[xwiki-devs] [Solr] What do we search for?

Hi devs, This is a very important question so think carefully. Let me explain: In XWiki (model) we have a few entity types. There are *wikis* which have *spaces* which have *documents*. A document can have *objects* and *attachments*. A document can also define a *class*. At the same time we like to say that in XWiki "everything is a document" because everything revolves around documents. The document is the central notion. We can query the database (using HQL or XWQL) for any of the previously mentioned entities but what should a Solr query return (semantically)? In other words: * are you searching for an object without caring about the document that holds the object? Same for an object property. * how often are you searching for an attachment without caring about the document that holds the attachment? * are you searching for a class or for the document that defines that class? * are you searching for a wiki without caring about the documents it contains? Same for a space. IMO the result of a Solr query should be, semantically, a list of documents. But maybe I'm wrong. ----------------------- Technical Details ----------------------- Unlike a relational database, Solr/Lucene index has a single 'table'. So normally you index a single entity type. Each row in the index represents an entity of that type. As a consequence the result of a Solr query is semantically a list of entities of that type. In our case the entity type is (naturally) *document*. If you want to index more entity types (e.g. index attachments and objects _separately_, not as part of a document) then, since there is only one 'table' in the index, you need to add a 'type' column that specifies the type of entity you have on each row (e.g. type=document, type=attachment, type=object etc.). The result of a Solr query is now, semantically, a list of different entity types, unless you filter by a specific type. It smells like a hack to me. Let's imagine what happens if we want to search for blog posts that has a specific tag. With the first approach this is easy because all the (indexed) information is on a single row. With the second approach this is considerably more complex because the information is spread on multiple rows: * one row with type=document for the blog post document * one row with type=object for the blog post object * one row with type=object for the tab object In a relational database when you have the information spread in multiple places (tables) you do joins. Fortunately (you would says) Solr supports joins. In this particular case we would have to perform 2 joins which means: index X index X index where X represents the cartesian product. The document name would be the join key. Pretty complex even before trying to write this in Solr query syntax.. So basically the question becomes: is it worth indexing more entities _separately_ instead of indexing just documents (with info about their objects and attachments) considering the complexity that it brings in writing Solr queries? Do we search for objects and attachments alone as separate entities often enough to justify this complexity? My answer is no. Thanks, Marius