As I understand it, primary entity storage is the same as other MediaWiki page content storage, but using JSON blobs, with slightly different [content] types for items and properties - the formats described at the second URL you gave.

Wikibase does not attempt to fully decompose an entity into SQL normal form, as you might expect from a typical SQL data store. The same applies to references, qualifiers, etc. A corollary of this is that it is inadvisable to store too much data on any single entity if it can be avoided, as it requires JSON parsing and creation proportional to the entity size, even if just one aspect is modified.

There are however secondary tables to support entities' different roles within Wikibase's editing and display interface (e.g. property information required to turn external links into URLs), and for performance - some shared between items and properties where requirements overlap, such as for labels; somewhat analogous to indexes on the JSON, some of which are updated after the blob itself. (If PostgreSQL's hstore or jsonb had been used, I'd imagine some being real indexes or at least automatically-derived fields.)

This storage is separate from any representation in WDQS, that derives from higher-level access to Wikibase entities, and which is more suitable for querying details otherwise stored within JSON. I believe this is described more in https://mediawiki.org/wiki/Wikibase/Indexing/RDF_Dump_Format - although said format may well differ from what is actually inside Blazegraph.

From: versant.2612@gmail.com <versant.2612@gmail.com>
Sent: Friday, April 1, 2022 11:34:53 PM
To: wikibaseug@lists.wikimedia.org <wikibaseug@lists.wikimedia.org>
Subject: [Wikibase] Wikibase relational schema

Are there any document that contains the mapping of Wikidata Data Model into a Wikibase relational schema?

I want know, for example, if Item is mapped to one relational table of MariaDB and Property to another one or if both are mapped to only one table (as Entity), if ItemDescriptions has a separate table, how many tables support the Statement structure, including references and qualifiers, which column datatype is used to represent an IRI, and so on. Or even if the entire model is stored as a JSON document, one for each entity.

https://www.mediawiki.org/wiki/Wikibase/DataModel
https://doc.wikimedia.org/Wikibase/master/php/md_docs_topics_json.html
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