL/SQL function:

CREATE OR REPLACE FUNCTION cs_fmt_browser_version(v_name 
  varchar, v_version varchar)
RETURN varchar IS
BEGIN
    IF v_version IS NULL THEN
        RETURN v_name;
    END IF;
    RETURN v_name || '/' || v_version;
END;
/
show errors;

Let's go through this function and see the differences compared to PL/pgSQL:

The RETURN key word in the function prototype (not the function body) becomes RETURNS in PostgreSQL. Also, IS becomes AS, and you need to add a LANGUAGE clause because PL/pgSQL is not the only possible function language.
In PostgreSQL, the function body is considered to be a string literal, so you need to use quote marks or dollar quotes around it. This substitutes for the terminating / in the Oracle approach.
The show errors command does not exist in PostgreSQL, and is not needed since errors are reported automatically.

This is how this function would look when ported to PostgreSQL:

CREATE OR REPLACE FUNCTION cs_fmt_browser_version(v_name 
  varchar, v_version varchar)
RETURNS varchar AS $$
BEGIN
    IF v_version IS NULL THEN
        RETURN v_name;
    END IF;
    RETURN v_name || '/' || v_version;
END;
$$ LANGUAGE plpgsql;

section 9.12.1 Porting Examples shows how to port a function that creates another function and how to handle the ensuing quoting problems.

Porting a Function that Creates Another Function from PL/SQL to PL/pgSQL:

The following procedure grabs rows from a SELECT statement and builds a large function with the results in IF statements, for the sake of efficiency.

This is the Oracle version:

CREATE OR REPLACE PROCEDURE cs_update_referrer_type_proc IS
    CURSOR referrer_keys IS
        SELECT * FROM cs_referrer_keys
        ORDER BY try_order;
    func_cmd VARCHAR(4000);
BEGIN
    func_cmd := 'CREATE OR REPLACE FUNCTION 
  cs_find_referrer_type(v_host IN VARCHAR, v_domain IN VARCHAR, 
  v_url IN VARCHAR) RETURN VARCHAR IS BEGIN';

    FOR referrer_key IN referrer_keys LOOP
        func_cmd := func_cmd ||
          ' IF v_' || referrer_key.kind
          || ' LIKE ”' || referrer_key.key_string
          || ”' THEN RETURN ”' || referrer_key.referrer_type
          || ”'; END IF;';
    END LOOP;

    func_cmd := func_cmd || ' RETURN NULL; END;';

    EXECUTE IMMEDIATE func_cmd;
END;
/
show errors;

Here is how this function would end up in PostgreSQL:

CREATE OR REPLACE FUNCTION cs_update_referrer_type_proc() 
  RETURNS void AS $func$
DECLARE
    referrer_keys CURSOR IS
        SELECT * FROM cs_referrer_keys
        ORDER BY try_order;
    func_body text;
    func_cmd text;
BEGIN
    func_body := 'BEGIN';

    FOR referrer_key IN referrer_keys LOOP
        func_body := func_body ||
          ' IF v_' || referrer_key.kind
          || ' LIKE ' || quote_literal(referrer_key.key_string)
          || ' THEN RETURN '
          || quote_literal(referrer_key.referrer_type)
          || '; END IF;' ;
    END LOOP;

    func_body := func_body || ' RETURN NULL; END;';

    func_cmd :=
      'CREATE OR REPLACE FUNCTION cs_find_referrer_type(v_host 
  varchar, v_domain varchar, v_url varchar)
        RETURNS varchar AS '
      || quote_literal(func_body)
      || ' LANGUAGE plpgsql;' ;

    EXECUTE func_cmd;
END;
$func$ LANGUAGE plpgsql;

Notice how the body of the function is built separately and passed through quote_literal to double any quote marks in it. This technique is needed because we cannot safely use dollar quoting for defining the new function: we do not know for sure what strings will be interpolated from the referrer_key.key_string field. (We are assuming here that referrer_key.kind can be trusted to always be host, domain, or url, but referrer_key.key_string might be anything, in particular it might contain dollar signs.) This function is actually an improvement on the Oracle original, because it will not generate broken code when referrer_key.key_string or referrer_key.referrer_type contain quote marks.

section 9.12.1 Porting Examples shows how to port a function with OUT parameters and string manipulation. PostgreSQL does not have a built-in instr function, but you can create one using a combination of other functions. In section 9.12.3 Appendix there is a PL/pgSQL implementation of instr that you can use to make your porting easier.

Porting a Procedure With String Manipulation and OUT Parameters from PL/SQL to PL/pgSQL:

The following Oracle PL/SQL procedure is used to parse a URL and return several elements (host, path, and query).

