How To Access MySQL from Oracle With ODBC and SQL

The Oracle gateway for ODBC provides an almost seamless data integration between Oracle and other RDBMS. I won’t argue about its performance, limits, or relevance. It serves a few purposes; set it up and you’ll be able, for example, to create database links between Oracle and MySQL. After all, wouldn’t it be nice if you could run some of the following SQL statements?

• select o.col1, m.col1 from oracle_tab
o, mysql_tab@mysql m where o.col1=m.col1;
• insert into oracle_tab (select * from mysql_tab@mysql);

This post is intended to share, the same way Karun did it for SQL Server last year, some tips related to the setup of the Oracle Gateway for ODBC with MySQL Connector ODBC on Linux.

Prerequisites

I’ve installed all the configuration on my laptop to test it. It’s running Ubuntu Intrepid Ibex 32bits, but I won’t dig into the challenge of installing the MySQL Connector ODBC 5.1 on it. All I’ll tell you is that, if I understand correctly, the version of iodbc that comes with Intrepid doesn’t support MySQL Connector ODBC 5.1 too well, and the messages that it returns are not quite explicit. To be frank, what I did is put that monkey on Augusto’s back. He sorted out everything in a few minutes. I guess I have to thank Augusto twice, just for this post! I’ve also followed his “Installing Oracle 11gR1 on Ubuntu 8.10 Intrepid Ibex” post to install the Oracle part.

But let’s talk about the prerequisites! You need to have installed and configured the following components:

• Oracle database SE1, SE or EE; I installed 11.1.0.7 but it should work with 10g too. You can check in ORACLE_HOME/bin, it has the dg4odbc executable.
• MySQL 4.1, 5.0, 5.1 or 6.0. According to the documentation, those are the MySQL versions supported by the Connector ODBC 5.1.
• MySQL
  Connector ODBC 5.1. The Oracle Gateway for ODBC checks/relies on some features, such as the ODBC descriptor, that are not available in 3.51.

Creating a MySQLDEMO database, user and table

For the purpose of the demonstration, I’ve created a database, a user and a table named DEMO with the mysql client. You’ll find the script below.

Important Note:
The gateway for ODBC doesn’t look to work correctly when data are stored in utf8 in MySQL, whether or not the Connector/ODBC does the transformation into a non-utf8 character set. For this reason, I set the MySQL database default character set to latin1. I suspect somehow the issue is related to the ODBC driver: if I use latin1 on the client side, Oracle should not see any difference, whatever the storing character set is. Anyway, there is also a limitation on the Oracle side, and it doesn’t handle utf8 correctly with the Connector (see MySupport note 756186.1)

$ mysql -uroot -p

create database demo character set latin1;
grant all privileges on demo.* to 'demo'@'localhost'
   identified by 'demo' with grant option;
flush privileges;
exit;

$ mysql -udemo -pdemo -Ddemo

create table demo (
   col1 integer,
   col2 date,
   col3 varchar(10),
   col4 varchar(10) character set utf8,
   col5 varbinary(10)) engine innodb;

insert into demo(col1, col2, col3, col4, col5)
   values(1, cast(now() as date), '0123456789', '0123456789', '0123456789');

select * from demo \G

*********** 1. row ************
col1: 1
col2: 2009-03-11
col3: 0123456789
col4: 0123456789
col5: 0123456789

exit;

Creating an ODBC DSN to access the DEMO database

Once the database was created, I created a user DSN in the Oracle owner, so that the the listener can get it via the dg4odbc program. By default the file that store the user DSN is $HOME/.odbc.ini, but you can change it to any file/location that fits your needs. This is how the file looks like on my server:

$ cat ~oracle/.odbc.ini 
[ODBC Data Sources]
demo = MySQL ODBC Driver 5.1

[demo]
Driver      = /home/oracle/mysql515/lib/libmyodbc5.so
DATABASE    = demo
DESCRIPTION = MySQL ODBC 5.1.5 Connector Sample
PORT        = 3306
SERVER      = 127.0.0.1
# UID         = demo
# PWD         = demo
CHARSET     = latin1
TRACEFILE   = /tmp/myodbc-demodsn.trc
TRACE       = OFF

Make sure the CHARSET parameter is set so that it doesn’t use utf8.

