commit afe5c5cb5438f0b6a3240b816ef7bd0629b98a69
author Shawn Pearce <sop@google.com> Sat Jul 01 20:11:59 2017 -0700
committer Shawn Pearce <sop@google.com> Mon Aug 07 07:25:56 2017 -0700
tree 32afbd119aba7be8bec0201f4b632299c4be12e8
parent 48373ee948023894603aae9675817c8b4cc36381

Optimize conflicts:id to send predicates to index

The conflicts operator was creating a complex predicate tree that
confused the query rewriter, preventing it from pushing indexed
predicates to the index query, resulting in full index scans being
performed for all open changes.

On a server with 100k open changes and a sluggish index and storage
backend, this bad behavior results in 12s per change screen view to
identify potential conflicts.

Simplify the code by building the OR graph for multiple change matches
at a higher level in ChangeQueryBuilder. ConflictsPredicate now only
has to deal with a single change at a time.

Create a standard AST of just AndPredicate/OrPredicate. The custom
subclass of AndPredicate used by ConflictsPredicate confuses the query
rewriter and makes it harder to join the (status:new OR status:draft)
part of the tree sent by the frontend with the (project:foo ref:bar)
contributed by ConflictsPredicate. That join is necessary to restrict
candidates from the change index to a feasible number.

Drop the IsMergePredicate, introduced in eb9b8a8c77ae by Saša as a fix
for issue 3052. This is not an indexed property and forces the query
rewriter to split the search into several full index scans:

parsed tree with IsMergePredicate:

  (status:new OR status:draft) mergeable2:1
  project:src ref:refs/heads/master
  -change:33
  (path:b.txt OR path:hi.txt OR merge:true)
  conflicts:33

executing predicate tree:

  AND
  + index{p=((status:new OR status:draft) mergeable2:1
          project:src ref:refs/heads/master -change:33)}
  + OR
    - index{p=(path:b.txt OR path:hi.txt)}
    - merge:true
  + conflicts:33

Refixing issue 3052 will require an index schema bump, either
including files different between 1st and 2nd parent in files
field within the index, or indexing a new is:merge boolean field.
Both are out of scope for this commit.

Dropping IsMergePredicate creates an optimal query for the index,
as intended:

  AND
  + index{p=((status:new OR status:draft) mergeable2:1
          project:src ref:refs/heads/master -change:33
          (path:b.txt OR path:hi.txt))}
  + conflicts:33

That addresses the immediate performance impact of the conflicts
query, even though it may omit some potentially conflicting merges.

Change-Id: I38c703f184f8018c1253e28a5365e22c1e7716d0