planner: enhance the rule max_min_eliminate to support multiple agg

planner/core/physical_plan_test.go

@@ -1265,10 +1265,34 @@ func (s *testPlanSuite) TestAggEliminator(c *C) {
 			sql:  "select max(a+1) from t;",
 			best: "IndexReader(Index(t.f)[[NULL,+inf]]->Sel([not(isnull(plus(test.t.a, 1)))])->TopN([plus(test.t.a, 1) true],0,1))->Projection->TopN([col_1 true],0,1)->Projection->Projection->StreamAgg",
 		},
-		// Do nothing to max+min.
+		// Min + Max to Limit + Sort + Join.
 		{
 			sql:  "select max(a), min(a) from t;",
-			best: "IndexReader(Index(t.f)[[NULL,+inf]]->StreamAgg)->StreamAgg",
+			best: "LeftHashJoin{TableReader(Table(t)->Limit)->Limit->StreamAgg->TableReader(Table(t)->Limit)->Limit->StreamAgg}",
+		},
+		// Min + Max with range condition.
+		{
+			sql:  "select max(a), min(a) from t where a > 10",
+			best: "LeftHashJoin{TableReader(Table(t)->Limit)->Limit->StreamAgg->TableReader(Table(t)->Limit)->Limit->StreamAgg}",
+		},
+		{
+			sql:  "select max(a), max(c), min(f) from t",
+			best: "LeftHashJoin{LeftHashJoin{TableReader(Table(t)->Limit)->Limit->StreamAgg->IndexLookUp(Index(t.c_d_e)[[NULL,+inf]]->Limit, Table(t))->Limit->StreamAgg}->IndexLookUp(Index(t.f)[[NULL,+inf]]->Limit, Table(t))->Limit->StreamAgg}",
+		},
+		// Do nothing if any column has no index.
+		{
+			sql:  "select max(a), max(b) from t",
+			best: "TableReader(Table(t)->StreamAgg)->StreamAgg",
+		},
+		// Do nothing if any column has a non-range condition.
+		{
+			sql:  "select max(a), max(c) from t where c > 10",
+			best: "IndexReader(Index(t.c_d_e)[(10,+inf]]->StreamAgg)->StreamAgg",
+		},
+		// Do nothing if the condition cannot be pushed down to range.
+		{
+			sql:  "select max(a), min(a) from t where a * 3 + 10 < 100",
+			best: "IndexReader(Index(t.f)[[NULL,+inf]]->Sel([lt(plus(mul(test.t.a, 3), 10), 100)])->StreamAgg)->StreamAgg",
 		},
 		// Do nothing to max with groupby.
 		{

planner/core/rule_max_min_eliminate.go

@@ -18,71 +18,188 @@ import (
 	"github.com/pingcap/parser/ast"
 	"github.com/pingcap/parser/mysql"
 	"github.com/pingcap/tidb/expression"
+	"github.com/pingcap/tidb/expression/aggregation"
 	"github.com/pingcap/tidb/types"
+	"github.com/pingcap/tidb/util/ranger"
 )
 
 // maxMinEliminator tries to eliminate max/min aggregate function.
 // For SQL like `select max(id) from t;`, we could optimize it to `select max(id) from (select id from t order by id desc limit 1 where id is not null) t;`.
 // For SQL like `select min(id) from t;`, we could optimize it to `select max(id) from (select id from t order by id limit 1 where id is not null) t;`.
+// For SQL like `select max(id), min(id) from t;`, we could optimize it to the cartesianJoin result of the two queries above if `id` has an index.
 type maxMinEliminator struct {
 }
 
 func (a *maxMinEliminator) optimize(ctx context.Context, p LogicalPlan) (LogicalPlan, error) {
-	a.eliminateMaxMin(p)
-	return p, nil
+	return a.eliminateMaxMin(p), nil
 }
 
