Parametric schema inference for massive JSON datasets


In recent years, JSON established itself as a very popular data format for representing massive data collections. JSON data collections are usually schemaless. While this ensures several advantages, the absence of schema information has important negative consequences as well: Data analysts and programmers cannot exploit a schema for a reliable description of the structure of the dataset, the correctness of complex queries and programs cannot be statically checked, and many schema-based optimizations are not possible. In this paper, we deal with the problem of inferring a schema from massive JSON datasets. We first identify a JSON type language which is simple and, at the same time, expressive enough to capture irregularities and to give complete structural information about input data. We then present our contributions, which are the design of a parametric and parallelizable schema inference algorithm, its theoretical study, and its implementation based on Spark, enabling reasonable schema inference time for massive collections. Our algorithm is parametric as the analyst can specify a parameter determining the level of precision and conciseness of the inferred schema. Finally, we report about an experimental analysis showing the effectiveness of our approach in terms of execution time, conciseness of inferred schemas, and scalability.

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    We are here ignoring empty structural types, which are record types where one mandatory field has type \(\emptyset \), since they are never inferred, and we could even forbid them in the syntax.

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    The inferred types for each dataset are reported in [19].

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    We may be more formal, as follows: Consider n keys and a space where every point is a set of shapes, that is, a set of subsets of . In this setting, every \({\mathcal {L}}\)-reduced type exactly indicates one point of a space whose size is \(2^{2^n}\); hence, each \({\mathcal {L}}\)-reduced type brings exactly the same amount of information: \(2^n\) bits. On the other side, a \({\mathcal {K}}\)-reduced type is, in general, compatible with many different points in this space; hence, it brings a lower number of bits, which depends on the number of optional keys, and may be computed for each specific \({\mathcal {K}}\)-reduced type. We may compare this number with \(2^n\) in order to mathematically quantify the information gain. We do not pursue this avenue because this model embeds the unrealistic idea that every distribution of shapes has the same probability, and because we do not believe that this model, although mathematically coherent, is a useful model of the information needs of the data analyst.

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    In the case of VK, we multiplied the original datasets 4, 6, ..., 20 times to reach a minimum size of 100 GB as the largest size.


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Correspondence to Mohamed-Amine Baazizi.

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Baazizi, MA., Colazzo, D., Ghelli, G. et al. Parametric schema inference for massive JSON datasets. The VLDB Journal 28, 497–521 (2019).

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  • JSON
  • Schema inference
  • Map-reduce
  • Spark
  • Big data collections