fix: scope HashJoinExec build_time to hash-table construction only - #24271
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ulrichurriola-parada-ops wants to merge 1 commit into
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fix: scope HashJoinExec build_time to hash-table construction only#24271ulrichurriola-parada-ops wants to merge 1 commit into
ulrichurriola-parada-ops wants to merge 1 commit into
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build_time previously wrapped the whole collect_left_input future, including the build-side child's own stream-draining work, so its compute got double-counted into this join's elapsed_compute. Time only the post-fold hash-map construction instead.
ulrichurriola-parada-ops
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August 11, 2026 14:48
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Rationale for this change
HashJoinExec's elapsed_compute metric (as reported by EXPLAIN ANALYZE and ExecutionPlanMetricsSet) includes time spent executing the entire build-side subtree, not just the join's own work. Any tool that sums elapsed_compute across a physical plan to estimate total CPU time (e.g. to build a cost/telemetry model) will double-count that subtree's compute: once under the child operator(s) that did the work, and again under HashJoinExec.
What changes are included in this PR?
build_time was previously timed around the entire collect_left_input future (via left_fut.get_shared(cx) in collect_build_side). That future both drains the build-side child's stream and builds the hash table, so the child's own compute got billed to build_time whenever the driving partition executed both phases inline.
Are these changes tested?
Covered by the existing hash_join test suite (correctness of join output and other metrics is unaffected). No new test was added to assert the exact build_time/elapsed_compute value, since there isn't currently a way to attribute deterministic wall-clock cost to a specific subtree in a unit test.
Are there any user-facing changes?
elapsed_compute reported for HashJoinExec (e.g. via EXPLAIN ANALYZE) will be smaller and more accurate, reflecting only the join's own compute rather than including its build-side subtree. No public API changes.