How to use variable inside quoted string in postgres dynamic SQL - postgresql

Question

I created the following function with plpgsql which takes 3 parameters.
CREATE OR REPLACE FUNCTION public.most_service_calls(
comp_id integer,
calls integer,
months integer)
RETURNS TABLE(state character varying, city character varying, cust_name character varying, num_calls bigint, cost numeric)
LANGUAGE 'plpgsql'
COST 100
VOLATILE
ROWS 1000
AS $BODY$
Begin
return query execute
'select * from
(select l.state, l.city, l.cust_name, count(distinct a.svc_ord_nbr) num_calls,
round(avg(a.std_labr_net_amt_dcrncy) + avg(a.travel_net_amt_dcrncy), 2)
from dmt_mas_svc_ord_fact a
inner join dmt_mas_cust_dim b on a.shipto_cust_id = b.cust_id
inner join store_location l on b.cust_name = l.cust_name
inner join company co on b.cust_lvl_2_nbr = co.company_nbr
where b.sap_sls_org_name like ''%Stanley US%''
and a.create_dtm >= now() - interval '' $3 months''
and co.company_id = $1
group by l.state, l.city, l.cust_name
order by l.state, l.city, l.cust_name ) q
where num_calls >= $2'
using comp_id, calls, months;
end;
$BODY$;
Since the query is quoted, all the single quoted strings are double quoted. Three variables are represented by $1, $2, $3. It is the variable inside a string that is causing the trouble. a.create_dtm >= now() - interval '' $3 months''
When I run the function, it seems to ignore whatever third parameter I provided. Therefore, all the following return the same result.
select * from most_service_calls(1,5,1)
select * from most_service_calls(1,5,12)
select * from most_service_calls(1,5,24)
And it turned out, $3 inside '' '' is taken as 3 since the result matches that of the query with 3 months hardcoded in the query.
What is the correct way to include the variable inside a string in a quoted query like this?

Answer 1

Your problem is not specific to dynamic SQL - you can't refer to a placeholder within a quoted string even in a normal SQL query.
Instead you could use:
$3 * interval '1 month'
or:
($3 || ' months')::interval
The first form multiplies your supplied numeric value by a one month interval. The second constructs a string specifying the number of months and then casts it to an interval.