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// Copyright 2018 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. | ||
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package core | ||
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import ( | ||
"math" | ||
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"github.com/pingcap/tidb/ast" | ||
"github.com/pingcap/tidb/expression" | ||
"github.com/pingcap/tidb/expression/aggregation" | ||
"github.com/pingcap/tidb/mysql" | ||
"github.com/pingcap/tidb/sessionctx" | ||
"github.com/pingcap/tidb/types" | ||
) | ||
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type aggregationEliminator struct { | ||
aggregationEliminateChecker | ||
} | ||
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type aggregationEliminateChecker struct { | ||
} | ||
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// tryToEliminateAggregation will eliminate aggregation grouped by unique key. | ||
// e.g. select min(b) from t group by a. If a is a unique key, then this sql is equal to `select b from t group by a`. | ||
// For count(expr), sum(expr), avg(expr), count(distinct expr, [expr...]) we may need to rewrite the expr. Details are shown below. | ||
// If we can eliminate agg successful, we return a projection. Else we return a nil pointer. | ||
func (a *aggregationEliminateChecker) tryToEliminateAggregation(agg *LogicalAggregation) *LogicalProjection { | ||
schemaByGroupby := expression.NewSchema(agg.groupByCols...) | ||
coveredByUniqueKey := false | ||
for _, key := range agg.children[0].Schema().Keys { | ||
if schemaByGroupby.ColumnsIndices(key) != nil { | ||
coveredByUniqueKey = true | ||
break | ||
} | ||
} | ||
if coveredByUniqueKey { | ||
// GroupByCols has unique key, so this aggregation can be removed. | ||
proj := a.convertAggToProj(agg) | ||
proj.SetChildren(agg.children[0]) | ||
return proj | ||
} | ||
return nil | ||
} | ||
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func (a *aggregationEliminateChecker) convertAggToProj(agg *LogicalAggregation) *LogicalProjection { | ||
proj := LogicalProjection{ | ||
Exprs: make([]expression.Expression, 0, len(agg.AggFuncs)), | ||
}.init(agg.ctx) | ||
for _, fun := range agg.AggFuncs { | ||
expr := a.rewriteExpr(agg.ctx, fun) | ||
proj.Exprs = append(proj.Exprs, expr) | ||
} | ||
proj.SetSchema(agg.schema.Clone()) | ||
return proj | ||
} | ||
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// rewriteExpr will rewrite the aggregate function to expression doesn't contain aggregate function. | ||
func (a *aggregationEliminateChecker) rewriteExpr(ctx sessionctx.Context, aggFunc *aggregation.AggFuncDesc) expression.Expression { | ||
switch aggFunc.Name { | ||
case ast.AggFuncCount: | ||
if aggFunc.Mode == aggregation.FinalMode { | ||
return a.wrapCastFunction(ctx, aggFunc.Args[0], aggFunc.RetTp) | ||
} | ||
return a.rewriteCount(ctx, aggFunc.Args, aggFunc.RetTp) | ||
case ast.AggFuncSum, ast.AggFuncAvg, ast.AggFuncFirstRow, ast.AggFuncMax, ast.AggFuncMin, ast.AggFuncGroupConcat: | ||
return a.wrapCastFunction(ctx, aggFunc.Args[0], aggFunc.RetTp) | ||
case ast.AggFuncBitAnd, ast.AggFuncBitOr, ast.AggFuncBitXor: | ||
return a.rewriteBitFunc(ctx, aggFunc.Name, aggFunc.Args[0], aggFunc.RetTp) | ||
default: | ||
panic("Unsupported function") | ||
} | ||
} | ||
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func (a *aggregationEliminateChecker) rewriteCount(ctx sessionctx.Context, exprs []expression.Expression, targetTp *types.FieldType) expression.Expression { | ||
// If is count(expr), we will change it to if(isnull(expr), 0, 1). | ||
// If is count(distinct x, y, z) we will change it to if(isnull(x) or isnull(y) or isnull(z), 0, 1). | ||
isNullExprs := make([]expression.Expression, 0, len(exprs)) | ||
for _, expr := range exprs { | ||
isNullExpr := expression.NewFunctionInternal(ctx, ast.IsNull, types.NewFieldType(mysql.TypeTiny), expr) | ||
isNullExprs = append(isNullExprs, isNullExpr) | ||
} | ||
innerExpr := expression.ComposeDNFCondition(ctx, isNullExprs...) | ||
newExpr := expression.NewFunctionInternal(ctx, ast.If, targetTp, innerExpr, expression.Zero, expression.One) | ||
return newExpr | ||
} | ||
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func (a *aggregationEliminateChecker) rewriteBitFunc(ctx sessionctx.Context, funcType string, arg expression.Expression, targetTp *types.FieldType) expression.Expression { | ||
// For not integer type. We need to cast(cast(arg as signed) as unsigned) to make the bit function work. | ||
innerCast := expression.WrapWithCastAsInt(ctx, arg) | ||
outerCast := a.wrapCastFunction(ctx, innerCast, targetTp) | ||
var finalExpr expression.Expression | ||
if funcType != ast.AggFuncBitAnd { | ||
finalExpr = expression.NewFunctionInternal(ctx, ast.Ifnull, targetTp, outerCast, expression.Zero.Clone()) | ||
} else { | ||
finalExpr = expression.NewFunctionInternal(ctx, ast.Ifnull, outerCast.GetType(), outerCast, &expression.Constant{Value: types.NewUintDatum(math.MaxUint64), RetType: targetTp}) | ||
} | ||
return finalExpr | ||
} | ||
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// wrapCastFunction will wrap a cast if the targetTp is not equal to the arg's. | ||
func (a *aggregationEliminateChecker) wrapCastFunction(ctx sessionctx.Context, arg expression.Expression, targetTp *types.FieldType) expression.Expression { | ||
if arg.GetType() == targetTp { | ||
return arg | ||
} | ||
return expression.BuildCastFunction(ctx, arg, targetTp) | ||
} | ||
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func (a *aggregationEliminator) optimize(p LogicalPlan) (LogicalPlan, error) { | ||
newChildren := make([]LogicalPlan, 0, len(p.Children())) | ||
for _, child := range p.Children() { | ||
newChild, _ := a.optimize(child) | ||
newChildren = append(newChildren, newChild) | ||
} | ||
p.SetChildren(newChildren...) | ||
agg, ok := p.(*LogicalAggregation) | ||
if !ok { | ||
return p, nil | ||
} | ||
if proj := a.tryToEliminateAggregation(agg); proj != nil { | ||
return proj, nil | ||
} | ||
return p, nil | ||
} |
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