A Lower Bound on Supporting Predecessor Search in $k$ sorted Arrays

TL;DR

This paper presents an optimal data structure for predecessor queries across multiple sorted arrays, achieving efficient query and construction times, and establishes a matching lower bound for such operations.

Contribution

It introduces a data structure supporting predecessor queries in $O( ext{log } n)$ time with near-linear construction, and proves this is optimal via a new lower bound using Ben-Or's technique.

Findings

Supports predecessor queries in $O( ext{log } n)$ time.

Construction time is $O(n ext{log}(rac{k}{ ext{log } n}))$.

Establishes a matching lower bound for strict predecessor queries.

Abstract

We seek to perform efficient queries for the predecessor among $n$ values stored in $k$ sorted arrays. Evading the $\Omega(n \log k)$ lower bound from merging $k$ arrays, we support predecessor queries in $O(\log n)$ time after $O(n \log(\frac{k}{\log n}))$ construction time. By applying Ben-Or's technique, we establish that this is optimal for strict predecessor queries, i.e., every data structure supporting $O(\log n)$-time strict predecessor queries requires $\Omega(n \log(\frac{k}{\log n}))$ construction time. Our approach generalizes as a template for deriving similar lower bounds on the construction time of data structures with some desired query time.