Re: lag_until_you_get_something() OVER () window function

This is a pretty elegant way of getting there.  

It also does a better job of respecting the window frame.

I'll use this until this https://commitfest.postgresql.org/action/patch_view?id=1096 shows up.

Thanks

On 2014-10-28 17:35, Merlin Moncure wrote:

On Tue, Oct 28, 2014 at 12:40 PM, Kirk Roybal <kirk@webfinish.com> wrote:

Hi Guys, I propose a lag (and/or lead) window function that propagates the last non-null value to the current row. Here's an example of what I mean by that: CREATE TABLE lag_test (id serial primary key, natural_key integer, somebody text); INSERT INTO lag_test(natural_key, somebody) VALUES (1, NULL), (1, 'Kirk'), (1, NULL), (2, 'Roybal'), (2, NULL), (2, NULL); /* Creates this data in the table. id natural_key somebody -- ----------- -------- 1 1 NULL 2 1 Kirk 3 1 NULL 4 2 Roybal 5 2 NULL 6 2 NULL lag_until_you_get_something(text) function should return this in the "somebody" column: id natural_key somebody -- ----------- -------- 1 1 NULL 2 1 Kirk 3 1 Kirk 4 2 Roybal 5 2 Roybal 6 2 Roybal Notice that row 6 has a value "Roybal", when the last known value was in row 4. Also, Row 1 did not get a value. */ -- Query that gets the right result for limited example data: CREATE FUNCTION last_elem (text[]) RETURNS text AS $$ SELECT $1[array_upper($1,1)]; $$ LANGUAGE SQL; SELECT id, natural_key, last_elem(string_to_array(string_agg(somebody, '|') OVER (ORDER BY natural_key, id)::text, '|')) lag_hard FROM lag_test ORDER BY natural_key, id;

Here's a more efficient and cleaner version of same:

CREATE OR REPLACE FUNCTION GapFillInternal(   s anyelement,   v anyelement) RETURNS anyelement AS
$$
BEGIN RETURN COALESCE(v,s);
END;
$$ LANGUAGE PLPGSQL IMMUTABLE;

CREATE AGGREGATE GapFill(anyelement) ( SFUNC=GapFillInternal, STYPE=anyelement
);

postgres=# select id, natural_key, gapfill(somebody) OVER (ORDER BY
natural_key, id) from lag_test;id │ natural_key │ gapfill
────┼─────────────┼───────── 1 │           1 │ 2 │           1 │ Kirk 3 │           1 │ Kirk 4 │           2 │ Roybal 5 │           2 │ Roybal 6 │           2 │ Roybal
(6 rows)

merlin