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Consider the case where:

  • Address A has some BTC.
  • Address B is empty/zero.
  • Address C is the intended recipient.

B sending to C is invalid on its own, but does make sense if associated with A sending to B first

Can two dependent transactions be put into the same block and accepted?

  • A ==> B
  • B ==> C

(Since the net result will be OK, even though the second transaction does not make sense until the first one is completed)

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6

Yes, but only if A -> B is placed before B -> C in the list of transactions.

Here is a graph of the percentage of transactions spending each other in the same block, it is usually around 10%.

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3
  • So even within a block, there is a strict ordering of transactions? I had been under the impression that blocks were ordered, but transactions within a block were unordered! Very interesting. Thanks.
    – abelenky
    Commented Jan 7, 2021 at 16:56
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    Actually, there are a substantial amount of such transactions :) Slightly related: eprint.iacr.org/2019/611 discusses those transactions as UTXOs created and spent in the same block reduce the size of the proofs
    – Antoine Poinsot
    Commented Jan 7, 2021 at 17:02
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    @abelenky No, there is no strict ordering, at least for Bitcoin. The answer states one of the three requirements: 1) this answer 2) the coinbase tx comes first 3) no duplicate/invalid/double-spend transactions. There are usually multiple possible orderings satisfying these requirements.
    – MCCCS
    Commented Jan 7, 2021 at 20:46

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