I am using rails 3 and ruby 1.9.2 in my application. For development environment, I am using MySQL and for staging environment, I am using PostgreSQL Database.
After hosting, we have faced some issues.

1) Quoting styles: MySQL allows you to quote table and column names with backquotes, whereas PostgreSQL uses double quotes. For ex: One of our tables has a column in it called when, which must be quoted whenever we use it. Rails will of course handle the quoting for you if you do something like

Meeting.find_by_when(Time.now)

But if you are constructing your own SQL conditions then you have to handle the quoting problem.

In MySQL, it would be like

Meeting.where("`when` < ?", Time.now)

In PostgreSQL, it would be like

Meeting.where("\"when\" < ?", Time.now)

Solution:

Meeting.where("#{Meeting.connection.quote_column_name(when)} < ?", Time.now)

2) Boolean type: MySQL lacks a native BOOLEAN type, so if you create a boolean column in Rails, you will end up with a TINYINT(1) column which has values of 0 and 1 for false and true respectively. PostgreSQL has a native BOOLEAN type, it will accept only false/true unlike MySQL.

In MySQL, it would be like

Meeting.where("import=1") OR Meeting.where("import=?", true)

In PostgreSQL, it would be like

Meeting.where("import=?", true)

Solution:

so replace 0 and 1 with false and true in your all files then it work in both 
MySQL and PostgreSQL.

3) Other differences:

1. Fulltext Search: PostgreSQL is case sensitive. MySQL is not case sensitive.
2. To select random records from DB, Mysql has a function called “rand()” and PostgreSQL has a function called “random()”.
3. PostgreSQL ALTER TABLE supports ADD COLUMN, RENAME COLUMN and RENAME TABLE only. MySQL has all options in ALTER TABLE.
4. In PostgreSQL, attribute name starting with numbers, like “360_degree” are not allowed.

Good luck!