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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