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Struggling with set-returning functions, seeking advice

From: Jason Topaz <topaz(at)panix(dot)com>
To: pgsql-novice(at)postgresql(dot)org
Subject: Struggling with set-returning functions, seeking advice
Date: 2003-07-23 08:57:45
Message-ID: 1058950663.2979.6.camel@localhost.localdomain (view raw or flat)
Thread:
Lists: pgsql-novice
Greetings.  I've successfully gotten some code to work, but wondering
whether anybody can offer advice on a better way to do this.  I've
read everything I can think of, including the recent paper on
PostgreSQL 7.3 set-returning functions.  Also, can anybody comment
whether I have found a PostgreSQL bug?  (though I highly doubt
it... I'm a newbie!).  Here's the story...

I have data in a table:

db=> SELECT * FROM ranges;
 id | startindex | endindex | otherdata 
----+------------+----------+-----------
  1 |          5 |        9 |        42
  2 |          4 |        6 |        93
(2 rows)


I'm trying to create a view that transforms this table into multiple
rows
per source row - one for each "index" between startindex and last
index.  It also needs to do calculations based on the index and
otherdata.  (For sample purposes, I am just saying the calculation is
index+otherdata).  So the view I would like to see is:

db=> SELECT * FROM ranges_setview;
 id | index | calc 
----+-------+------
  1 |     5 |   47
  1 |     6 |   48
  1 |     7 |   49
  1 |     8 |   50
  1 |     9 |   51
  2 |     4 |   97
  2 |     5 |   98
  2 |     6 |   99
(8 rows)


I did get this to work with the enclosed code using a set-returning
function in plpgsql.  But I have quite a few questions. Can anybody help
out?

1) Note that my first attempt (view "ranges_setview_broken", referring
   to a pgplsql set function "make_rows") fails.  But when I make a
   second function with identical signature, but in language 'sql'
   (it's just a passthrough to my original pgplsql function), suddenly
   the server error goes away.  This seems strange to me.  Is there
   a reason my pgplsql function should behave differently than the
   sql function?  Or is this a PostgreSQL bug?  version() is:

     PostgreSQL 7.3.2 on i386-redhat-linux-gnu, compiled by GCC
i386-redhat-linux-gcc (GCC) 3.2.2 20030213 (Red Hat Linux 8.0 3.2.2-1)

2) Any advice on efficiency?  When using the view I create, my
   function make_rows gets called twice - with the same parameters -
   for every source row in table "ranges", since I extract two fields
   from the returned data.  Confirmed this with "RAISE INFO".  It
   seems like there should be a way to call make_rows only once per
   source row, but I can't figure out how to organize the query to
   do this.

   I would love for the make_rows() call to appear in the FROM clause,
   but each time I try, PostgreSQL seems to be unhappy with the fact
   that the input parameters need to come from another target table.
   Hmm... any way to rig this up with a correlated subquery?


3) Any recommendations for a completely different way to approach this
   table transformation?  Incidentally, I have simplified a lot for
   purposes of this posting, but the real application involves
transforming
   time-series data (stored in an array field) into a expanded table
   where there is one row per original element in the array.

4) The approach I'm using - putting set-value functions as target
   columns of the select statement - is described in the manual as
   deprecated and likely to go away.  What other way can I accomplish
   my task?  (maybe somebody's clever answer to quetion #2 or #3 will
   solve this problem anyway).


CREATE TABLE ranges (
	id INTEGER PRIMARY KEY,
	startindex INTEGER NOT NULL,
	endindex INTEGER NOT NULL,
	otherdata INTEGER NOT NULL
);

INSERT INTO ranges VALUES(1, 5, 9, 42);
INSERT INTO ranges VALUES(2, 4, 6, 93);


CREATE TYPE myrecord_type AS (
	index INTEGER,
	calc INTEGER
);

-- Generate a table with rows having 'index' numbered
--  from $1 to $2, and 'calc' set to 'index' + $3
CREATE OR REPLACE FUNCTION make_rows(INTEGER, INTEGER, INTEGER)
  RETURNS SETOF myrecord_type
  LANGUAGE 'plpgsql'
  AS '
    DECLARE
      rec myrecord_type%ROWTYPE;
    BEGIN
      FOR index IN $1..$2 LOOP
	SELECT index, index + $3 INTO rec;
        RETURN NEXT rec;
      END LOOP;
      RETURN;
    END
  ';


-- Using this view generates an error:
--   "Set-valued function called in context that cannot accept a set"

CREATE OR REPLACE VIEW ranges_setview_broken AS (
	SELECT 
	  id,
	  (make_rows(startindex, endindex, otherdata)).index,
	  (make_rows(startindex, endindex, otherdata)).calc
	FROM ranges
);

-- SQL wrapper function passing through to plpgsql function

CREATE OR REPLACE FUNCTION make_rows_sql(INTEGER, INTEGER, INTEGER)
  RETURNS SETOF myrecord_type
  LANGUAGE 'sql'
  AS 'select * from make_rows($1, $2, $3)';


-- Identical view to last time, but using wrapper 
-- instead of original function.  Works fine.

CREATE OR REPLACE VIEW ranges_setview_notbroken AS (
	SELECT
          id,
	  (make_rows_sql(startindex, endindex, otherdata)).index,
	  (make_rows_sql(startindex, endindex, otherdata)).calc
	FROM ranges
);


Thanks in advance for any pointers!

--
Jason Topaz
topaz(at)panix(dot)com



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Next:From: A.BhuvaneswaranDate: 2003-07-23 09:38:51
Subject: Re: Need clarifications......
Previous:From: Wolfgang DrotschmannDate: 2003-07-23 08:21:33
Subject: Re: CREATE TYPE with array

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