Re: mysql with postgres

​This is a bit confusing on the first read. Do you really want to "hack" the source code to the PostgreSQL software? Or do you mean that you have some other software (what?) which uses the PostgreSQL data base (perhaps using libpg) and you want to modify that source code so that it will access a MySQL database instead of a PostgreSQL database?

The first case makes no sense to me, so I assume you really are doing the second case: You want some other software to interface with MySQL instead of PostgreSQL. This case makes more sense to me. And I would say that the answer is "it depends". Well, actually the answer is "yes, if you're knowledgeable about both MySQL and PostgreSQL as well as your application". Both MySQL and PostgreSQL have multiple software APIs that can be used to access them. If your software is using the "low level" API, such as libpg for PostgreSQL, then you have a lot of work to do because the API are _NOT_ identical. But if your software uses an API such as ODBC, then it should be simple. 

However, in this latter case, I think it would be better for you to talk with the developer(s) of the other software. Many developers write their code such that the database access is encapsulated in a single module so that supporting multiple database systems only requires modifying that one module. The people here have no idea because you didn't mention the name of the product which is using PostgreSQL which you want to use MySQL instead.

In general, you can't just "pop in" a new database interface to existing software when the software in question is using a low level API such as "libpg" for PostgreSQL or libmysqlclient / libmysqld for MySQL. The low level APIs are not identical. I don't know if there is anyone here who has a "how to convert from the PostgreSQL to MySQL API" type document which would be needed to address this.