Varchar Vs. Text index matching - why different?

I just noticed that comparing a text to a varchar results in an index not 
being used without a cast in some circumstances. Given that they have 
identical internal representations (or so I believe), that seems a little odd.

Can anyone shed some light on this for me? I assume it has something to do 
with believing text->varchar is a down-cast due to the size limitation on 
varchar, but don't we collect size stats of text fields (stawidth)?

The following illustrates the behaviour:

create table tt(id serial, tf text);
create unique index tfi on tt(tf);
create unique index ttpk on tt(id);

insert into tt(tf) values('fred' || currval('tt_id_seq'));
insert into tt(tf) select 'fred' || currval('tt_id_seq') from tt;



create table vt(id serial, vf varchar(31));
create unique index vfi on vt(vf);
create unique index vtpk on vt(id);

insert into vt(vf) select 'fred' || currval('vt_id_seq') from tt;

analyze;

explain select * from tt,vt where tt.id=12345 and vt.vf = tt.tf;                                 QUERY PLAN
----------------------------------------------------------------------------- Nested Loop
(cost=100000000.00..100000944.29rows=1 width=32)   Join Filter: (("inner".vf)::text = "outer".tf)   ->  Index Scan
usingttpk on tt  (cost=0.00..3.01 rows=1 width=16)         Index Cond: (id = 12345)   ->  Seq Scan on vt
(cost=100000000.00..100000531.68rows=32768 width=16)
 
(5 rows)

rt3=# explain select * from tt,vt where tt.id=12345 and vt.vf = cast(tt.tf 
as varchar);                              QUERY PLAN
---------------------------------------------------------------------- Nested Loop  (cost=0.00..7.92 rows=1 width=32)
-> Index Scan using ttpk on tt  (cost=0.00..3.01 rows=1 width=16)         Index Cond: (id = 12345)   ->  Index Scan
usingvfi on vt  (cost=0.00..4.89 rows=1 width=16)         Index Cond: (vt.vf = ("outer".tf)::character varying)
 
(5 rows)





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