Partitioned table scanning all pertitions when the where clause specifies the partition key

All;


I created a table that includes a lastname column that is a varchar(50) :


CREATE TABLE customers (
cust_id      bigint,
custinfo   text,
cust_dob   date,
lastname   varchar(50),
firstname varchar(50),
custaddr   varchar(200),
city            varchar(100),
state        varchar(100),
zipcode   varchar(5)
)  PARTITION BY LIST (lastname) ;


Then we created 4,000 partitions with each partition hosting a specific 
set of lastname's (each partition has 100 lastname's)

The partitions are all based on UPPER(lastname) like this:


CREATE TABLE cust_part1
PARTITION OF customer
FOR VALUES IN ('SMITH', 'JONES','REX', 'ROBINSON', 'ROBINSON JR' ... )


and all the partitions have been loaded up with all the data that lines 
up with the partition key (100 lastnames per partition)


Each partition has an INDEX on upper(lastname)



When I run something like :

EXPLAIN SELECT * FROM customers where lastname = 'BORRIS';


The explain plan does an index scan on each partition even though 
constraint_exclusion os set to "partition"


Thanks in advance for any advice