-// Try to convert max/min to Limit+Sort operators.
-func (a *maxMinEliminator) eliminateMaxMin(p LogicalPlan) {
-	// We don't need to guarantee that the child of it is a data source. This transformation won't be worse than previous.
-	if agg, ok := p.(*LogicalAggregation); ok {
-		// We only consider case with single max/min function.
-		if len(agg.AggFuncs) != 1 || len(agg.GroupByItems) != 0 {
-			return
+// Compose the scalar aggregations by cartesianJoin.
+func (a *maxMinEliminator) composeAggsByInnerJoin(aggs []*LogicalAggregation) (plan LogicalPlan) {
+	plan = aggs[0]
+	sctx := plan.SCtx()
+	for i := 1; i < len(aggs); i++ {
+		join := LogicalJoin{JoinType: InnerJoin}.Init(sctx)
+		join.SetChildren(plan, aggs[i])
+		join.schema = buildLogicalJoinSchema(InnerJoin, join)
+		join.cartesianJoin = true
+		plan = join
+	}
+	return
+}
+
+// checkColCanUseIndex checks the following conditions:
+// 1. whether the col is the prefix of an index.
+// 2. whether all of the selection conditions can be pushed down to the index range.
+func (a *maxMinEliminator) checkColCanUseIndex(plan LogicalPlan, col *expression.Column) bool {
+	switch p := plan.(type) {
+	case *LogicalSelection:
+		// Check whether all of the conditions can be pushed down as accessConds.
+		if _, filterConds := ranger.DetachCondsForColumn(p.ctx, p.Conditions, col); len(filterConds) != 0 {
+			return false
 		}
-		f := agg.AggFuncs[0]
-		if f.Name != ast.AggFuncMax && f.Name != ast.AggFuncMin {
-			return
+		return a.checkColCanUseIndex(p.children[0], col)
+	case *DataSource:
+		// Check whether there is an accessPath can use index for col.
+		for _, path := range p.possibleAccessPaths {
+			if path.isTablePath {
+				if col == p.handleCol {
+					return true
+				}
+			} else {
+				if col.ColName.L == path.index.Columns[0].Name.L && path.index.Columns[0].Length == types.UnspecifiedLength {
+					return true
+				}
+			}
 		}
+		return false
+	default:
+		return false
+	}
+}
 
-		child := p.Children()[0]
-		ctx := p.SCtx()
-
-		// If there's no column in f.GetArgs()[0], we still need limit and read data from real table because the result should NULL if the below is empty.
-		if len(expression.ExtractColumns(f.Args[0])) > 0 {
-			// If it can be NULL, we need to filter NULL out first.
-			if !mysql.HasNotNullFlag(f.Args[0].GetType().Flag) {
-				sel := LogicalSelection{}.Init(ctx)
-				isNullFunc := expression.NewFunctionInternal(ctx, ast.IsNull, types.NewFieldType(mysql.TypeTiny), f.Args[0])
-				notNullFunc := expression.NewFunctionInternal(ctx, ast.UnaryNot, types.NewFieldType(mysql.TypeTiny), isNullFunc)
-				sel.Conditions = []expression.Expression{notNullFunc}
-				sel.SetChildren(p.Children()[0])
-				child = sel
-			}
+// cloneSubPlans clones the subPlan. We only consider `Selection` and `DataSource` here,
+// because we have restricted the subPlan in `checkColCanUseIndex`.
+func (a *maxMinEliminator) cloneSubPlans(plan LogicalPlan) LogicalPlan {
+	switch p := plan.(type) {
+	case *LogicalSelection:
+		conditions := make([]expression.Expression, 0, len(p.Conditions))
+		for _, cond := range p.Conditions {
+			conditions = append(conditions, cond.Clone())
+		}
+		sel := LogicalSelection{Conditions: conditions}.Init(p.ctx)
+		sel.SetChildren(a.cloneSubPlans(p.children[0]))
+		return sel
+	case *DataSource:
+		// This is a shallow clone which may not be safe.
+		newDs := *p
+		newDs.self = &newDs
+		return &newDs
+	}
+	// This won't happen, because we have checked the subtree.
+	return nil
+}
 
-			// Add Sort and Limit operators.
-			// For max function, the sort order should be desc.
-			desc := f.Name == ast.AggFuncMax
-			// Compose Sort operator.
-			sort := LogicalSort{}.Init(ctx)
-			sort.ByItems = append(sort.ByItems, &ByItems{f.Args[0], desc})
-			sort.SetChildren(child)
-			child = sort
+// splitAggFuncAndCheckIndices splits the agg to multiple aggs and check whether each agg needs a sort
+// after the transformation. For example, we firstly split the sql: `select max(a), min(a), max(b) from t` ->
+// `select max(a) from t` + `select min(a) from t` + `select max(b) from t`.
+// Then we check whether `a` and `b` have indices. If any of the used column has no index, we cannot eliminate
+// this aggregation.
+func (a *maxMinEliminator) splitAggFuncAndCheckIndices(agg *LogicalAggregation) (aggs []*LogicalAggregation, canEliminate bool) {
+	aggs = make([]*LogicalAggregation, 0, len(agg.AggFuncs))
+	for _, f := range agg.AggFuncs {
+		// We must make sure the args of max/min is a simple single column.
+		col, ok := f.Args[0].(*expression.Column)
+		if !ok {
+			return nil, false
 		}
+		if !a.checkColCanUseIndex(agg.children[0], col) {
+			return nil, false
+		}
+	}
+	// we can split the aggregation only if all of the aggFuncs pass the check.
+	for i, f := range agg.AggFuncs {
+		newAgg := LogicalAggregation{AggFuncs: []*aggregation.AggFuncDesc{f}}.Init(agg.ctx)
+		newAgg.SetChildren(a.cloneSubPlans(agg.children[0]))
+		newAgg.schema = expression.NewSchema(agg.schema.Columns[i])
+		aggs = append(aggs, newAgg)
+	}
+	return aggs, true
+}
 
