The exception raised when a backend fails to connect to a resource. Due to pooled connections, this may be raised by describe and query functions as well as the connect function.

The exception raised when query preparation fails.

The exception raised when query execution failes.

This exception may be raised by a backend when something unexpected happen during the communication process. It signifies a programming error or at least an incompatibility somewhere:

• A query returns a different number of tuples than expected.
• A query returns tuples of a different type then expected.
• A query returns a different kind of result than expected.
• A networked backend does not understand the result from the server, e.g. due to a different protocol version.