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replace_cross_joins.go
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replace_cross_joins.go
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// Copyright 2020-2021 Dolthub, 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,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package analyzer
import (
"github.com/gabereiser/go-mysql-server/sql"
"github.com/gabereiser/go-mysql-server/sql/expression"
"github.com/gabereiser/go-mysql-server/sql/plan"
"github.com/gabereiser/go-mysql-server/sql/transform"
)
// comparisonSatisfiesJoinCondition checks a) whether a comparison is a valid join predicate,
// and b) whether the Left/Right children of a comparison expression covers the dependency trees
// of a plan.CrossJoin's children.
func comparisonSatisfiesJoinCondition(expr expression.Comparer, j *plan.JoinNode) bool {
lCols := j.Left().Schema()
rCols := j.Right().Schema()
var re, le *expression.GetField
switch e := expr.(type) {
case *expression.Equals, *expression.NullSafeEquals, *expression.GreaterThan,
*expression.LessThan, *expression.LessThanOrEqual, *expression.GreaterThanOrEqual:
ce, ok := e.(expression.Comparer)
if !ok {
return false
}
le, ok = ce.Left().(*expression.GetField)
if !ok {
return false
}
re, ok = ce.Right().(*expression.GetField)
if !ok {
return false
}
default:
return false
}
return lCols.Contains(le.Name(), le.Table()) && rCols.Contains(re.Name(), re.Table()) ||
rCols.Contains(le.Name(), le.Table()) && lCols.Contains(re.Name(), re.Table())
}
// expressionCoversJoin checks whether a subexpressions's comparison predicate
// satisfies the join condition. The input conjunctions have already been split,
// so we do not care which predicate satisfies the expression.
func expressionCoversJoin(c sql.Expression, j *plan.JoinNode) (found bool) {
return transform.InspectExpr(c, func(expr sql.Expression) bool {
switch e := expr.(type) {
case expression.Comparer:
return comparisonSatisfiesJoinCondition(e, j)
}
return false
})
}
// replaceCrossJoins recursively replaces filter nested cross joins with equivalent inner joins.
// There are 3 phases after we identify a Filter -> ... -> CrossJoin pattern.
// 1) Build a list of disjunct predicate expressions by top-down splitting conjunctions (AND).
// 2) For every CrossJoin, check whether a subset of predicates covers as join conditions,
// and create a new InnerJoin with the matching predicates.
// 3) Remove predicates from the parent Filter that have been pushed into InnerJoins.
func replaceCrossJoins(ctx *sql.Context, a *Analyzer, n sql.Node, scope *Scope, sel RuleSelector) (sql.Node, transform.TreeIdentity, error) {
if !n.Resolved() {
return n, transform.SameTree, nil
}
return transform.Node(n, func(n sql.Node) (sql.Node, transform.TreeIdentity, error) {
f, ok := n.(*plan.Filter)
if !ok {
return n, transform.SameTree, nil
}
predicates := splitConjunction(f.Expression)
movedPredicates := make(map[int]struct{})
newF, _, err := transform.Node(f, func(n sql.Node) (sql.Node, transform.TreeIdentity, error) {
cj, ok := n.(*plan.JoinNode)
if !ok || !cj.Op.IsCross() {
return n, transform.SameTree, nil
}
joinConjs := make([]int, 0, len(predicates))
for i, c := range predicates {
if expressionCoversJoin(c, cj) {
joinConjs = append(joinConjs, i)
}
}
if len(joinConjs) == 0 {
return n, transform.SameTree, nil
}
newExprs := make([]sql.Expression, len(joinConjs))
for i, v := range joinConjs {
movedPredicates[v] = struct{}{}
newExprs[i] = predicates[v]
}
return plan.NewInnerJoin(cj.Left(), cj.Right(), expression.JoinAnd(newExprs...)), transform.NewTree, nil
})
if err != nil {
return f, transform.SameTree, err
}
// only alter the Filter expression tree if we transferred predicates to an InnerJoin
if len(movedPredicates) == 0 {
return f, transform.SameTree, nil
}
// remove Filter if all expressions were transferred to joins
if len(predicates) == len(movedPredicates) {
return newF.(*plan.Filter).Child, transform.NewTree, nil
}
newFilterExprs := make([]sql.Expression, 0, len(predicates)-len(movedPredicates))
for i, e := range predicates {
if _, ok := movedPredicates[i]; ok {
continue
}
newFilterExprs = append(newFilterExprs, e)
}
newF, err = newF.(*plan.Filter).WithExpressions(expression.JoinAnd(newFilterExprs...))
return newF, transform.NewTree, err
})
}