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Type:
Improvement
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Resolution: Unresolved
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Priority:
Trivial - P5
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None
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Affects Version/s: None
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Component/s: Aggregation Framework
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Query Optimization
There are many special cases where the change streams opt out from the normal targeting, and some cases where they require special code that just lives alongside the more "normal" path. As some examples, look at these grep results:
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git grep -P '(\$change|change stream|change streams|ChangeStream)' src/mongo/s/query/cluster_aggregate.cpp src/mongo/db/pipeline/sharded_agg_helpers.* src/mongo/s/query/cluster_aggregation_planner.cpp src/mongo/db/pipeline/sharded_agg_helpers.cpp: // $changeStream aggregations ignore NamespaceNotFound in order to allow streams to be opened on src/mongo/db/pipeline/sharded_agg_helpers.cpp: // present, then $changeStream should immediately return an empty cursor just as other src/mongo/db/pipeline/sharded_agg_helpers.cpp: // - Any aggregation which begins with a $changeStream stage. src/mongo/db/pipeline/sharded_agg_helpers.cpp: return nss.isCollectionlessAggregateNS() || litePipe.hasChangeStream(); src/mongo/db/pipeline/sharded_agg_helpers.cpp: // If this is a $changeStream, we swallow NamespaceNotFound exceptions and continue. src/mongo/db/pipeline/sharded_agg_helpers.cpp: if (!(litePipe.hasChangeStream() && src/mongo/db/pipeline/sharded_agg_helpers.cpp: // In order for a $changeStream to work reliably, we need the shard registry to be at least as src/mongo/db/pipeline/sharded_agg_helpers.cpp: // that must run on all shards (e.g. $currentOp) because, unlike $changeStream, those pipelines src/mongo/db/pipeline/sharded_agg_helpers.cpp: if (litePipe.hasChangeStream()) { src/mongo/s/query/cluster_aggregate.cpp: invariant(!litePipe.hasChangeStream()); src/mongo/s/query/cluster_aggregate.cpp: } catch (const ExceptionFor<ErrorCodes::CloseChangeStream>&) { src/mongo/s/query/cluster_aggregate.cpp: // This exception is thrown when a $changeStream stage encounters an event src/mongo/s/query/cluster_aggregate.cpp: // Set the postBatchResumeToken. For non-$changeStream aggregations, this will be empty. src/mongo/s/query/cluster_aggregate.cpp: // If this is a change stream or a collectionless aggregation, we immediately return the user- src/mongo/s/query/cluster_aggregate.cpp: if (litePipe.hasChangeStream() || nss.isCollectionlessAggregateNS()) { src/mongo/s/query/cluster_aggregate.cpp: // If we are not merging on mongoS, then this is not a $changeStream aggregation, and we src/mongo/s/query/cluster_aggregate.cpp: // $changeStream, we allow the operation to continue so that stream cursors can be established src/mongo/s/query/cluster_aggregate.cpp: // does not exist and this is not a $changeStream, then we return an empty cursor. src/mongo/s/query/cluster_aggregate.cpp: } else if (!(litePipe.hasChangeStream() && src/mongo/s/query/cluster_aggregate.cpp: // If we don't have a routing table, then this is a $changeStream which must run on all shards. src/mongo/s/query/cluster_aggregate.cpp: invariant(routingInfo || (mustRunOnAll && litePipe.hasChangeStream())); src/mongo/s/query/cluster_aggregate.cpp: auto tailMode = liteParsedPipeline.hasChangeStream() src/mongo/s/query/cluster_aggregation_planner.cpp: // For change streams, we need to set up a custom stage to establish cursors on new shards when src/mongo/s/query/cluster_aggregation_planner.cpp: if (liteParsedPipeline.hasChangeStream()) {
I'm not sure this is a good idea, but I'd like to explore the hypothesis that the regular path and the change stream path have less in common than is worth combining code paths.
Something similar might be true on mongod, but it's more obvious to me on mongos. SERVER-42942 and SERVER-42723 both had to add or maintain special code paths for change streams on mongos which seem to further compound the problem.
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