  Most NoSQL systems are schema-on-read: data can be stored without first
having to declare a Schema that imposes a structure. This schemaless feature
offers flexibility to evolve data-intensive applications when data frequently
change. However, freeing from declaring schemas does not mean their absence,
but rather that they are implicit in data and code. Therefore, diagramming
tools similar to those available for relational systems are also needed to help
developers and administrators understanding NoSQL schemas.
  Visualizing diagrams is not practical if schemas contain hundreds of database
entities, and exploration or query facilities are then needed. In schemaless
NoSQL stores, data of the same entity can be stored with different structure
which can increase the difficulty of having readable diagrams.
  NoSQL schema management tools should therefore have three main components:
schema extraction, schema visualization, and schema query. Since that there
exist four main NoSQL data models, it is convenient that such tools can be
built on a generic data model that provide platform-independence to query and
visualize schemas. With the aim of favoring the creation of generic database
tools, the authors of this paper defined the U-Schema unified data model that
integrates the four main NoSQL data models and the relational model.
  This paper is focused on querying NoSQL and relational schemas which are
represented as U-Schema models. We present the SkiQL language designed on
U-Schema to achieve a platform-independent schema query service. SkiQL provides
two constructs: schema-query and relationship-query. The former allows to
obtain information of entity or relationship types, and the latter that of the
aggregations or references (relations among types). We will show how SkiQL was
evaluated by calculating well-known metrics for languages and using a survey.
