Count (select 1) subquery as constant

Hello Pgsql-performance,

To not flood network with many parameters I send only one and use `WITH` hack to reuse value inside query:

WITH
_app_period AS ( select app_period() ),
ready AS (
SELECT

min( lower( o.app_period ) ) OVER ( PARTITION BY agreement_id ) <@ (select * from _app_period) AS new_order,
max( upper( o.app_period ) ) OVER ( PARTITION BY agreement_id ) <@ (select * from _app_period) AS del_order
,o.*
FROM "order_bt" o
LEFT JOIN acc_ready( 'Usage', (select * from _app_period), o ) acc_u ON acc_u.ready
LEFT JOIN acc_ready( 'Invoice', (select * from _app_period), o ) acc_i ON acc_i.ready

LEFT JOIN agreement a ON a.id = o.agreement_id
LEFT JOIN xcheck c ON c.doc_id = o.id and c.doctype = 'OrderDetail'

WHERE o.sys_period @> sys_time() AND o.app_period && (select * from _app_period)
)
SELECT * FROM ready

https://explain.depesz.com/s/kDCJ3#query

but becaues of this `acc_ready` is not inlined and I get perfomance
downgrade.

Can we mark here (select * from _app_period) subquery as constant and
allow to pass inline condition:

>none of the actual arguments contain volatile expressions or subselects
https://wiki.postgresql.org/wiki/Inlining_of_SQL_functions

this subselect is not volatile and could be expanded to constant

What do you think about this proposition?

I expect it to spent 0.5ms instead of 14ms like here (I put app_period() explicitly)
https://explain.depesz.com/s/iNTw 30 times faster!

EXPLAIN( ANALYSE, FORMAT JSON, VERBOSE, settings, buffers )
WITH ready AS (
SELECT

min( lower( o.app_period ) ) OVER ( PARTITION BY agreement_id ) <@ app_period() AS new_order,
max( upper( o.app_period ) ) OVER ( PARTITION BY agreement_id ) <@ app_period() AS del_order
,o.*
FROM "order_bt" o
LEFT JOIN acc_ready( 'Usage', app_period(), o ) acc_u ON acc_u.ready
LEFT JOIN acc_ready( 'Invoice', app_period(), o ) acc_i ON acc_i.ready

LEFT JOIN agreement a ON a.id = o.agreement_id
LEFT JOIN xcheck c ON c.doc_id = o.id and c.doctype = 'OrderDetail'

WHERE o.sys_period @> sys_time() AND o.app_period && app_period()
)
SELECT * FROM ready

--
Best regards,
Eugen Konkov