postgres access a cte inside a view - sql

Question

is it possible, in postgresql - to access a cte defined in a view?
By that I mean - if you have the following:
create view my_view as
with
blah as (select 1 as x, 2 as y, 3 as z)
select
x*x as x_squared,
y*y as y_squared,
z*z as z_squared
from
blah
is there any way from outside of getting at blah? eg looking for something like:
select * from my_view.blah
Basically we have large views that use a number of complicated CTE's - and it's quite difficult sometimes to troubleshoot them without splitting them all out into separate smaller views [yes, I would prefer to just keep it like that, but I don't have that option]
I know I will be able to do it by making a stored proc that pulls out the view definition - extracts the with clauses, parses up to the blah definition, changes that to the main select, gets rid of the rest, and then does the query - but that all seems like a lot of work. Am hoping there's a built-in way?

Answer 1

You can create the CTE as a view by itself. For example:
create table a (b int);
insert into a (b) values (1), (50), (200), (350), (1000);
create view blah as
select * from a where b > 100;
Anf then base your original view on this new intermediate one to avoid repeating code:
create view my_view as
select * from blah where b < 500;
See running example at DB Fiddle.

Answer 2

ok - so I have a sort of solution - it's a not a function - it's a procedure that turns a cte into a materialized_view.
I first wanted to formulate it as function, so you could say:
select * from cte_from( 'my_real_view', 'the_cte' )
but it appears that a function needs its schema defined in advance, which is obviously impossible in this case. If anyone can suggest a hack to make it closer to above, I'd apperciate it. But anyway - bottom line this works:
create procedure from_cte(view_schema text, view_name text, cte_name text, materialized_view_name text) as
$func$
declare
_code text;
_others text;
_script text;
begin
execute format('drop materialized view if exists %s', materialized_view_name);
with recursive
string_provider as (
select view_definition as the_string,
position(concat(cte_name, ' as (') in lower(view_definition)) + length(cte_name) + 5 as start_location
from information_schema.views
where table_name = view_name
and table_schema = view_schema
),
string_walk as (
select start_location as x,
1 as brackets
from string_provider
union
select x + 1,
new_brackets
from string_provider,
string_walk,
lateral (select case
when substring(the_string from x + 1 for 1) = '(' then brackets + 1
when substring(the_string from x + 1 for 1) = ')' then brackets - 1
else brackets
end as new_brackets
) calculated
where new_brackets != 0
and x < length(the_string)
)
select substring(the_string from start_location for 1 + max(x) - start_location),
trim(substring(the_string from 0 for start_location - length(cte_name) - 5))
into _code, _others
from string_walk,
string_provider
group by the_string, start_location;
if length(_others) < 5 then
select _code into _script;
else
select concat(substring(_others from 0 for length(_others)), ' ', _script) into _script;
end if;
execute format('create materialized view %s as ( %s )', materialized_view_name, _code);
end
$func$ language plpgsql;