Whats happen here?

Whats happen here?
-------------------------

root=> \dt

Database = root
+------------------+----------------------------------+----------+
| Owner | Relation | Type |
+------------------+----------------------------------+----------+
| root | x | table |
| root | y | table |
+------------------+----------------------------------+----------+
root=> \d x

Table = x
+----------------------------------+----------------------------------+-----
--+
| Field | Type |
Length|
+----------------------------------+----------------------------------+-----
--+
| a | char() |
20 |
+----------------------------------+----------------------------------+-----
--+
root=> \d y

Table = y
+----------------------------------+----------------------------------+-----
--+
| Field | Type |
Length|
+----------------------------------+----------------------------------+-----
--+
| a | varchar() |
5 |
+----------------------------------+----------------------------------+-----
--+
root=>select * from x a, y b where a.a = b.a;
ERROR: There is more than one possible operator '=' for types 'bpchar' and
'varchar'
You will have to retype this query using an explicit cast

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