Re: Showing matrix with single query

On Thu, Oct 20, 2016 at 6:07 AM, Арсений Нестюк <arseniy(dot)nestyuk(at)gmail(dot)com>
wrote:

> Hello everyone.
>
> I'm trying to solve the following problem:
> There's a table (row int, col int, val int) that represents some matrix.
> The number of columns in the matrix is arbitrary. I need to show the matrix
> in the usual form, with columns being the columns of the matrix. So, from
> the table {(0,0,1), (0,1,2), (1,0,3),(1,1,4)} I want to get the table (row
> int, col0 int, col1 int) {(0,1,2), (1,3,4)}.
> I'm trying to solve it with one query, as user-friendly as possible.
> Ideally, it should take the form
> SELECT * FROM show_matrix('name_of_matrix');
>
> Currently, I'm generating the required table with the crosstab function:
> cols = 'row_n int, ' || (SELECT string_agg(col_name, ', ') FROM
> (SELECT 'c' || generate_series(0, col_max) || ' int' AS col_name) AS Subq);
> query = 'CREATE TEMPORARY TABLE Temp_show AS SELECT * FROM
> CROSSTAB(''SELECT row_n, col, val FROM ' || matrix_name || ' ORDER BY 1'',
> ''SELECT generate_series(0, ' || col_max || ')'') AS (' || cols || ');';
>
> I have two solutions, but they need two queries. The first looks like this:
> SELECT show_matrix('matrix');
> SELECT * FROM Temp_show;
> where show_matrix essentially executes the query defined above and returns
> void.
>
> The second is
> SELECT prepare_matrix('matrix');
> SELECT * FROM show_matrix('matrix');
> Here show_matrix returns setof needed row type; prepare_matrix creates
> that type and the show_matrix with the right type in "returns ...".
> (EXECUTE 'CREATE TYPE needed_type AS (' || cols ||');';
> CREATE FUNCTION show_matrix(text) RETURNS SETOF needed_type AS $$ ...
> RETURN QUERY SELECT * FROM Temp_show;)
>
> Both solutions need two subsequent queries, because objects from the
> second query don't exist before the first is executed. The trouble is that
> the function's returning type should be defined as soon as function starts
> execution, but that type depends on the name of a matrix - on an argument.
> Triggers, to my knowledge, aren't fired on plain selects, so they can't be
> used to compress two lines into one; rules on select, on the other hand,
> allow only one action. An anyelement on the output requires an anyelement
> of the same type on the input, which I can't get without redefining
> function or some element of that type to pass as an argument to a
> polymorphic function...
>
> So, is there any way to solve the problem in one query?
>
>
​A self-contained example probably would have been more effective here...I
can barely follow along but understand the two core issues:

1. A query must define all of its columns at execution time.
2. An anyelement output function must have at least one anyelement input.

​Focusing on that alone one convention is to define a type and then pass in
a "null" of that type as the anyelement input. You then expect to get a
non-null value of that type upon function completion.

<not tested>
CREATE TYPE pair (x int, y int);
CREATE FUNCTION random_item(anyelement) RETURNS anyelement;
SELECT random_item(null::pair); -- returns pair (2, 6)

This is one more tool for the belt. I don't understand the details of your
requirements to suggest the best way to combine pseudo type functions and
dynamic SQL.

At some point you have to detect the column count from an initial query and
dynamically incorporate it into a second query. Anything beyond that is
because you are trying to make things better (faster, more understandable).

David J.