-		// Compose Limit operator.
-		li := LogicalLimit{Count: 1}.Init(ctx)
-		li.SetChildren(child)
+// Try to convert a single max/min to Limit+Sort operators.
+func (a *maxMinEliminator) eliminateSingleMaxMin(agg *LogicalAggregation) *LogicalAggregation {
+	f := agg.AggFuncs[0]
+	child := agg.Children()[0]
+	ctx := agg.SCtx()
 
-		// If no data in the child, we need to return NULL instead of empty. This cannot be done by sort and limit themselves.
-		// Since now it's almost one row returned, a agg operator is okay to do this.
-		p.SetChildren(li)
-		return
+	// If there's no column in f.GetArgs()[0], we still need limit and read data from real table because the result should NULL if the below is empty.
+	if len(expression.ExtractColumns(f.Args[0])) > 0 {
+		// If it can be NULL, we need to filter NULL out first.
+		if !mysql.HasNotNullFlag(f.Args[0].GetType().Flag) {
+			sel := LogicalSelection{}.Init(ctx)
+			isNullFunc := expression.NewFunctionInternal(ctx, ast.IsNull, types.NewFieldType(mysql.TypeTiny), f.Args[0])
+			notNullFunc := expression.NewFunctionInternal(ctx, ast.UnaryNot, types.NewFieldType(mysql.TypeTiny), isNullFunc)
+			sel.Conditions = []expression.Expression{notNullFunc}
+			sel.SetChildren(agg.Children()[0])
+			child = sel
+		}
+
+		// Add Sort and Limit operators.
+		// For max function, the sort order should be desc.
+		desc := f.Name == ast.AggFuncMax
+		// Compose Sort operator.
+		sort := LogicalSort{}.Init(ctx)
+		sort.ByItems = append(sort.ByItems, &ByItems{f.Args[0], desc})
+		sort.SetChildren(child)
+		child = sort
+	}
+
+	// Compose Limit operator.
+	li := LogicalLimit{Count: 1}.Init(ctx)
+	li.SetChildren(child)
+
+	// If no data in the child, we need to return NULL instead of empty. This cannot be done by sort and limit themselves.
+	// Since now it's almost one row returned, a agg operator is okay to do this.
+	agg.SetChildren(li)
+	return agg
+}
+
+// Try to convert max/min to Limit+Sort operators.
+func (a *maxMinEliminator) eliminateMaxMin(p LogicalPlan) LogicalPlan {
+	if agg, ok := p.(*LogicalAggregation); ok {
+		if len(agg.GroupByItems) != 0 {
+			return agg
+		}
+		// Make sure that all of the aggFuncs are Max or Min.
+		for _, aggFunc := range agg.AggFuncs {
+			if aggFunc.Name != ast.AggFuncMax && aggFunc.Name != ast.AggFuncMin {
+				return agg
+			}
+		}
+		if len(agg.AggFuncs) == 1 {
+			// If there is only one aggFunc, we don't need to guarantee that the child of it is a data
+			// source, or whether the sort can be eliminated. This transformation won't be worse than previous.
+			return a.eliminateSingleMaxMin(agg)
+		}
+		// If we have more than one aggFunc, we can eliminate this agg only if all of the aggFuncs can benefit from
+		// their column's index.
+		aggs, canEliminate := a.splitAggFuncAndCheckIndices(agg)
+		if !canEliminate {
+			return agg
+		}
+		for i := range aggs {
+			aggs[i] = a.eliminateSingleMaxMin(aggs[i])
+		}
+		return a.composeAggsByInnerJoin(aggs)
 	}
 
+	newChildren := make([]LogicalPlan, 0, len(p.Children()))
 	for _, child := range p.Children() {
-		a.eliminateMaxMin(child)
+		newChildren = append(newChildren, a.eliminateMaxMin(child))
 	}
+	p.SetChildren(newChildren...)
+	return p
 }
 
 func (*maxMinEliminator) name() string {