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This paper was written as the companion paper of the ICDT 2020 invited tutorial. Query determinacy is a broad topic, with literally hundreds of papers published since late 1980s. This paper is not going to be a "survey" but rather a personal perspective of a person somehow involved in the recent developments in the area. First I explain how, in the last 30+ years, the question of determinacy was formalized. There are many parameters here: obviously one needs to choose the query language of the available views and the query language of the query itself. But - surprisingly - there is also some choice regarding what the word "to compute" actually means in this context. Then I concentrate on certain variants of the decision problem of determinacy (for each choice of parameters there is one such problem) and explain how I understand the mechanisms rendering such variants of determinacy decidable or undecidable. This is on a rather informal level. No really new theorems are presented, but I show some improvements of existing theorems and also simplified proofs of some of the earlier results.
@InProceedings{marcinkowski:LIPIcs.ICDT.2020.2,
author = {Marcinkowski, Jerzy},
title = {{What Makes a Variant of Query Determinacy (Un)Decidable?}},
booktitle = {23rd International Conference on Database Theory (ICDT 2020)},
pages = {2:1--2:20},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-139-9},
ISSN = {1868-8969},
year = {2020},
volume = {155},
editor = {Lutz, Carsten and Jung, Jean Christoph},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ICDT.2020.2},
URN = {urn:nbn:de:0030-drops-119265},
doi = {10.4230/LIPIcs.ICDT.2020.2},
annote = {Keywords: database theory, query, view, determinacy}
}