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planner: support stable result mode (#25971)
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| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,125 @@ | ||
| // Copyright 2021 PingCAP, Inc. | ||
| // | ||
| // Licensed under the Apache License, Version 2.0 (the "License"); | ||
| // you may not use this file except in compliance with the License. | ||
| // You may obtain a copy of the License at | ||
| // | ||
| // http://www.apache.org/licenses/LICENSE-2.0 | ||
| // | ||
| // Unless required by applicable law or agreed to in writing, software | ||
| // distributed under the License is distributed on an "AS IS" BASIS, | ||
| // See the License for the specific language governing permissions and | ||
| // limitations under the License. | ||
|
|
||
| package core | ||
|
|
||
| import ( | ||
| "context" | ||
|
|
||
| "github.com/pingcap/tidb/expression" | ||
| "github.com/pingcap/tidb/planner/util" | ||
| ) | ||
|
|
||
| /* | ||
| resultsStabilizer stabilizes query results. | ||
| NOTE: it's not a common rule for all queries, it's specially implemented for a few customers. | ||
| Results of some queries are not stable, for example: | ||
| create table t (a int); insert into t values (1), (2); select a from t; | ||
| In the case above, the result can be `1 2` or `2 1`, which is not stable. | ||
| This rule stabilizes results by modifying or injecting a Sort operator: | ||
| 1. iterate the plan from the root, and ignore all input-order operators (Sel/Proj/Limit); | ||
| 2. when meeting the first non-input-order operator, | ||
| 2.1. if it's a Sort, update it by appending all output columns into its order-by list, | ||
| 2.2. otherwise, inject a new Sort upon this operator. | ||
| */ | ||
| type resultsStabilizer struct { | ||
| } | ||
|
|
||
| func (rs *resultsStabilizer) optimize(ctx context.Context, lp LogicalPlan) (LogicalPlan, error) { | ||
| stable := rs.completeSort(lp) | ||
| if !stable { | ||
| lp = rs.injectSort(lp) | ||
| } | ||
| return lp, nil | ||
| } | ||
|
|
||
| func (rs *resultsStabilizer) completeSort(lp LogicalPlan) bool { | ||
| if rs.isInputOrderKeeper(lp) { | ||
| return rs.completeSort(lp.Children()[0]) | ||
| } else if sort, ok := lp.(*LogicalSort); ok { | ||
| cols := sort.Schema().Columns // sort results by all output columns | ||
| if handleCol := rs.extractHandleCol(sort.Children()[0]); handleCol != nil { | ||
| cols = []*expression.Column{handleCol} // sort results by the handle column if we can get it | ||
| } | ||
| for _, col := range cols { | ||
| exist := false | ||
| for _, byItem := range sort.ByItems { | ||
| if col.Equal(nil, byItem.Expr) { | ||
| exist = true | ||
| break | ||
| } | ||
| } | ||
| if !exist { | ||
| sort.ByItems = append(sort.ByItems, &util.ByItems{Expr: col}) | ||
| } | ||
| } | ||
| return true | ||
| } | ||
| return false | ||
| } | ||
|
|
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| func (rs *resultsStabilizer) injectSort(lp LogicalPlan) LogicalPlan { | ||
| if rs.isInputOrderKeeper(lp) { | ||
| lp.SetChildren(rs.injectSort(lp.Children()[0])) | ||
| return lp | ||
| } | ||
|
|
||
| byItems := make([]*util.ByItems, 0, len(lp.Schema().Columns)) | ||
| cols := lp.Schema().Columns | ||
| if handleCol := rs.extractHandleCol(lp); handleCol != nil { | ||
| cols = []*expression.Column{handleCol} | ||
| } | ||
| for _, col := range cols { | ||
| byItems = append(byItems, &util.ByItems{Expr: col}) | ||
| } | ||
| sort := LogicalSort{ | ||
| ByItems: byItems, | ||
| }.Init(lp.SCtx(), lp.SelectBlockOffset()) | ||
| sort.SetChildren(lp) | ||
| return sort | ||
| } | ||
|
|
||
| func (rs *resultsStabilizer) isInputOrderKeeper(lp LogicalPlan) bool { | ||
| switch lp.(type) { | ||
| case *LogicalSelection, *LogicalProjection, *LogicalLimit: | ||
| return true | ||
| } | ||
| return false | ||
| } | ||
|
|
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| // extractHandleCols does the best effort to get the handle column. | ||
| func (rs *resultsStabilizer) extractHandleCol(lp LogicalPlan) *expression.Column { | ||
| switch x := lp.(type) { | ||
| case *LogicalSelection, *LogicalLimit: | ||
| handleCol := rs.extractHandleCol(lp.Children()[0]) | ||
| if x.Schema().Contains(handleCol) { | ||
| // some Projection Operator might be inlined, so check the column again here | ||
| return handleCol | ||
| } | ||
| case *DataSource: | ||
| if x.tableInfo.IsCommonHandle { | ||
| // Currently we deliberately don't support common handle case for simplicity. | ||
| return nil | ||
| } | ||
| handleCol := x.getPKIsHandleCol() | ||
| if handleCol != nil { | ||
| return handleCol | ||
| } | ||
| } | ||
| return nil | ||
| } | ||
|
|
||
| func (rs *resultsStabilizer) name() string { | ||
| return "stabilize_results" | ||
| } |
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