Do not attempt to push down calls with offsets

We don't correctly handle pushdowns with offsets, so we just skip them
instead of returning erroneous results.

Author: JamesGuthrie

CHANGELOG.md

@@ -19,6 +19,7 @@ We use the following categories for changes:
 
 ### Fixed
 - Fix spans with end < start. Start and end are swapped in this case. [#1096]
+- Disable push downs which use `offset`, as they are broken [#1129]
 
 ## [0.9.0] - 2022-02-02
 

pkg/pgmodel/querier/query_builder.go

@@ -242,7 +242,25 @@ func canAttemptPushdown(metadata *promqlMetadata) bool {
 	path := metadata.path // PromQL AST.
 	queryHints := metadata.queryHints
 	selectHints := metadata.selectHints
-	return extension.ExtensionIsInstalled && queryHints != nil && !hasSubquery(path) && selectHints != nil
+
+	switch {
+	// We can't push down without hints.
+	case queryHints == nil || selectHints == nil:
+		return false
+	// We can't do pushdowns without the extension.
+	case hasSubquery(path):
+		return false
+	// We can't handle subqueries in pushdowns.
+	case !extension.ExtensionIsInstalled:
+		return false
+	default:
+		// We can't handle offsets in pushdowns.
+		vs, isVectorSelector := queryHints.CurrentNode.(*parser.VectorSelector)
+		if isVectorSelector && vs.Offset != 0 {
+			return false
+		}
+	}
+	return true
 }
 
 // tryPushDown inspects the AST above the current node to determine if it's

pkg/tests/end_to_end_tests/promql_query_endpoint_test.go

@@ -2506,6 +2506,10 @@ func TestPromQLQueryEndpoint(t *testing.T) {
 			name:  "two pushdowns, same metric different matchers",
 			query: `sum(rate(metric_2{foo = "bar"}[5m]))/sum(rate(metric_2[5m]))`,
 		},
+		{
+			name:  "delta function over range selector with offset",
+			query: `delta(metric_2[1m] offset 3m)`,
+		},
 	}
 	start := time.Unix(startTime/1000, 0)
 	end := time.Unix(endTime/1000, 0)