feat(frontend): redirect cast char to varchar (#20532)

e2e_test/batch/functions/cast.slt.part

@@ -38,3 +38,19 @@ select date(1);
 
 query error unexpected arguments number
 select date();
+
+query ?
+select cast('a' as char(1));
+----
+a
+
+# since we don't support char type, cast to char(2) will be the same as cast to varchar, so its behavior is the same as cast to varchar and not the same as postgresql.
+query ?
+select 'a'::char(2) = 'a '::char(2);
+----
+f
+
+query ?
+select 'a'::varchar = 'a '::varchar;
+----
+f

src/frontend/src/binder/expr/mod.rs

@@ -937,6 +937,20 @@ impl Binder {
                     Err(ErrorCode::BindError(format!("Can't cast {} to regproc", lhs_ty)).into())
                 }
             }
+            // Redirect cast char to varchar to make system like Metabase happy.
+            // Char is not supported in RisingWave, but some ecosystem tools like Metabase will use it.
+            // Notice that the behavior of `char` and `varchar` is different in PostgreSQL.
+            // The following sql result should be different in PostgreSQL:
+            // ```
+            // select 'a'::char(2) = 'a '::char(2);
+            // ----------
+            // t
+            //
+            // select 'a'::varchar = 'a '::varchar;
+            // ----------
+            // f
+            // ```
+            AstDataType::Char(_) => self.bind_cast_inner(expr, DataType::Varchar),
             _ => self.bind_cast_inner(expr, bind_data_type(&data_type)?),
         }
     }