ProjectionStage will create a SimpleMemoryUsageTracker using createChunkedSimpleMemoryUsageTrackerForStage. No per-stage limit is configured, the default INT_MAX is used. The tracker is threaded down through TransformerInterface::applyTransformation() --> ProjectionExecutor::applyProjection() --> ProjectionNode::applyExpressions(). The flags gFeatureFlagExpressionMemoryTracking and gFeatureFlagMemoryTracking must be checked before passing the tracker.

As part of the expression memory tracking work, ProjectionStage will serialise the tracker into the executionStats explain output.