On Mon, Dec 5, 2011 at 2:45 PM, David Johnston
<polobo@yahoo.com> wrote:
On Dec 4, 2011, at 22:28, Maxim Boguk <
maxim.boguk@gmail.com> wrote:
> Hi.
>
> Is here any way to combine WITH and WITH RECURSIVE into single query?
>
> Something like:
>
> WITH t AS (some complicated select to speed up recursive part),
> RECURSIVE r AS
> (
> ...
> UNION ALL
> ...
> )
>
> ?
>
> --
> Maxim Boguk
> Senior Postgresql DBA.
Add RECURSIVE after the WITH; it then applies to any/all the CTEs.
Look at the specification (and description) in the SELECT documentation closely.
David J.
Trouble is I trying to precalculate some data through WITH syntax (non recursive).
To be used later in WITH RECURSIVE part (and keep a single of that data instead of N).
Something like:
WITH _t AS (some complicated select to speed up recursive part),
RECURSIVE r AS
(
...
UNION ALL
SELECT * FROM r
JOIN t ON ...
)
So I need have precalculated t table before I start an iterator.
Now instead of _t I using record[] + unnest but that appoach very memory hungry for long iterations:
WITH RECURSIVE r AS
(
SELECT ...
ARRAY(SELECT ROW(t.*) FROM some complicated select to speed up recursive part) as _t_array
FROM ...
UNION ALL
SELECT
...,
_t_array
FROM r
JOIN (unnest(_t_array) ...) ON something
)
However that approach lead to having copy of the _t_array per each final row, so can use a lot of memory.
PS: Yes I know about pl/pgsql but WITH RECURSIVE iterators can give 2-10 time performance gains over implemenation of the same algorythm inside pl/pgsql.
--
Maxim Boguk
Senior Postgresql DBA.