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Type:
Improvement
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Resolution: Fixed
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Priority:
Major - P3
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Affects Version/s: None
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Component/s: None
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Query Integration
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Fully Compatible
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v8.0
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QI 2024-04-01, QI 2024-04-15
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33
In SERVER-87589, we limited the number of rewritten $first/$last accumulators only to 2 as overhead of generating the same sort key multiple times for multiple rewritten $top/$bottom could be big if there are too many $first/$last.
{$sort: {sortField: 1}},
{$group: {_id: ..., firstX: {$first: "$x"}, firstY: {$first: "$y"},
lastX: {$last: "$x"}, lastY: {$last: "$y"}}}
--> becomes -->
{$group: {_id: ...,
tops: {$top: {sortBy: {sortField:1}}, outputs: {x: "$x", y: "$y"}},
bottoms: {$bottom: {sortyBy: {sortField:1}}, outputs: {x: "$x", y:"$y"}}},
{$project: {
firstX: "$tops.x", firstY: "$tops.y", lastX: "$bottoms.x", lastY: "$bottoms.y"}}
Update #1:
anna.wawrzyniak@mongodb.com suggested to implement a generalized optimization like this where, if there are several topNs/bottomNs on the same patterns, we use a single sorter instead of having a different sorter for each accumulator.
Update #2:
As part of this improvement, we should be able to increase internalQueryMaxFirstLastRewrites's default value to a much higher one.
- causes
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SERVER-92146 Rewrite to group topN/bottomN sorts generates incorrect projection
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- Closed
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- is related to
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- Closed
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- related to
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SERVER-88613 Introduce a new syntax to $group to define the common sort key for multiple $top & $bottom accumulators
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- Backlog
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- split from
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SERVER-85213 Rewrite $sort+$group with $first/$last to use $top/$bottom
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- Backlog
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