How is a post-transaction state calculated?

Question

A receipt from rpc returns a root which I believe is the "post-transaction state" from the yellow paper. The internet seems to say this is the same thing as the full stateRoot at the exact moment after the transaction.

However. I tested these assumptions by picking a transaction which threw an error, and looking at the receipt roots immediately before and after. When a transaction throws an error the state should not be updated, so I expected the roots to be equal. They are not.

What in the state would change after an error transaction?

Am I misunderstanding which value is which?

Answer 1

The state trie contains the balance of each account. Even transactions that throw an exception result in a reduction in the sender's balance through gas costs, which means that the resulting state is different from the initial state.

An interesting question would be to see if a transaction with a gas price of 0 can be excluded from a block without changing the resulting state root. I believe this would be the case.

EDIT: On second thought, the state trie also contains account nonces, so even if a transaction used zero ETH for gas fees, the nonce would still cause the state root to change.