Add transaction buffer filter.

impl/memory/datastore_index_selection.go

Index: impl/memory/datastore_index_selection.go
diff --git a/impl/memory/datastore_index_selection.go b/impl/memory/datastore_index_selection.go
index ad76bb7c04ddaa4f2bda4d4d7d9beb126e84e81c..c9d876f974535550ff78e067af9156230d153416 100644
--- a/impl/memory/datastore_index_selection.go
+++ b/impl/memory/datastore_index_selection.go
@@ -167,6 +167,9 @@ func (idxs *indexDefinitionSortableSlice) maybeAddDefinition(q *reducedQuery, s
 		if len(q.eqFilters) > 1 || (len(q.eqFilters) == 1 && q.eqFilters["__ancestor__"] == nil) {
 			return false
 		}
+		if len(sortBy) > 1 && q.eqFilters["__ancestor__"] != nil {
+			return false
+		}
 	}
 
 	// Make sure the equalities section doesn't contain any properties we don't
@@ -387,7 +390,7 @@ func generate(q *reducedQuery, idx *indexDefinitionSortable, c *constraints) *it
 					"start suffix for implied ancestor doesn't start with ancestor! start:%v ancestor:%v",
 					def.start, chopped))
 			}
-			def.start = def.start[:offset+len(chopped)]
+			def.start = def.start[offset+len(chopped):]

[Vadim Sh., 2015/09/28 18:52:56]

diffs like this look scary.. Was there a bug here?

[iannucci, 2015/09/29 03:21:37]

Yeah there was. I fixed the tests for this too. Basically if you had an
inequality on __key__ and also had an ancestor and also ended up selecting a
built-in index, then this would incorrectly set the start/end points.

I actually had tests for these, but the expectations in the tests were totally
wrong >_<.

Now that you pointed it out, however, I was able to delete some more code here
which was an impossible case (the if statement above would only happen if you
had an inequality and an ancestor and a built-in index... but that's impossible!
if you look at the index listing on real appengine
(https://cloud.google.com/appengine/docs/go/datastore/indexes#Go_Index_configu...),
you can see that "Queries with ancestor and inequality filters" require new
indexes).

I also added some clarifying comment to this section.

 		}
 		if def.end != nil {
 			offset := 0
@@ -400,7 +403,7 @@ func generate(q *reducedQuery, idx *indexDefinitionSortable, c *constraints) *it
 					"end suffix for implied ancestor doesn't start with ancestor! end:%v ancestor:%v",
 					def.end, chopped))
 			}
-			def.end = def.end[:offset+len(chopped)]
+			def.end = def.end[offset+len(chopped):]
 		}
 	}