Scope & Purpose

This is an experimental feature in draft status.

This PR adds an optimizer rule that attempts to replace certain(*) 1-step-traversals by a join. The rationale is that for some (commonly encountered) queries this transformation yields good speed improvements.

The rule replaces

FOR v,e,p IN 1..1 {IN/OUT}BOUND $startVertex GRAPH $G 
   ...

by

FOR e IN edgeColl
  FILTER e._from == $startVertex
  FOR v IN vertexColl
    FILTER e._to == v._id
    LET p = { vertices: [$startVertex, v], edges: [ e ] }

This is not without challenges to get semantically equivalent, and should currently be considered incorrect and broken often. The current conditions under which this is considered correct are

• The traversal only uses 1..1 as depth
• The traversal involves precisely one edge collection
• The traversal involves precisely one vertex collection
• The traversal direction is either INBOUND or OUTBOUND
• TODO: Combination of uniqueness levels that are permissible.
• TODO: StartVertex can be a document or a string id; if it is a string ID, the document needs to be materialized appropriately