Date: Tue, 08 Aug 2023 01:34:56 +0000
To: John Light <bitcoin-dev@lightco.in>
From: ZmnSCPxj <ZmnSCPxj@protonmail.com>
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Cc: bitcoin-dev@lists.linuxfoundation.org
Subject: Re: [bitcoin-dev] Pull-req to remove the arbitrary limits on
	OP_Return outputs
Precedence: list

Good morning John Light,

> 2. Documentation about OP_RETURN says that OP_RETURN outputs are "provabl=
y-prunable".[2] A question I had about this was, are there any tools availa=
ble that a full node operator could use to prune this data from their nodes=
?

As I understand it, `OP_RETURN` outputs are already pruned, in the sense th=
at they are never put in the UTXO database in the first place.
Indeed, as I understand it, "pruning" in this context is about removing (or=
 not inserting at all, in the case of `OP_RETURN`) from the UTXO database.
UTXO database is best to keep small to reduce lookups of UTXOs being spent,=
 as that impacts speed of validation.

Archival nodes still retain the raw `OP_RETURN` data as part of the raw blo=
ck data, as it is necessary to prove that those transactions are (1) valid =
transactions and (2) part of (i.e. in the Merkle tree of) a valid block on =
the block header chain.
Block-pruning nodes also still retain this data, as they can at least serve=
 recent blocks with the same requirement of proving that transactions conta=
ining `OP_RETURN` are valid transactions in a valid block on the block head=
er chain.

If you want to prove that a block is valid, you need to present even `OP_RE=
TURN` data, as you need to be able to show the actual transaction containin=
g it, so that the verifier can see that the transaction is correctly format=
ted and its txid matches the supposed location in the Merkle tree.
Block relay requires that the node relaying a block prove that that block i=
s indeed valid, thus you need to retain the `OP_RETURN` data.
Thus, in this context "pruning" refers to not keeping `OP_RETURN` TXOs in t=
he UTXO database.

Regards,
ZmnSCPxj