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$unionWIth $search explain errors out when using $search and $$SEARCH_META in the subpipeline since SEARCH_META is used in the original run of the pipeline which seems to add it in the let variables. Then when $unionWith reruns the pipeline for explain, it errors due to SEARCH_META not being allowed to be in the letVariables. pipeline: [{ $unionWith: { coll: searchColl.getName(), pipeline: [ {$search: mongotQuery}, { $project: { "_id": 0, "ref_id": "$_id", "title": "$title", "searchMeta": "$$SEARCH_META", } } ] } }] A possible solution is to clear SEARCH_META from the let variables.

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xgen-internal-githook commented on Tue, 13 Aug 2024 16:03:22 +0000: Author: {'name': 'erinzhu001', 'email': 'erin.zhu@mongodb.com', 'username': 'erinzhu001'} Message: SERVER-92337 Reset variables for $unionWith during second execution of subpipeine for explain (#26029) GitOrigin-RevId: 3706ed48ec7ff9eb606181d300d65757350606d4 Branch: master https://github.com/mongodb/mongo/commit/28261ea89f9321cee72e8462101067aea5657841

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