  Since the introduction of retroactive data structures at SODA 2004, a major
unsolved problem has been to bound the gap between the best partially
retroactive data structure (where changes can be made to the past, but only the
present can be queried) and the best fully retroactive data structure (where
the past can also be queried) for any problem. It was proved in 2004 that any
partially retroactive data structure with operation time $T(n,m)$ can be
transformed into a fully retroactive data structure with operation time
$O(\sqrt{m} \cdot T(n,m))$, where $n$ is the size of the data structure and $m$
is the number of operations in the timeline [Demaine 2004], but it has been
open for 14 years whether such a gap is necessary.
  In this paper, we prove nearly matching upper and lower bounds on this gap
for all $n$ and $m$. We improve the upper bound for $n \ll \sqrt m$ by showing
a new transformation with multiplicative overhead $n \log m$. We then prove a
lower bound of $\min\{n \log m, \sqrt m\}^{1-o(1)}$ assuming any of the
following conjectures:
  - Conjecture I: Circuit SAT requires $2^{n - o(n)}$ time on $n$-input
circuits of size $2^{o(n)}$. (Far weaker than the well-believed SETH
conjecture, which asserts that CNF SAT with $n$ variables and $O(n)$ clauses
already requires $2^{n-o(n)}$ time.)
  - Conjecture II: Online $(\min,+)$ product between an integer $n\times n$
matrix and $n$ vectors requires $n^{3 - o(1)}$ time.
  - Conjecture III (3-SUM Conjecture): Given three sets $A,B,C$ of integers,
each of size $n$, deciding whether there exist $a \in A, b \in B, c \in C$ such
that $a + b + c = 0$ requires $n^{2 - o(1)}$ time.
  Our lower bound construction illustrates an interesting power of fully
retroactive queries: they can be used to quickly solve batched pair evaluation.
We believe this technique can prove useful for other data structure lower
bounds, especially dynamic ones.