Configuring dg4odbc to use the DSN

dg4odbc gets its settings from a file named init[SID].ora located in $ORACLE_HOME/hs/admin. In this case, SID is an arbitrary parameter we’ll configure in the listener.orafile (see next section). I’ve used SID=mysql for this demo, and the initmysql.ora file looks like the one below:

$ cat $ORACLE_HOME/hs/admin/initmysql.ora
#
# HS init parameters
#
HS_FDS_CONNECT_INFO=demo
HS_FDS_TRACE_LEVEL=0
HS_FDS_SHAREABLE_NAME=/home/oracle/mysql515/lib/libmyodbc5.so
HS_LANGUAGE=AMERICAN_AMERICA.WE8ISO8859P15
# HS_NLS_NCHAR=AL32UTF8
#
# ODBC specific environment variables
#
set ODBCINI=/home/oracle/.odbc.ini
set LD_LIBRARY_PATH=/home/oracle/mysql515/lib

#
# Environment variables required for the non-Oracle system
#
set HOME=/home/oracle

As you can see above:

• ODBCINI is the location odbc.ini the file.
• HS_FDS_CONNECT_INFO points to the right DSN.
• HS_FDS_SHAREABLE_NAME points to ODBC driver shared library.
• HS_LANGUAGE is set to avoid the problem described in Oracle MySupport “756186.1: Error Ora-28500 and Sqlstate I Issuing Selects From a Unicode Oracle RDBMS With Dg4odbc To Mysql”.

Configuring the listener

To configure the listener, I had to change the listener.ora file to add the SID defined in the previous section and associate it with the gateway for ODBC; here is a copy of my setup used; I’ve kept all the settings (host, port, dynamic registration) default:

$ cat $ORACLE_HOME/network/admin/listener.ora

SID_LIST_LISTENER =
  (SID_LIST =
    (SID_DESC =
      (ORACLE_HOME = /u01/app/oracle/product/11.1.0/db_1)
      (SID_NAME = mysql)
      (PROGRAM = dg4odbc)
      (ENVS ="LD_LIBRARY_PATH=/home/oracle/mysql515/lib:/usr/lib:$ORACLE_HOME/lib")
    )
  )

Once the listener is setup, you can bounce or reload it; if it’s not started, just start it:

$ lsnrctl start

And add an entry in the listener.ora file like the one below; make sure you’ve added HS=OK and that it’s not in the CONNECT_DATA clause:

MYSQL =
  (DESCRIPTION =
    (ADDRESS_LIST =
      (ADDRESS = (PROTOCOL = TCP)
                 (HOST = localhost)
                 (PORT = 1521)
      )
    )
    (CONNECT_DATA =
      (SID = mysql)
    )
    (HS = OK)
  )

Creating a database link to connect to MySQL from Oracle

At this point, and after a little debugging of the different layers, I was able to create a database link from Oracle to MySQL. To do so, I connected to Oracle and used the CREATE DATABASE LINK command:

$ sqlplus / as sysdba

create database link mysql 
  connect to "demo" 
  identified by "demo" 
  using 'mysql';

select "col3" from "demo"@mysql;

col3
---------------------------------
0123456789

Note:
The case policy differs between Oracle and MySQL, and you must
always surround the table and columns name with double quotes.

Using the Gateway for ODBC

If you remember correctly, I stored the string “0123456789” in col3, col4, and col5. You’ll see some of the issues with the character set by querying a datum stored in utf8:

select "col4" from "demo"@mysql;

col4
----------------------------------------
0 1 2 3 4

and one stored in a varbinary:

select "col5" from "demo"@mysql;

col5
--------------------
30313233343536373839

But you’ll also be able to enjoy some of the features of your new Oracle/MySQL integrated environment, and be able to create a table in Oracle using data from MySQL:

create table demo as 
  select "col1" col1, "col2" col2, "col3" col3 
    from "demo"@mysql;

select * from demo;

COL1 COL2      COL3
---- --------- ----------
   1 11-MAR-09 0123456789

Unfortunately, you cannot insert data directly from Oracle into MySQL with an insert as select:

insert into "demo"@mysql("col1").
  select 2 from dual;

ERROR at line 2:
ORA-02025: all tables in the SQL statement must be at the remote database

But you can workaround that issue with some PL/SQL (I’m not saying it’s efficient):

begin
  for i in (select col1, col2, col3 from demo) loop
     insert into "demo"@mysql("col1","col2", "col3")
       values (2,i.col2, i.col3);
  end loop;
end;
/

select "col1","col2", "col3" 
  from "demo"@mysql;

col1 col2      col3
---- --------- ----------
   1 11-MAR-09 0123456789
   2 11-MAR-09 0123456789

To prevent the access to MySQL from Oracle, you can drop the database link:

drop database link mysql;

That is it. It works pretty well so far and, despite the limits of such an approach, it can be quite useful for those that want to migrate from MySQL to Oracle.