Syntax error in SQL statement of PL/pgSQL function - sql

Question

I have written a stored procedure with the help of the SO community. I have cobbled/hacked together the answers of various questions to write my function.
However, when I try to create my function in the db (PostgreSQL 8.4), I get the following errors:
ERROR: syntax error at or near "$4"
LINE 1: ...ank() OVER (ORDER BY $1 , $2 ) / $3 )::int as $4 , $1 ,...
^
AND
QUERY: SELECT ceil(rank() OVER (ORDER BY $1 , $2 ) / $3 )::int as $4 , $1 , $2 , $5 , $6 , $7 , $8 , $9 , $10 FROM foobar WHERE $2 BETWEEN $11 AND $12 AND $1 = ANY( $13 ) ORDER BY $1 , $2 , $4
CONTEXT: SQL statement in PL/PgSQL function "custom_group" near line 9
This is the code for the function I am trying to create:
CREATE OR REPLACE FUNCTION custom_group(start_date DATE, end_date DATE, grouping_period INTEGER, _ids int[] DEFAULT '{}')
RETURNS TABLE (
grp INTEGER,
id INTEGER,
entry_date DATE,
pop REAL,
hip REAL,
lop REAL,
pcl REAL,
vop BIGINT,
poi BIGINT) AS
$BODY$
BEGIN
IF _ids <> '{}'::int[] THEN -- excludes empty array and NULL
RETURN QUERY
SELECT ceil(rank() OVER (ORDER BY id, entry_date) / $3)::int as grp
,id, entry_date, pop, hip, lop, pcl, vop, poi
FROM foobar
WHERE entry_date BETWEEN start_date AND end_date AND id = ANY(_ids)
ORDER BY id, entry_date, grp ;
ELSE
RETURN QUERY
SELECT ceil(rank() OVER (ORDER BY id, entry_date) / $3)::int as grp
,id, entry_date, pop, hip, lop, pcl, vop, poi
FROM foobar
WHERE entry_date BETWEEN start_date AND end_date
ORDER BY id, entry_date, grp ;
END IF;
END;
$BODY$ LANGUAGE plpgsql;
Can anyone understand why I am getting these errors - and how do I fix them?

Answer 1

The error comes from a naming conflict.
The variable grp is defined implicitly by the RETURNS TABLE clause. In the function body, you try to use the same identifier as column alias, which conflicts.
Just use a different name for grp - the column alias will not be visible outside of the function anyway.
And table-qualify the other columns:
CREATE OR REPLACE FUNCTION custom_group(_start_date DATE
,_end_date DATE
,_grouping_period INTEGER,
,_ids int[] DEFAULT '{}')
RETURNS TABLE (grp int, id int, entry_date date, pop real, hip real,
lop real, pcl real, vop bigint, poi bigint) AS
$BODY$
BEGIN
IF _ids <> '{}'::int[] THEN -- excludes empty array and NULL
RETURN QUERY
SELECT ceil(rank() OVER (ORDER BY f.id, f.entry_date) / $3)::int AS _grp
,f.id, f.entry_date, f.pop, f.hip, f.lop, f.pcl, f.vop, f.poi
FROM foobar f
WHERE f.entry_date BETWEEN _start_date AND _end_date AND id = ANY(_ids)
ORDER BY f.id, f.entry_date, _grp;
ELSE
RETURN QUERY
SELECT ceil(rank() OVER (ORDER BY f.id, f.entry_date) / $3)::int -- no alias
,f.id, f.entry_date, f.pop, f.hip, f.lop, f.pcl, f.vop, f.poi
FROM foobar f
WHERE f.entry_date BETWEEN _start_date AND _end_date
ORDER BY f.id, f.entry_date, 1; -- ordinal pos. instead of col alias
END IF;
END;
$BODY$ LANGUAGE plpgsql;
The reason why I prefix IN parameters with _ is the same: avoid such naming conflicts.
You don't even have to use an alias for the computed column at all in this case. You could use the ordinal position in ORDER BY like I demonstrate in the second query. I quote the manual here:
Each expression can be the name or ordinal number of an output column
(SELECT list item), or it can be an arbitrary expression formed from
input-column values.