Pass binary expressions to the underlying query

Binary math inside of a where condition was previously disallowed. Now,
these types of queries are just passed verbatim down to the underlying
query engine which can handle it.

We may want to revisit this when it comes to tags at some point as it
prevents the more efficient filtering of tags that a simple expression
allows, but it allows a query like this to be done:

    SELECT * FROM cpu WHERE value + 2 < 5

So while it can be better, this is a good initial implementation to
provide this functionality. There are very rare situations where a tag
may be used appropriately in one of these circumstances.

Fixes #3558.

CHANGELOG.md

@@ -15,6 +15,7 @@
 - [#1856](https://github.com/influxdata/influxdb/issues/1856): Add `elapsed` function that returns the time delta between subsequent points.
 - [#5502](https://github.com/influxdata/influxdb/issues/5502): Add checksum verification to TSM inspect tool
 - [#6444](https://github.com/influxdata/influxdb/pull/6444): Allow setting the config path through an environment variable and default config path.
+- [#3558](https://github.com/influxdata/influxdb/issues/3558): Support field math inside a WHERE clause.
 
 ### Bugfixes
 

tsdb/meta.go

@@ -747,6 +747,15 @@ func mergeSeriesFilters(op influxql.Token, ids SeriesIDs, lfilters, rfilters Fil
 // idsForExpr will return a collection of series ids and a filter expression that should
 // be used to filter points from those series.
 func (m *Measurement) idsForExpr(n *influxql.BinaryExpr) (SeriesIDs, influxql.Expr, error) {
+	// If this binary expression has another binary expression, then this
+	// is some expression math and we should just pass it to the underlying query.
+	if _, ok := n.LHS.(*influxql.BinaryExpr); ok {
+		return m.seriesIDs, n, nil
+	} else if _, ok := n.RHS.(*influxql.BinaryExpr); ok {
+		return m.seriesIDs, n, nil
+	}
+
+	// Retrieve the variable reference from the correct side of the expression.
 	name, ok := n.LHS.(*influxql.VarRef)
 	value := n.RHS
 	if !ok {