You may have been routed here from other recipes, so you might not even know what prepared transactions are, let alone what an old prepared transaction looks like.
The good news is that prepared transactions don't just happen; they happen in certain situations. If you don't know what I'm talking about, it's OK! You won't need to, and better still, you probably don't have any prepared transactions either.
Prepared transactions are part of the two-phase commit feature, also known as 2PC. A transaction commits in two stages rather than one, allowing multiple databases to have synchronized commits. Its typical use is to combine multiple so-called resource managers using the XA protocol, which is usually provided by a Transaction Manager (TM), as used by the Java Transaction API (JTA) and others. If none of this means anything to you, then you probably don't have any prepared transactions.