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  1. Core Server
  2. SERVER-99773

Bucket-level $group rewrite on sharded TS collection is not performed on shards

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    • Query Integration
    • Fully Compatible
    • ALL
    • v8.1, v8.0
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      import {ShardingTest} from "jstests/libs/shardingtest.js";

const dbName = 'testDB';
const collName = 'testColl';
const st = new ShardingTest({shards: 2, rs: {nodes: 2}});
const sDB = st.s.getDB(dbName);
assert.commandWorked(sDB.adminCommand({enableSharding: dbName, primaryShard: st.shard0.shardName}));

assert.commandWorked(sDB.adminCommand({
    shardCollection: `${dbName}.${collName}`,
    key: {meta: 1, time: 1},
    timeseries: {timeField: 'time', metaField: 'meta', granularity: "hours"}
}));

const coll = sDB.getCollection(collName);
assert.commandWorked(coll.insert({time: new Date(), m: 1}));
jsTestLog(coll.explain().aggregate([{$group: {_id: '$meta', max: {$max: '$time'}}}]))
// Shows no $group rewrite on the shards

st.stop();
      Show
      import {ShardingTest} from "jstests/libs/shardingtest.js" ; const dbName = 'testDB' ; const collName = 'testColl' ; const st = new ShardingTest({shards: 2, rs: {nodes: 2}}); const sDB = st.s.getDB(dbName); assert .commandWorked(sDB.adminCommand({enableSharding: dbName, primaryShard: st.shard0.shardName})); assert .commandWorked(sDB.adminCommand({ shardCollection: `${dbName}.${collName}`, key: {meta: 1, time: 1}, timeseries: {timeField: 'time' , metaField: 'meta' , granularity: "hours" } })); const coll = sDB.getCollection(collName); assert .commandWorked(coll.insert({time: new Date(), m: 1})); jsTestLog(coll.explain().aggregate([{$group: {_id: '$meta' , max: {$max: '$time' }}}])) // Shows no $group rewrite on the shards st.stop();
    • QI 2025-02-03, QI 2025-02-17
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      In the repro provided in the shards section of the explain, the $group fields are not rewritten to reference the bucket-level fields. This is done as a performance optimization.

      This issue began in SERVER-93874, but after reviewing the logic I don't see why the rewrite doesn't occur. After investigating I found that on the shards this condition returns false because the _fieldSet in the bucket spec doesn't contain the meta field. This is odd because the unpack bucket stage shows us including the meta field as output.

            Assignee:
            chris.wolff@mongodb.com Chris Wolff
            Reporter:
            matt.boros@mongodb.com Matt Boros
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