How to get the type of array elements?

Question

I am writing a PL / pgSQL polymorphic function that iterates through an array. I am interested in using FOREACH, however, I cannot figure out how to declare a temporary variable with the correct type.

My function is below, for more information see comment on line 4.

CREATE OR REPLACE FUNCTION uniq(ary anyarray) RETURNS anyarray AS $$
DECLARE
  ret ary%TYPE := '{}';
  v ???; -- how do I get the element type of @ary@?
BEGIN
  IF ary IS NULL THEN
    return NULL;
  END IF;

  FOREACH v IN ARRAY ary LOOP
    IF NOT v = any(ret) THEN
      ret = array_append(ret, v);
    END IF;
  END LOOP;

  RETURN ret;
END;
$$ LANGUAGE plpgsql;

Answer 1

The answer to the main question

AFAIK, you cannot declare a variable of a polymorphic type without a variable or template parameter.

, : IN, INOUT OUT ANYELEMENT . (ab) :

CREATE OR REPLACE FUNCTION uniq1(ary ANYARRAY, v ANYELEMENT = NULL)
  RETURNS anyarray AS
$func$
DECLARE
   ret      ary%TYPE := '{}';
   some_var v%TYPE;  -- we could declare more variables now
                     -- but we don't need to
BEGIN
   IF ary IS NULL THEN
      RETURN NULL;
   END IF;

   FOREACH v IN ARRAY ary LOOP  -- instead, we can use v directly
      IF NOT v = any(ret) THEN
         ret := array_append(ret, v);
      END IF;
   END LOOP;

   RETURN ret;
END
$func$  LANGUAGE plpgsql;

:

• plpgsql ?

DECLARE . .

, : ANYELEMENT = NULL

( ):

SELECT uniq1('{1,2,1}'::int[]);
SELECT uniq1('{foo,bar,bar}'::text[]);

OUT :

CREATE OR REPLACE FUNCTION uniq2(ary ANYARRAY, elem ANYELEMENT = NULL
                               , OUT ret ANYARRAY)
  RETURNS anyarray AS
$func$
BEGIN
   IF ary IS NULL
      THEN RETURN;
      ELSE ret := '{}';  -- init
   END IF;

   FOREACH elem IN ARRAY ary LOOP
      IF elem = ANY(ret) THEN  -- do nothing
      ELSE
         ret := array_append(ret, elem);
      END IF;
   END LOOP;
END
$func$  LANGUAGE plpgsql;

, NULL.

NULL:

CREATE OR REPLACE FUNCTION uniq3(ary ANYARRAY, elem ANYELEMENT = NULL
                               , OUT ret ANYARRAY)
  RETURNS anyarray AS
$func$
BEGIN
   IF ary IS NULL
      THEN RETURN;
      ELSE ret := '{}';  -- init
   END IF;

   FOREACH elem IN ARRAY ary LOOP
      IF elem IS NULL THEN  -- special test for NULL
         IF array_length(array_remove(ret, NULL), 1) = array_length(ret, 1) THEN
            ret := array_append(ret, NULL);
         END IF;
      ELSIF elem = ANY(ret) THEN  -- do nothing
      ELSE
         ret := array_append(ret, elem);
      END IF;
   END LOOP;
END
$func$  LANGUAGE plpgsql;

NULL - :

• , NULL Postgres?

. . :

SQL

Postgres 9.4 WITH ORDINALITY, . :

• PostgreSQL unsest()

:

SELECT ARRAY (
   SELECT elem
   FROM  (
      SELECT DISTINCT ON (elem) elem, i
      FROM   unnest('{1,2,1,NULL,4,NULL}'::int[]) WITH ORDINALITY u(elem, i)
      ORDER  BY elem, i
      ) sub
   ORDER  BY i) AS uniq;

:

uniq
------------
{1,2,NULL,4}

DISTINCT ON:

• GROUP BY?

:

SELECT *
FROM   test t
     , LATERAL (
   SELECT ARRAY (
      SELECT elem
      FROM  (
         SELECT DISTINCT ON (elem) elem, i
         FROM   unnest(t.arr) WITH ORDINALITY u(elem, i)
         ORDER  BY elem, i
         ) sub
      ORDER BY i) AS arr
   ) a;

: NULL. :

SELECT t.*, CASE WHEN t.arr IS NULL THEN NULL ELSE a.arr END AS arr
FROM   test t
     , LATERAL (
   SELECT ARRAY (
      SELECT elem
      FROM  (
         SELECT DISTINCT ON (elem) elem, ord
         FROM   unnest(t.arr) WITH ORDINALITY u(elem, ord)
         ORDER  BY elem, ord
         ) sub
      ORDER BY ord) AS arr
   ) a;

:

SELECT *
FROM   test t
LEFT   JOIN LATERAL (
   SELECT ARRAY (
      SELECT elem
      FROM  (
         SELECT DISTINCT ON (elem) elem, i
         FROM   unnest(t.arr) WITH ORDINALITY u(elem, i)
         ORDER  BY elem, i
         ) sub
      ORDER BY i) AS arr
   ) a ON t.arr IS NOT NULL;

Postgres 9.3 generate_subscripts():

SELECT *
FROM   test t
     , LATERAL (
   SELECT ARRAY (
      SELECT elem
      FROM  (
         SELECT DISTINCT ON (t.arr[i]) t.arr[i] AS elem, i
         FROM   generate_subscripts(t.arr, 1) i
         ORDER  BY t.arr[i], i
         ) sub
      ORDER  BY i
      ) AS arr
   ) a;

sqlfiddle, pg 9.3, WITH ORDINALITY :

SQL Fiddle.

Answer 2

, anyarray ( ).

FOR LOOP :

CREATE OR REPLACE FUNCTION uniq(ary anyarray) RETURNS anyarray AS $$
DECLARE
  ret ary%TYPE := '{}';
  i int;
BEGIN
  IF ary IS NULL THEN
    return NULL;
  END IF;

  FOR i IN array_lower(ary, 1) .. array_upper(ary, 1) LOOP
    IF NOT ary[i] = any(ret) THEN
      ret = array_append(ret, ary[i]);
    END IF;
  END LOOP;

  RETURN ret;
END;
$$ LANGUAGE plpgsql;

:

create or replace function uniq_without_loop(arr anyarray)
returns anyarray language plpgsql as $$
begin
    return (
        select array_agg(distinct elem)
        from unnest(arr) elem);
end $$;

, :

create or replace function unsorted_uniq_without_loop(arr anyarray)
returns anyarray language plpgsql as $$
begin
    return (
        select array_agg(elem)
        from (
            select elem 
            from (
                select distinct on(elem) elem, row_number() over ()
                from unnest(array[arr]) elem
                ) sub
            order by row_number
            ) sub);
end $$;