Why not use PostgreSQL as a storage engine in MySQL?
Several programs demand that the underlying DB is MySQL. Similarly, software like GridSQL demand that the underlying database is PostgreSQL. If they were integrated then, there would be a wealth of advantages including:
1. Easier migration between the two.
2. Possibly the ability to use MySQL replication with PostgreSQL
3. Ability to use software that can only communicate with MySQL with PostgreSQL
4. A new transaction based storage engine in MySQL

I do understand that things like PostgreSQL arrays make it impossible to simply attach the MySQL interface to any PostgreSQL database, but the database was created only through the MySQL interface, then there would be a guarantee that it did not contain Arrays.

Now what are the other reasons why this is not possible?
Why not use PostgreSQL as a storage engine in MySQL?
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Several programs demand that the underlying DB is MySQL. Similarly, software like GridSQL demand that the underlying database is PostgreSQL. If they were integrated then, there would be a wealth of advantages including:
1. Easier migration between the two.
2. Possibly the ability to use MySQL replication with PostgreSQL
3. Ability to use software that can only communicate with MySQL with PostgreSQL
4. A new transaction based storage engine in MySQL

I do understand that things like PostgreSQL arrays make it impossible to simply attach the MySQL interface to any PostgreSQL database, but the database was created only through the MySQL interface, then there would be a guarantee that it did not contain Arrays.

Now what are the other reasons why this is not possible?