How is Database Migration done?

Question

i remember in my previous job, i needed to do data migration. in that case, i needed to migrate to a new system, i was to develop, so it has a different table schema. i think 1st, i should know:

• in general, how is data migrated (with the same schema) to a different DB engine. eg. MySQL -> MSSQL. in my case, my destination DB was MySQL and i used MySQL Migration Toolkit

• i am thinking, in an enterprise app, there may be stored procedures, triggers that also need to be imported.

• if table schema is different, how will i then go abt doing this? in my prev job, what i did was import data (in my case, from Access) into my destination (MySQL) leaving table structures. then use SQL to select data and manipulate as required into final destination tables.

• in my case, where i dont have documentation for the old db, and the columns was not named correctly, eg. it uses say 'field1', 'field2' etc. i needed to trace from the application code what the columns mean. any better way? or sometimes, columns contain multiple values in delimited data, is reading code the only way?

Answer 1

I really depends, but from your question I assume you want to hear what other people do. So here is what I do in my current project.

I have to migrate from Oracle to Oracle but to a completely different schema. The old system was 2-tier (old client, old database) the new system is 3-tier (new client, business logic, new database). We have more than 600 tables in the new schema.

After much pondering we scraped the idea of doing a migration from old database to new database in SQL. We decided that in our case i would be much easier to go:

old database -> old client -> business logic -> new database

In the old database much of the data is stored in strange ways and the old client mangles it in complex ways. We have access to the source code of the old client but it is a very large system.

We wrote a migration tool that sits above the old client and the business logic. We have some SQL before and some SQL after that but the bulk of data is migrated via old client and business logic.

The downside is that it is slow, a complete migration taking more than 190 hours in our case but otherwise it works well.

UPDATE

As far as stored procedures and triggers are concerned: Even as we use the same DBMS in old and new system (both Oracle) the procedures and triggers are written from scratch for the new system.