trying to build immutable function in index

Good afternoon,

We are still running postgresql 9.4 - still trying to convince management
to upgrade :<(

I am trying to create this index (the REGEXP portion is a date check)

create index custattr_value_includes on customer_attributes
using btree (value, account_id, group_num, id)
WHERE value::text ~
'^([0-9][0-9][0-9][0-9])-(([0]?[1-9])|([1][0-2]))-(([0-2]?[0-9])|([3][0-1]))$'::text
AND value::text ~
'([0-9][0-9][0-9][0-9])-(0[13578]|1[02])-(0[1-9]|[12][0-9]|3[01])|([0-9][0-9][0-9][0-9])-(0[469]|11])-(0[1-9]|[12][0-9]|30)|([0-9][0-9][0248][048]|[0-9][0-9][13579][26])-(02)-(0[1-9]|1[0-9]|2[0-9])|([0-9][0-9][0248][1235679]|[0-9][0-9][13579][01345789])-(02)-(0[1-9]|1[0-9]|2[0-8])'::text
AND value::date >= date_trunc('MONTH'::text, 'now'::text::date - '1
mon'::interval month)
AND value::date <= date_trunc('DAY'::text, 'now'::text::date::timestamp
with time zone)
AND account_id IS NOT NULL

Predictably the date_trunc portion errors out with this error:
ERROR: functions in index predicate must be marked IMMUTABLE
So I copy the source of the 3 flavors of date_trunc as follows:

CREATE OR REPLACE FUNCTION pg_catalog,date_trunc(
text,
timestamp with time zone,
text)
RETURNS timestamp with time zone
LANGUAGE 'internal'

COST 1
STABLE STRICT PARALLEL SAFE
AS $BODY$timestamptz_trunc_zone$BODY$;

ALTER FUNCTION public.my_date_trunc(text, timestamp with time zone, text)
OWNER TO postgres;

COMMENT ON FUNCTION public.my_date_trunc(text, timestamp with time zone,
text)

changing the STABLE behavior to IMMUTABLE and the name from
pg_catalog.date_trunc to public.my_date_trunc (all three flavors) and
recompile all three
Then I change the index source to reference the newly compiled functions
create index custattr_value_includes on customer_attributes
using btree (value, account_id, group_num, id)
WHERE value::text ~
'^([0-9][0-9][0-9][0-9])-(([0]?[1-9])|([1][0-2]))-(([0-2]?[0-9])|([3][0-1]))$'::text
AND value::text ~
'([0-9][0-9][0-9][0-9])-(0[13578]|1[02])-(0[1-9]|[12][0-9]|3[01])|([0-9][0-9][0-9][0-9])-(0[469]|11])-(0[1-9]|[12][0-9]|30)|([0-9][0-9][0248][048]|[0-9][0-9][13579][26])-(02)-(0[1-9]|1[0-9]|2[0-9])|([0-9][0-9][0248][1235679]|[0-9][0-9][13579][01345789])-(02)-(0[1-9]|1[0-9]|2[0-8])'::text
AND value::date >= my_date_trunc('MONTH'::text, 'now'::text::date - '1
mon'::interval month)
AND value::date <= my_date_trunc('DAY'::text, 'now'::text::date::timestamp
with time zone)
AND account_id IS NOT NULL

And I am still getting the error: functions in index predicate must be
marked IMMUTABLE

Am I missing something? Any comments welcome. Thank you.

--
*Mark Steben*
Database Administrator
@utoRevenue <http://www.autorevenue.com/> | Autobase
<http://www.autobase.net/>
CRM division of Dominion Dealer Solutions
95D Ashley Ave.
West Springfield, MA 01089
t: 413.327-3045
f: 413.383-9567

www.fb.com/DominionDealerSolutions
www.twitter.com/DominionDealer
www.drivedominion.com <http://www.autorevenue.com/>

<http://autobasedigital.net/marketing/DD12_sig.jpg>