In PipelineD::prepareExecutor, we are sometimes pushing down a computed projection past a sort+limit. This is incorrect because the projection can raise an error when it sees documents that the sort+limit would not return.
We should change buildProjectionForPushdown so that it never returns a computed projection:
/** * Given a dependency set and a pipeline, builds a projection BSON object to push down into the * PlanStage layer. The rules to push down the projection are as follows: * 1. If there is an inclusion projection at the front of the pipeline, it will be pushed down * as is. * 2. If there is no inclusion projection at the front of the pipeline, but there is a finite * dependency set, a projection representing this dependency set will be pushed down. * 3. Otherwise, an empty projection is returned and no projection push down will happen. */ auto buildProjectionForPushdown(const DepsTracker& deps, Pipeline* pipeline) {
Computed projections should be ineligible for case 1. For example, if the pipeline starts with {$project: {b: {$round: "$a"}}}, we'll fall through to case 2 and push down {a: 1}, based on the dependencies.
- related to
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SERVER-54768 Sort/project re-ordering is inconsistent between find() and aggregation
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- Backlog
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