[Bitcoin-development] A suggestion for reducing the size of the UTXO database

Bob McElrath bob_bitcoin at mcelrath.org
Sun May 10 13:35:25 UTC 2015

This is my biggest headache with practical bitcoin usage. I'd love to hear it if
anyone has any clever solutions to the wallet/utxo locked problem. Spending
unconfirmed outputs really requires a different security model on the part of
the receiver than #confirmations, but isn't inherently bad if the receiver has a
better security model and knows how to compute the probability that an
unconfirmed-spend will get confirmed. Of course the bigger problem is wallet
software that refuses to spend unconfirmed outputs.

I've thought a bit about a fork/merge design: if the change were computed by the
network instead of the submitter, two transactions having the same change
address and a common input could be straightforwardly merged or split (in a
reorg), where with bitcoin currently it would be considered a double-spend.  Of
course that has big privacy implications since it directly exposes the change
address, and is a hard fork, but is much closer to what people expect of a
debit-based "account" in traditional banking.

The fact of the matter is that having numerous sequential debits on an account
is an extremely common use case, and bitcoin is obtuse in this respect.

On May 9, 2015 1:09:32 PM EDT, Jim Phillips <jim at ergophobia.org> wrote:
>Forgive me if this idea has been suggested before, but I made this
>suggestion on reddit and I got some feedback recommending I also bring
>to this list -- so here goes.
>I wonder if there isn't perhaps a simpler way of dealing with UTXO
>What if, rather than deal with the issue at the protocol level, we deal
>with it at the source of the problem -- the wallets. Right now, the
>wallet selects only the minimum number of unspent outputs when building
>transaction. The goal is to keep the transaction size to a minimum so
>the fee stays low. Consequently, lots of unspent outputs just don't get
>used, and are left lying around until some point in the future.
>What if we started designing wallets to consolidate unspent outputs?
>selecting unspent outputs for a transaction, rather than choosing just
>minimum number from a particular address, why not select them ALL? Take
>of the UTXOs from a particular address or wallet, send however much
>to be spent to the payee, and send the rest back to the same address or
>change address as a single output? Through this method, we should wind
>shrinking the UTXO database over time rather than growing it with each
>transaction. Obviously, as Bitcoin gains wider adoption, the UTXO
>will grow, simply because there are 7 billion people in the world, and
>eventually a good percentage of them will have one or more wallets with
>spendable bitcoin. But this idea could limit the growth at least.
>The vast majority of users are running one of a handful of different
>apps: Core, Electrum; Armory; Mycelium; Breadwallet; Coinbase; Circle;
>Blockchain.info; and maybe a few others. The developers of all these
>wallets have a vested interest in the continued usefulness of Bitcoin,
>so should not be opposed to changing their UTXO selection algorithms to
>that reduces the UTXO database instead of growing it.
>>From the miners perspective, even though these types of transactions
>be larger, the fee could stay low. Miners actually benefit from them in
>that it reduces the amount of storage they need to dedicate to holding
>UTXO. So miners are incentivized to mine these types of transactions
>with a
>higher priority despite a low fee.
>Relays could also get in on the action and enforce this type of
>behavior by
>refusing to relay or deprioritizing the relay of transactions that
>use all of the available UTXOs from the addresses used as inputs.
>are not only the ones who benefit the most from a reduction of the UTXO
>database, they're also in the best position to promote good behavior.
>*James G. Phillips IV*
>*"Don't bunt. Aim out of the ball park. Aim for the company of
>-- David Ogilvy*
>*This message was created with 100% recycled electrons. Please think
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Sent from my Android device with K-9 Mail. Please excuse my brevity.
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