Monday, November 08, 2010

The trouble of getting APEX on MySQL to work...

In theory it should be so easy...running an APEX application on an Oracle instance with database links to MySQL for retrieving and modifying data. In theory that is, because in practice it is somewhat more cumbersome. I finally got there, but it took some steps  - and patience...

I started off with the steps provided from a couple of websites. Like these from Birijan, from Pythian, from WebAJ and from Easysoft. They´re all not the most recent ones, so I downloaded the latest versions of MySQL (5.1.51), MySQL Connector for ODBC (5.1.7) and unixODBC (2.3.0) and installed this stuff on top of the Oracle Developer Days Virtual Machine. Following the steps provided on the sites earlier mentioned I had a MySQL database with a demo table within minutes. So far so good!

After changing the odbcinst.ini, odbc.ini, tlistener.ora, tnsnames.ora and the initmysql.ora (in $ORACLE_HOME/hs/admin) file I fired up SQL*Plus and created a databaselink using 
create database link mysql
  connect to "demo"
  identified by "demo"
  using 'mysql';
So I expected a nice return set when I entered
select * from "employees"@mysql;
But instead of a list of employees I got this (very unclear) message:
ERROR at line 1:
ORA-28500: connection from ORACLE to a 
non-Oracle system returned this message:
[
So I Googled around, tried this and that, traced everything traceable. But still couldn't get any other result than the ORA-28500. So I turned to the forum for help, which resulted in this thread. In the last post you can see what the (simple) solution was: in the initmysql.ora (or whatever that file is called), I had a line for the language/region/characterset setting and there was a very tiny typo ...
HS_LANGUAGE=AMERICAN.AMERICA.WE8ISO8859P15
should be
HS_LANGUAGE=AMERICAN_AMERICA.WE8ISO8859P15
So from that moment on, I could enter a SQL statement in SQL*Plus and retrieve data from my MySQL database! But the happiness didn't last that long ;-(
Over and over and whatever I tried, I only retrieved just one row from my employees table - the first one according to the ordering of the result set. And sometimes even I just got one - the first - column back, and the other columns where just "gone"....

So, again, I called to the Oracle Forums to the rescue, which resulted in this thread. Again, a lot of tracing and after some time it seemed to be a problem in the version of the MySQL ODBC Connnector, because version 5.1.7 didn't work (and 5.1.0 had the same problem), while 5.1.5 did work. So I deinstalled my current version and reinstalled the previous GA version: 3.51.27. And from that moment on it worked flawlessly! I could query, insert, update and delete records in my MySQL database from Oracle without a hitch.

The last step - using an APEX application to do the data manipulation - is well written by Ilmar Kerm in this blog post, including some code to implement optimistic locking and to overcome the ORA-02047 error. And now - finally - I got an APEX application running on MySQL (in fact, it stills runs on Oracle, but uses MySQL data of course).

So after all you can do it in an hour or two:
- if you don't make any stupid typos and
- if you're "lucky" enough to use (or get) the working drivers immediately!
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