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

Pieter Wuille pieter.wuille at gmail.com
Sat May 9 20:38:56 UTC 2015

Miners do not care about the age of a UTXO entry, apart for two exceptions.
It is also economically irrelevant.
* There is a free transaction policy, which sets a small portion of block
space aside for transactions which do not pay sufficient fee. This is
mostly an altruistic way of encouraging Bitcoin adoption. As a DoS
prevention mechanism, there is a requirement that these free transactions
are of sufficient priority (computed as BTC-days-destroyed per byte),
essentially requiring these transactions to consume another scarce
resource, even if not money.
* Coinbase transaction outputs can, as a consensus rule, only be spent
after 100 confirmations. This is to prevent random reorganisations from
invalidating transactions that spend young coinbase transactions (which
can't move to the new chain). In addition, wallets also select more
confirmed outputs first to consume, for the same reason.
On May 9, 2015 1:20 PM, "Raystonn" <raystonn at hotmail.com> wrote:

> That policy is included in Bitcoin Core.  Miners use it because it is the
> default.  The policy was likely intended to help real transactions get
> through in the face of spam.  But it favors those with more bitcoin, as the
> priority is determined by amount spent multiplied by age of UTXOs.  At the
> very least the amount spent should be removed as a factor, or fees are
> unlikely to ever be paid by those who can afford them.  We can reassess the
> role age plays later.  One change at a time is better.
>  On 9 May 2015 12:52 pm, Jim Phillips <jim at ergophobia.org> wrote:
> On Sat, May 9, 2015 at 2:43 PM, Raystonn <raystonn at hotmail.com> wrote:
> How about this as a happy medium default policy: Rather than select UTXOs
> based solely on age and limiting the size of the transaction, we select as
> many UTXOs as possible from as few addresses as possible, prioritizing
> which addresses to use based on the number of UTXOs it contains (more being
> preferable) and how old those UTXOs are (in order to reduce the fee)?
> If selecting older UTXOs gives higher priority for a lesser (or at least
> not greater) fee, that is an incentive for a rational user to use the older
> UTXOs.  Such policy needs to be defended or removed.  It doesn't support
> privacy or a reduction in UTXOs.
> Before starting this thread, I had completely forgotten that age was even
> a factor in determining which UTXOs to use. Frankly, I can't think of any
> reason why miners care how old a particular UTXO is when determining what
> fees to charge. I'm sure there is one, I just don't know what it is. I just
> tossed it in there as homage to Andreas who pointed out to me that it was
> still part of the selection criteria.
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