Backport to 2.28.x: #10209: Fix potential incorrect stddev(float8/4) in columnar pipeline - #10269
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Postgres uses the same float8_accum transition function for both avg() and stddev() for these types. TimescaleDB used to use a simplified implementation for avg(), because it doesn't require tracking sum of squares. However, Postgres uses a shared transition state for aggregates sharing a transition functions, as given by AggRef.aggtransno. This leads to potentially wrong result when both avg() and stddev() of same variable are computed in one query. Change the aggregation states in columnar aggregation to follow aggtransno. Remove the "float8 accum with no squares" implementation because it cannot be safely used in general case, and checking for special cases is complicated. (cherry picked from commit 58e9840)
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July 16, 2026 08:57
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This is an automated backport of #10209: Fix potential incorrect stddev(float8/4) in columnar pipeline.
The original issue is #10203.
This PR will be merged automatically after all the relevant CI checks pass. If this fix should not be backported, or will be backported manually, just close this PR. You can use the backport branch to add your changes, it won't be modified automatically anymore.
For more details, please see the documentation
Original description
Fix potential incorrect stddev(float8/4) in columnar pipeline
Postgres uses the same float8_accum transition function for both avg() and stddev() for these types. TimescaleDB used to use a simplified implementation for avg(), because it doesn't require tracking sum of squares. However, Postgres uses a shared transition state for aggregates sharing a transition functions, as given by AggRef.aggtransno. This leads to potentially wrong result when both avg() and stddev() of same variable are computed in one query.
Change the aggregation states in columnar aggregation to follow aggtransno. Remove the "float8 accum with no squares" implementation because it cannot be safely used in general case, and checking for special cases is complicated.
Fixes #10203