logicalplan: fix edgecase with scalar() in distribution (#511)

scalar() is not really distributive from our experience. This is because
it returns NaN even if the given vector selector doesn't match anything
so as a consequence it is impossible to know whether the NaN is expected
or not from the PoV of the root querier.

Signed-off-by: Giedrius Statkevičius <giedrius.statkevicius@vinted.com>

logicalplan/distribute.go

@@ -552,6 +552,12 @@ func isDistributive(expr *Node, skipBinaryPushdown bool, engineLabels map[string
 				return false
 			}
 		}
+		// scalar() returns NaN if the vector selector returns nothing
+		// so it's not possible to know which result is correct. Hence,
+		// it is not distributive.
+		if e.Func.Name == "scalar" {
+			return false
+		}
 	}
 
 	return true

logicalplan/distribute_test.go

@@ -42,6 +42,11 @@ func TestDistributedExecution(t *testing.T) {
 			expr:     `(http_requests_total)`,
 			expected: `dedup(remote((http_requests_total)), remote((http_requests_total)))`,
 		},
+		{
+			name:     "scalar",
+			expr:     `scalar(redis::shard_price_per_month)`,
+			expected: `scalar(dedup(remote(redis::shard_price_per_month), remote(redis::shard_price_per_month)))`,
+		},
 		{
 			name:     "rate",
 			expr:     `rate(http_requests_total[5m])`,