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An Intensional Expressiveness Gap of Comprehension Syntax

Author Limsoon Wong

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Author Details

Limsoon Wong
  • School of Computing, National University of Singapore, Singapore

Funding

  • Wong, Limsoon: This work was supported in part by a Singapore Ministry of Education tier-2 grant, MOE-T2EP20221-0005; and in part by a Kwan Im Thong Hood Cho Temple Chair Professorship.

Acknowledgements

During Val Tannen’s visit to Singapore in 2019, we developed the concept of synchrony iterators [Stefano Perna et al., 2022]. Synchrony iterators enables efficient algorithms for synchronized iterations on multiple collections to be expressed elegantly using comprehension syntax. Notably, they simplify the creation of efficient algorithms for low-selectivity non-equijoins. During our discussions, I was struck by an apparent gap: the difference between the efficiency of database queries in comprehension syntax and their real-world implementation in relational database systems. I am grateful for Val’s friendship and mentorship over the past three and a half decades. I am honoured and glad to contribute this chapter, which bridges the above gap in our understanding of comprehension syntax, as a tribute to Val in this festschrift volume.

Cite AsGet BibTex

Limsoon Wong. An Intensional Expressiveness Gap of Comprehension Syntax. In The Provenance of Elegance in Computation - Essays Dedicated to Val Tannen. Open Access Series in Informatics (OASIcs), Volume 119, pp. 11:1-11:13, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2024)
https://doi.org/10.4230/OASIcs.Tannen.11

Abstract

Comprehension syntax is widely adopted in modern programming languages as a means for manipulating collection types. This paper proves that all subquadratic algorithms which are expressible in comprehension syntax, do not compute low-selectivity joins. As database systems support these joins efficiently, this confirms an intensional expressiveness gap between comprehension syntax and relational database systems. The proof of this intensional expressiveness gap relies on a "limited-mixing" lemma which states that subquadratic algorithms expressible using comprehension syntax have limited ability for mixing atomic objects in their inputs.

Subject Classification

ACM Subject Classification
  • Information systems → Relational database query languages
Keywords
  • Comprehension syntax
  • intensional expressive power
  • limited-mixing lemma

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