This is the Oracle version:

CREATE OR REPLACE PROCEDURE cs_parse_url(
    v_url IN VARCHAR,
    v_host OUT VARCHAR,  -- This will be passed back
    v_path OUT VARCHAR,  -- This one too
    v_query OUT VARCHAR) -- And this one
IS
    a_pos1 INTEGER;
    a_pos2 INTEGER;
BEGIN
    v_host := NULL;
    v_path := NULL;
    v_query := NULL;
    a_pos1 := instr(v_url, '//');

    IF a_pos1 = 0 THEN
        RETURN;
    END IF;
    a_pos2 := instr(v_url, '/', a_pos1 + 2);
    IF a_pos2 = 0 THEN
        v_host := substr(v_url, a_pos1 + 2);
        v_path := '/';
        RETURN;
    END IF;

    v_host := substr(v_url, a_pos1 + 2, a_pos2 - a_pos1 - 2);
    a_pos1 := instr(v_url, '?', a_pos2 + 1);

    IF a_pos1 = 0 THEN
        v_path := substr(v_url, a_pos2);
        RETURN;
    END IF;

    v_path := substr(v_url, a_pos2, a_pos1 - a_pos2);
    v_query := substr(v_url, a_pos1 + 1);
END;
/
show errors;

Here is a possible translation into PL/pgSQL:

CREATE OR REPLACE FUNCTION cs_parse_url(
    v_url IN VARCHAR,
    v_host OUT VARCHAR,  -- This will be passed back
    v_path OUT VARCHAR,  -- This one too
    v_query OUT VARCHAR) -- And this one
AS $$
DECLARE
    a_pos1 INTEGER;
    a_pos2 INTEGER;
BEGIN
    v_host := NULL;
    v_path := NULL;
    v_query := NULL;
    a_pos1 := instr(v_url, '//');

    IF a_pos1 = 0 THEN
        RETURN;
    END IF;
    a_pos2 := instr(v_url, '/', a_pos1 + 2);
    IF a_pos2 = 0 THEN
        v_host := substr(v_url, a_pos1 + 2);
        v_path := '/';
        RETURN;
    END IF;

    v_host := substr(v_url, a_pos1 + 2, a_pos2 - a_pos1 - 2);
    a_pos1 := instr(v_url, '?', a_pos2 + 1);

    IF a_pos1 = 0 THEN
        v_path := substr(v_url, a_pos2);
        RETURN;
    END IF;

    v_path := substr(v_url, a_pos2, a_pos1 - a_pos2);
    v_query := substr(v_url, a_pos1 + 1);
END;
$$ LANGUAGE plpgsql;

This function could be used like this:

SELECT * FROM cs_parse_url('http://foobar.com/query.cgi?baz');

section 9.12.1 Porting Examples shows how to port a procedure that uses numerous features that are specific to Oracle.

Porting a Procedure from PL/SQL to PL/pgSQL:

The Oracle version:

CREATE OR REPLACE PROCEDURE cs_create_job(v_job_id IN INTEGER) IS
    a_running_job_count INTEGER;
    PRAGMA AUTONOMOUS_TRANSACTION; (note 1)
BEGIN
    LOCK TABLE cs_jobs IN EXCLUSIVE MODE; (note 2)

    SELECT count(*) INTO a_running_job_count FROM cs_jobs 
  WHERE end_stamp IS NULL;

    IF a_running_job_count > 0 THEN
        COMMIT; -- free lock (note 3)
        raise_application_error(-20000,
                 'Unable to create a new job: a job is 
  currently running.');
    END IF;

    DELETE FROM cs_active_job;
    INSERT INTO cs_active_job(job_id) VALUES (v_job_id);

    BEGIN
        INSERT INTO cs_jobs (job_id, start_stamp) VALUES 
  (v_job_id, sysdate);
    EXCEPTION
        WHEN dup_val_on_index THEN NULL; -- don't worry if it 
  already exists
    END;
    COMMIT;
END;
/
show errors

Procedures like this can easily be converted into PostgreSQL functions returning void. This procedure in particular is interesting because it can teach us some things:

Note 1:: There is no PRAGMA statement in PostgreSQL.
Note 2:: If you do a LOCK TABLE in PL/pgSQL, the lock will not be released until the calling transaction is finished.
Note 3:: You cannot issue COMMIT in a PL/pgSQL function. The function is running within some outer transaction and so COMMIT would imply terminating the function's execution. However, in this particular case it is not necessary anyway, because the lock obtained by the LOCK TABLE will be released when we raise an error.

This is how we could port this procedure to PL/pgSQL:

CREATE OR REPLACE FUNCTION cs_create_job(v_job_id integer) 
  RETURNS void AS $$
DECLARE
    a_running_job_count integer;
BEGIN
    LOCK TABLE cs_jobs IN EXCLUSIVE MODE;

    SELECT count(*) INTO a_running_job_count FROM cs_jobs 
  WHERE end_stamp IS NULL;

    IF a_running_job_count > 0 THEN
        RAISE EXCEPTION 'Unable to create a new job: a job is
 currently running'; (note 1)
    END IF;

    DELETE FROM cs_active_job;
    INSERT INTO cs_active_job(job_id) VALUES (v_job_id);

    BEGIN
        INSERT INTO cs_jobs (job_id, start_stamp) VALUES 
  (v_job_id, now());
    EXCEPTION
        WHEN unique_violation THEN  (note 2)
            -- don't worry if it already exists
    END;
END;
$$ LANGUAGE plpgsql;

Note 1:: The syntax of RAISE is considerably different from Oracle's statement, although the basic case RAISE exception_name works similarly.
Note 2:: The exception names supported by PL/pgSQL are different from Oracle's. The set of built-in exception names is much larger (see Volume 1A: A PostgreSQL Error Codes). There is not currently a way to declare user-defined exception names, although you can throw user-chosen SQLSTATE values instead.

The main functional difference between this procedure and the Oracle equivalent is that the exclusive lock on the cs_jobs table will be held until the calling transaction completes. Also, if the caller later aborts (for example due to an error), the effects of this procedure will be rolled back.

The PostgreSQL 9.0 Reference Manual - Volume 2 - Programming Guide

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