Query Design — juxt/crux 19.04-1.0.2-alpha

Query Design

Introduction

This document serves as a guide for understand the query internals of Crux.

Index

Crux has the fundamental notion of an index.

link:../src/crux/db.clj[role=include]

The two operations are seek and next.

Layered Index

The layered index exists to faciliate the idea of navigating up and down an index, in a tree like manner.

link:../src/crux/db.clj[role=include]

For example the index attribute+value+entity+content-hash is the following tree:

digraph G
{
attribute->value
value->entity
entity->content

}

open-level gives instructions to open and move down a level. In the above example if could be moving the index down to point at the values within a given attribute. That is to say that if we have an attribute :name, the index will interate across all values for that attribute, until there are no more name values.

close-level moves the index back-up, so in the above example, we can iterate at the higher level of attribute.

Virtual Index

A Virtual Index comprises together multiple child indices. This is to join indices together, returning key/value pairs on where they match.

A join condition in a query could be reflected by a Virtual Index. A Virtual Index will maintain state as to where Index is currently positioned.

UnaryJoinVirtualIndex comprises of multiple child indices and implements both Index and LayeredIndex. Calling seek-values on it will advance all the child indices internally until they all contain the same key. This involves calling seek-values on each child index until the indices match at the same level. Calling next-values would move all the indices along until the next common key that all the indices share.

Table 1. Virtual Index Example
`a`	0	1		3	4	5	6
`b`	0		2	3		5
`c`			2	3	4	5	6

In the above example, where UnaryJoinVirtualIndex joins three RelationVirtualIndexs (a,b,c). Calling seek-values would return 3 as the first value found.

Calling next-values would jump ahead to the value 5.

TODO: JP: I need to understand the Nary vs Unary testing index-test.