Problem

The legacy-format SBE explain omits numKeysEstimate, numDocsEstimate and indexSeekEstimate for cost-based ranker (CBR) estimates, while the classic engine's legacy explain and SBE's V3 explain both report them.

Details

• Classic (statsToBSON in src/mongo/db/query/plan_explainer_impl.cpp) calls QSNEstimate::serialize() and then additionally appends numKeysEstimate (IXSCAN) or numDocsEstimate (other stages) from inCE, and indexSeekEstimate from indexSeekCE.
• SBE V3 explain (appendCostBasedStatsV3) emits the same three fields under statistics.costBased.
• SBE legacy explain (qsnToBson in src/mongo/db/query/plan_explainer_sbe.cpp, non-V3 branch) calls only estimates.at(node)->serialize(*bob), which emits costEstimate, cardinalityEstimate and estimatesMetadata.

The estimates are computed regardless of engine (indexSeekCE is set in cardinality_estimator.cpp); they are just not serialized in the SBE legacy path.

Reproduction

Run jstests/noPassthroughWithMongod/query/cbr/cbr_stage_costing.js with {{--mongodSetParameters='

{internalQueryFrameworkControl: trySbeEngine}

'}} (or featureFlagSbeFull) and the checkSbeFullyEnabled early exit removed. It fails at the indexSeekEstimate assertion in runTest: the IXSCAN stage has costEstimate, cardinalityEstimate and estimatesMetadata but no indexSeekEstimate.

Proposed fix

In the legacy branch of qsnToBson, append numKeysEstimate/numDocsEstimate and indexSeekEstimate the same way classic does, ideally via a helper shared with statsToBSON. Add coverage in plan_explainer_test.cpp and remove the SBE early exits from the CBR jstests that depend on these fields.

Related

• SERVER-130179 (plumbed estimates into SBE explain for the release config)
• SERVER-136769 (stale TODOs referencing SERVER-130179)