The same problem is repeatable with an MS SQL Server data source.

Your create table is incorrect:

"create table t1 (a varchar(10)) engine=connect table_type=odbc connection='DSN=oraodbc;UID=system;PWD=manager' catfunc=Columns;"

The column definition is for a catfunc table, it is not the one of the table that is queried. The simpler is to leave CONNECT define them by not specifying it:

create table ct1 engine=connect table_type=odbc tabname='t1' connection='DSN=oraodbc;UID=system;PWD=manager' catfunc=Columns;

CREATE TABLE `ct1` (
`Table_Qualif` char(128) NOT NULL,
`Table_Owner` char(128) NOT NULL,
`Table_Name` char(128) NOT NULL,
`Column_Name` char(128) NOT NULL,
`Data_Type` smallint(6) NOT NULL,
`Type_Name` char(20) NOT NULL,
`Precision` int(10) NOT NULL,
`Length` int(10) NOT NULL,
`Scale` smallint(6) NOT NULL,
`Radix` smallint(6) NOT NULL,
`Nullable` smallint(6) NOT NULL,
`Remarks` char(128) NOT NULL
) ENGINE=CONNECT DEFAULT CHARSET=latin1 CONNECTION='DSN=ORACLE_TEST;UID=system;PWD=manager' `TABLE_TYPE`='odbc' `TABNAME`='T1' `CATFUNC`='Columns';

and "select * from ct1;" returns:

Table_Qualif Table_Owner Table_Name Column_Name Data_Type Type_Name Precision Length Scale Radix Nullable Remarks
<null> SYSTEM T1 A 3 DECIMAL 38 40 0 10 1 <null>

Note 1: On windows the wrong catfunc table does not cause a "lost connection" but returns an error message saying:
mysql_store_result failed: Got error 174
'[Oracle][ODBC][ORA]ORA-01406: fetched column value truncated' from CONNECT

Note 2: The Oracle t1 table was created as "create table t1 (a int)". Oracle makes a DECIMAL 38 column a. This is probably the cause of the MDEV-5340 bug.

Note 3: This obviously applies also to any data source including MS SQL Server.

This query without a table definition crashes with Oracle with the same symptoms:

drop table if exists t1;
create table t1 engine=connect table_type=odbc connection='DSN=oraodbc;UID=system;PWD=manager' catfunc=Columns;
select * from t1;

ERROR 2013 (HY000): Lost connection to MySQL server during query

The problem is repeatable with any data source having many tables (400+).

Fixed in 10.0-connect.