app/vlselect/logsql: skip rows without _stream reference

_stream field can be empty for the recently ingested rows because respective entry in indexdb is not yet searchable as it haven't been flushed to storage yet.
This change just skips such items in the output response to make it more consistent.

See: https://github.com/VictoriaMetrics/VictoriaMetrics/issues/6042
Signed-off-by: Zakhar Bessarab <z.bessarab@victoriametrics.com>

app/vlselect/logsql/logsql.go

@@ -48,8 +48,29 @@ func ProcessQueryRequest(w http.ResponseWriter, r *http.Request, stopCh <-chan s
 		}
 		rowsCount := len(columns[0].Values)
 
+		// skip entries with empty _stream column
+		// _stream is empty in case indexdb entry was not flushed to the storage yet
+		// skipping such entries makes the result more consistent
+		streamCol := 0
+
+		// fast path
+		// _stream column is a built-in column and it is always supposed to be at the same position
+		if len(columns) >= 2 && columns[1].Name == "_stream" {
+			streamCol = 1
+		} else {
+			for i := 1; i < len(columns); i++ {
+				if columns[i].Name == "_stream" {
+					streamCol = i
+					break
+				}
+			}
+		}
+
 		bb := blockResultPool.Get()
 		for rowIdx := 0; rowIdx < rowsCount; rowIdx++ {
+			if columns[streamCol].Values[rowIdx] == "" {
+				continue
+			}
 			WriteJSONRow(bb, columns, rowIdx)
 		}