jiaqizho commented on PR #776:
URL: https://github.com/apache/cloudberry/pull/776#issuecomment-2539008062
The context about this PR: ORCA can create the 3-stage agg in
`agg(distinct)` path, but prohibit the distribution key in distinct. Because
ORCA think 2-stage agg is enough.
So u can see the 3-stage agg path but no cost generated in the memo. like
```
ROOT
Group 9 (#GExprs: 8):
0: CLogicalGbAgg( Global ) Grp Cols: [][Global], Minimal Grp Cols: [],
Generates Duplicates :[ 0 ] [ 0 8 ]
1: CLogicalGbAgg( Global ) Grp Cols: [][Global], Minimal Grp Cols: [],
Generates Duplicates :[ 0 ] , m_aggStage :[ Three Stage Scalar DQA ] [ 12 15
]
2: CLogicalGbAgg( Global ) Grp Cols: [][Global], Minimal Grp Cols: [],
Generates Duplicates :[ 0 ] , m_aggStage :[ Two Stage Scalar DQA ] [ 16 15 ]
3: CLogicalGbAgg( Global ) Grp Cols: [][Global], Minimal Grp Cols: [],
Generates Duplicates :[ 0 ] , m_aggStage :[ Two Stage Scalar DQA ] [ 20 25 ]
4: CPhysicalScalarAgg( Global, multi-stage ) Grp Cols: [], Minimal Grp
Cols:[], Generates Duplicates :[ 0 ] , m_aggStage :[ Two Stage Scalar DQA ] [
20 25 ]
Cost Ctxts:
main ctxt (stage 0)0.1, child ctxts:[0], rows:1.000000 (group), cost:
484.101386
5: CPhysicalScalarAgg( Global, multi-stage ) Grp Cols: [], Minimal Grp
Cols:[], Generates Duplicates :[ 0 ] , m_aggStage :[ Two Stage Scalar DQA ] [
16 15 ]
Cost Ctxts:
main ctxt (stage 0)0.1, child ctxts:[0], rows:1.000000 (group), cost:
679.103561
6: CPhysicalScalarAgg( Global, multi-stage ) Grp Cols: [], Minimal Grp
Cols:[], Generates Duplicates :[ 0 ] , m_aggStage :[ Three Stage Scalar DQA ]
[ 12 15 ]
Cost Ctxts:
7: CPhysicalScalarAgg( Global ) Grp Cols: [], Minimal Grp Cols:[],
Generates Duplicates :[ 0 ] [ 0 8 ]
Cost Ctxts:
main ctxt (stage 0)0.1, child ctxts:[0], rows:1.000000 (group), cost:
527.493765
Grp OptCtxts:
0 (stage 0): (req CTEs: [], req order: [<empty> match: satisfy ], req
dist: [SINGLETON (master) match: satisfy], req rewind: [], req rewind: [NONE
NO-MOTION match: satisfy], req partition propagation: [<empty> match: satisfy
]) => Best Expr:4
```
The GroupExpression#6 is the **Three Stage Scalar DQA**.
Details of why this path have no cost:
1. ORCA will create a `CPhysicalMotionHashDistribute HASHED`
GroupExpression(Redistribute Motion) below the GroupExpression#6. If current
physical agg enforce the distribution.
2. If current required distribution key(hashagg) is same as distribution
key, then `Redistribute Motion` won't get the cost(in
CPhysicalMotion::FValidContext)
3. So the root group(GroupExpression#6) won't be selected.
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