Improve CostHashAgg with single-column NDV and spill-aware cost model#1672
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yjhjstz wants to merge 1 commit intoapache:mainfrom
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Improve CostHashAgg with single-column NDV and spill-aware cost model#1672yjhjstz wants to merge 1 commit intoapache:mainfrom
yjhjstz wants to merge 1 commit intoapache:mainfrom
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Two enhancements to CostHashAgg in CCostModelGPDB: 1. Single-column NDV optimization for local partial HashAgg: When GROUP BY has exactly 1 column, use GetNDVs() (global NDV from column statistics) instead of pci->Rows() to estimate the output row count of the local partial aggregation stage. GetNDVs() returns the global NDV directly, so no * UlHosts() scaling is needed. This lets the optimizer distinguish high-NDV cases (partial agg streams nearly as many rows as input, 2-phase has little benefit) from low-NDV cases (partial agg significantly reduces data before redistribution, 2-phase is preferred). Multi-column GROUP BY falls back to the original behavior: num_output_rows = pci->Rows() * UlHosts(). 2. Spill-aware cost model: When num_output_rows * width exceeds the spilling memory threshold (EcpHJSpillingMemThreshold, 50 MB), apply higher cost unit values to reflect disk I/O overhead. Uses the existing HJ spilling cost parameters (EcpHJFeedingTupColumnSpillingCostUnit etc.) which are already tuned for spilling scenarios. TPC-H benchmark: -14.3% overall (Q17 -60%, Q03 -6%). TPC-DS benchmark: -0.4% overall (Q59 -29%).
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Changes
Two enhancements to
CostHashAggin GPORCA'sCCostModelGPDB:Single-column NDV optimization: When
GROUP BYhas exactly one column, useGetNDVs()(global NDV from column statistics) instead ofpci->Rows()to estimate the local partial aggregation output. This allows the optimizer to distinguish high-NDV cases (partial agg streams nearly as many rows as input → 1-phase preferred) from low-NDV cases (partial agg significantly reduces data → 2-phase preferred). Multi-columnGROUP BYfalls back to the originalpci->Rows() * UlHosts()logic.Spill-aware cost model: When
num_output_rows × widthexceeds the 50 MB spilling threshold, apply higher cost unit values to reflect disk I/O overhead.TPC-H: -13.0% overall (274,201ms → 238,438ms)
Only 2 queries changed plans:
Q17 is the dominant win.
l_partkeyis a high-NDV single-column group key (~6.6M distinct values); partial aggregation produced 19.6M output rows (nearly identical to input), causing 1.3 GB of disk spill in the Finalize stage. The optimizer now correctly chooses 1-phase, reducing spill to 490 MB and cutting latency by 36 seconds. No plan change caused a regression.TPC-DS: +0.1% overall (605,646ms → 606,421ms)
Only 2 queries changed plans:
Combined Assessment
Fixes #ISSUE_Number
What does this PR do?
Type of Change
Breaking Changes
Test Plan
make installcheckmake -C src/test installcheck-cbdb-parallelImpact
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User-facing changes:
Dependencies:
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