SELECT d."Selling_date",  d."Value_in_EUR", d."Value_in_currency", d."Site"
FROM report.get_sa001('2016-01-01'::date, '2017-03-31'::date, 32) AS d

Select a."Selling_currency" FROM report."Axis_RefCustomer" AS a

SELECT d."Selling_date",
a."Selling_currency",
d."Value_in_EUR",
d."Value_in_currency",
d."Site"
FROM report.get_sa001('2016-01-01'::date, '2017-03-31'::date, 32) AS d
LEFT JOIN report."Axis_RefCustomer"
AS a ON d."Site" = a."Site"
AND d."Internal_reference" = a."Reference_internal"
AND d."Customer_code" = a."Customer_code"

Function Scan on get_sa001 d  (cost=0.25..10.25 rows=1000 width=104) (actual time=2522.959..2534.987 rows=53446 loops=1)
Total runtime: 2537.926 ms

Hash Right Join  (cost=3527.82..5840.32 rows=74513 width=4) (actual time=47.363..71.317 rows=77965 loops=1)
    Hash Cond: ((("T12_RefCustomer"."Site")::text = ("T01_References"."Site")::text) AND (("T12_RefCustomer"."Internal_reference")::text = ("T01_References"."Internal_reference")::text))
    ->  Seq Scan on "T12_RefCustomer"  (cost=0.00..348.81 rows=13181 width=29) (actual time=0.002..2.350 rows=13182 loops=1)
    ->  Hash  (cost=1973.13..1973.13 rows=74513 width=22) (actual time=46.591..46.591 rows=74513 loops=1)
            Buckets: 2048  Batches: 4  Memory Usage: 1019kB
            ->  Seq Scan on "T01_References"  (cost=0.00..1973.13 rows=74513 width=22) (actual time=0.014..18.580 rows=74513 loops=1)
    Total runtime: 72.540 ms

Nested Loop Left Join  (cost=1.23..2375.17 rows=1000 width=108) (actual time=2406.131..42314.297 rows=53446 loops=1)
  ->  Function Scan on get_sa001 d  (cost=0.25..10.25 rows=1000 width=168) (actual time=2405.980..2429.250 rows=53446 loops=1)
  ->  Nested Loop  (cost=0.98..2.35 rows=1 width=28) (actual time=0.547..0.746 rows=1 loops=53446)
        Join Filter: (((d."Site")::text = ("T01_References"."Site")::text) AND ((d."Internal_reference")::text = ("T01_References"."Internal_reference")::text) AND (("T02_Customers"."Site")::text = ("T01_References"."Site")::text))
        Rows Removed by Join Filter: 126"
        ->  Nested Loop  (cost=0.57..1.42 rows=1 width=33) (actual time=0.011..0.057 rows=127 loops=53446)
              ->  Index Scan using "T02_Customers_pkey" on "T02_Customers"  (cost=0.28..0.77 rows=1 width=17) (actual time=0.004..0.004 rows=1 loops=53446)
                    Index Cond: ((d."Customer_code")::text = ("Customer_code")::text)
              ->  Index Scan using "T12_RefCustomer_pkey" on "T12_RefCustomer"  (cost=0.29..0.64 rows=1 width=29) (actual time=0.007..0.027 rows=99 loops=68927)
                    Index Cond: ((("Customer_code")::text = ("T02_Customers"."Customer_code")::text) AND (("Site")::text = ("T02_Customers"."Site")::text))
        ->  Index Scan using "T01_References_pkey" on "T01_References"  (cost=0.42..0.92 rows=1 width=22) (actual time=0.005..0.005 rows=1 loops=6795220)
              Index Cond: ((("Internal_reference")::text = ("T12_RefCustomer"."Internal_reference")::text) AND (("Site")::text = ("T12_RefCustomer"."Site")::text))
Total runtime: 42318.196 ms

with
    __d as(
        select
            "Selling_date",
            "Value_in_EUR",
            "Value_in_currency",
            "Site",
            "Internal_reference",
            "Customer_code"
        from
            report.get_sa001('2016-01-01'::date, '2017-03-31'::date, 32)
    ),
    __a as(
        select
            "Selling_currency",
            "Site",
            "Reference_internal" as "Internal_reference",
            "Customer_code"
        from
            report."Axis_RefCustomer"
        where
            ("Site", "Reference_internal", "Customer_code") in(
                select
                    "Site",
                    "Internal_reference",
                    "Customer_code"
                from
                    __d
            )
    )
select
    __d."Selling_date",
    __a."Selling_currency",
    __d."Value_in_EUR",
    __d."Value_in_currency",
    __d."Site"
from
    __d
    left join __a using("Site", "Internal_reference", "Customer_code")

SELECT d."Selling_date"
     , a."Selling_currency"
     , d."Value_in_EUR"
     , d."Value_in_currency"
     , d."Site"
FROM   (
   SELECT *
   FROM   report.get_sa001(date '2016-01-01', date '2017-03-31', 32)
   OFFSET 0  -- probably redundant
   ) d
LEFT   JOIN report."Axis_RefCustomer" a ON d."Site" = a."Site"
                                       AND d."Internal_reference" = a."Reference_internal"
                                       AND d."Customer_code" = a."Customer_code";

SELECT d."Selling_date"
     , ( SELECT "Selling_currency"
         FROM   report."Axis_RefCustomer"
         WHERE  d."Site"                = a."Site"
         AND    d."Internal_reference"  = a."Reference_internal"
         AND    d."Customer_code"       = a."Customer_code")
     , d."Value_in_EUR"
     , d."Value_in_currency"
     , d."Site"
FROM   report.get_sa001(date '2016-01-01', date '2017-03-31', 32) d;