Fix incorrect math when aggregates that emit different times are used

When using `non_negative_derivative()` and `last()` in a math aggregate
with each other, the math would not be matched with each other because
one of those aggregates would emit one fewer point than the others. The
math iterators have been modified so they now track the name and tags of
a point and match based on those.

This isn't necessarily ideal and may come to bite us in the future. We
don't necessarily have a defined structure for all iterators so it can
be difficult to know which of two points is supposed to come first in
the ordering. This uses the common ordering that usually makes sense,
but the query engine is getting complicated enough where I am not 100%
certain that this is correct in all circumstances.

CHANGELOG.md

@@ -7,6 +7,7 @@
 - [#7910](https://github.com/influxdata/influxdb/issues/7910): Fix EvalType when a parenthesis expression is used.
 - [#7929](https://github.com/influxdata/influxdb/issues/7929): Fix series tag iteration segfault. (#7922)
 - [#7906](https://github.com/influxdata/influxdb/issues/7906): Anchors not working as expected with case-insensitive regex
+- [#7895](https://github.com/influxdata/influxdb/issues/7895): Fix incorrect math when aggregates that emit different times are used.
 
 ## v1.2.0 [2017-01-24]
 

cmd/influxd/run/server_test.go

@@ -1684,6 +1684,15 @@ func TestServer_Query_SelectGroupByTimeDerivative(t *testing.T) {
 cpu value=15 1278010021000000000
 cpu value=20 1278010022000000000
 cpu value=25 1278010023000000000
+
+cpu0,host=server01 ticks=10,total=100 1278010020000000000
+cpu0,host=server01 ticks=30,total=100 1278010021000000000
+cpu0,host=server01 ticks=32,total=100 1278010022000000000
+cpu0,host=server01 ticks=47,total=100 1278010023000000000
+cpu0,host=server02 ticks=40,total=100 1278010020000000000
+cpu0,host=server02 ticks=45,total=100 1278010021000000000
+cpu0,host=server02 ticks=84,total=100 1278010022000000000
+cpu0,host=server02 ticks=101,total=100 1278010023000000000
 `)},
 	}
 
@@ -1789,6 +1798,11 @@ cpu value=25 1278010023000000000
 			command: `SELECT derivative(percentile(value, 50), 4s) from db0.rp0.cpu where time >= '2010-07-01 18:47:00' and time <= '2010-07-01 18:47:03' group by time(2s)`,
 			exp:     `{"results":[{"statement_id":0,"series":[{"name":"cpu","columns":["time","derivative"],"values":[["2010-07-01T18:47:02Z",20]]}]}]}`,
 		},
+		&Query{
+			name:    "calculate derivative of ticks divided by aggregate",
+			command: `SELECT non_negative_derivative(mean(ticks), 1s) / last(total) * 100 AS usage FROM db0.rp0.cpu0 WHERE time >= '2010-07-01 18:47:00' AND time <= '2010-07-01 18:47:03' GROUP BY host, time(1s)`,
+			exp:     `{"results":[{"statement_id":0,"series":[{"name":"cpu0","tags":{"host":"server01"},"columns":["time","usage"],"values":[["2010-07-01T18:47:00Z",null],["2010-07-01T18:47:01Z",20],["2010-07-01T18:47:02Z",2],["2010-07-01T18:47:03Z",15]]},{"name":"cpu0","tags":{"host":"server02"},"columns":["time","usage"],"values":[["2010-07-01T18:47:00Z",null],["2010-07-01T18:47:01Z",5],["2010-07-01T18:47:02Z",39],["2010-07-01T18:47:03Z",17]]}]}]}`,
+		},
 	}...)
 
 	for i, query := range test.queries {