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release-22.2.0: opt: don't add reordered join with extra filters to original memo group #89350

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Oct 5, 2022
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release-22.2.0: opt: don't add reordered join with extra filters to original memo group #89350

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126 changes: 108 additions & 18 deletions pkg/sql/opt/xform/join_order_builder.go
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Original file line number Diff line number Diff line change
Expand Up @@ -316,6 +316,14 @@ type JoinOrderBuilder struct {
// assembling filters.
equivs props.EquivSet

// rebuildAllJoins is true when the filters in the original matched join tree
// were not pushed down as far as possible. When this is true, all joins
// except the root join need to be re-built, possibly with additional filters
// pushed down. While technically it is sufficient to only do this for the
// joins that would be changed by a successful push-down, it is simpler to
// handle things this way (and the problem is rare).
rebuildAllJoins bool

onReorderFunc OnReorderFunc

onAddJoinFunc OnAddJoinFunc
Expand Down Expand Up @@ -354,6 +362,12 @@ func (jb *JoinOrderBuilder) Reorder(join memo.RelExpr) {
// the best plan.
jb.ensureClosure(join)

// Ensure that the JoinOrderBuilder will not add reordered joins to the
// original memo groups (apart from the root) in the case when doing so
// would add filters that weren't present in the original joins. See the
// validateEdges comment for more information.
jb.validateEdges()

if jb.onReorderFunc != nil {
// Hook for testing purposes.
jb.callOnReorderFunc(join)
Expand Down Expand Up @@ -466,6 +480,72 @@ func (jb *JoinOrderBuilder) ensureClosure(join memo.RelExpr) {
}
}

// validateEdges checks whether each edge applies to its original join. If any
// do not, normalization rules failed to synthesize and push a filter down as
// far as possible, and it is not valid to add new reordered joins to the
// original memo groups. When this is the case, all joins except for the root
// join need to be removed from the plans map. This prevents cases where a join
// is added to a memo group that isn't logically equivalent.
//
// This is necessary because the JoinOrderBuilder expects each join tree for a
// given set of relations to contain all filters that apply to those relations.
// When a new join is constructed, it doesn't contain "degenerate" filters -
// filters that only refer to one side of the join. So if the original join tree
// had an implicit filter that could have been synthesized and pushed down the
// tree, but wasn't, using the original join group that *should* have that
// filter when building a new join would cause a filter to be dropped.
//
// Take the following (simplified) example of a join tree where filter push-down
// rules have failed:
//
// (xy join ab on true) join uv on x = u and a = u
//
// Here, the JoinOrderBuilder will synthesize an 'x = a' filter that will be
// used to join xy and ab. If it was added to the original group, we would have
// a memo group that looks like this:
//
// group: (xy join ab on true), (xy join ab on x = a)
//
// Later joins that are constructed using this group would expect the 'x = a'
// filter to be present, and would avoid adding redundant filters. Therefore,
// a join tree like the following would be added to the memo.
//
// (xy join ab on true) join uv on x = u
//
// Notice how the 'a = u' filter has been dropped because it would be redundant
// when 'x = u' and 'x = a' are already present. We prevent this from happening
// by not reusing the original memo groups in the case when the JoinOrderBuilder
// is able to synthesize and/or push down filters that weren't in the original
// join tree.
func (jb *JoinOrderBuilder) validateEdges() {
for i := range jb.edges {
if jb.rebuildAllJoins {
break
}
e := &jb.edges[i]
if e.op.joinType == opt.InnerJoinOp {
jb.rebuildAllJoins = !e.checkInnerJoin(e.op.leftVertexes, e.op.rightVertexes)
} else {
jb.rebuildAllJoins = !e.checkNonInnerJoin(e.op.leftVertexes, e.op.rightVertexes)
}
}
if jb.rebuildAllJoins {
for vertexes := range jb.plans {
if vertexes.isSingleton() || vertexes == jb.allVertexes() {
// Do not remove the plan if it is for a base relation (not a join) or
// it is the root join. Adding to the root join group is correct because
// the JoinOrderBuilder will only consider filters that were present
// (even if only implicitly) in the root join tree. It is also necessary
// because the purpose of the JoinOrderBuilder is to add equivalent join
// plans to the root join group - otherwise, any new joins would be
// disconnected from the main query plan.
continue
}
delete(jb.plans, vertexes)
}
}
}

// dpSube carries out the DPSube algorithm (citations: [8] figure 4). All
// disjoint pairs of subsets of base relations are enumerated and checked for
// validity. If valid, the pair of subsets is used along with the edges
Expand Down Expand Up @@ -526,9 +606,10 @@ func (jb *JoinOrderBuilder) addJoins(s1, s2 vertexSet) {
continue
}
if !joinIsRedundant {
// If this edge was originally part of a join between relation sets s1 and
// s2, any other edges that apply will also be part of that original join.
joinIsRedundant = e.joinIsRedundant(s1, s2)
// If this edge was originally part of a join between relation sets s1
// and s2, any other edges that apply will also be part of that original
// join.
joinIsRedundant = jb.joinIsRedundant(e, s1, s2)
}
for j := range e.filters {
jb.equivs.AddFromFDs(&e.filters[j].ScalarProps().FuncDeps)
Expand All @@ -549,7 +630,7 @@ func (jb *JoinOrderBuilder) addJoins(s1, s2 vertexSet) {
// Construct a non-inner join. If any inner join filters also apply to the
// pair of relationSets, construct a select on top of the join with the
// inner join filters.
jb.addJoin(e.op.joinType, s1, s2, e.filters, innerJoinFilters, e.joinIsRedundant(s1, s2))
jb.addJoin(e.op.joinType, s1, s2, e.filters, innerJoinFilters, jb.joinIsRedundant(e, s1, s2))
return
}
if e.checkNonInnerJoin(s2, s1) {
Expand All @@ -575,7 +656,7 @@ func (jb *JoinOrderBuilder) addJoins(s1, s2 vertexSet) {
// 010 on the right. 101 is larger than 111 / 2, so we will not enumerate
// this plan unless we consider a join with s2 on the left and s1 on the
// right.
jb.addJoin(e.op.joinType, s2, s1, e.filters, innerJoinFilters, e.joinIsRedundant(s2, s1))
jb.addJoin(e.op.joinType, s2, s1, e.filters, innerJoinFilters, jb.joinIsRedundant(e, s2, s1))
return
}
}
Expand Down Expand Up @@ -642,6 +723,19 @@ func (jb *JoinOrderBuilder) makeTransitiveEdge(col1, col2 opt.ColumnID) {
return
}

originalJoin, ok := jb.plans[op.leftVertexes.union(op.rightVertexes)]
if !ok {
panic(errors.AssertionFailedf("failed to find expected join plan"))
}
if !originalJoin.Relational().FuncDeps.AreColsEquiv(col1, col2) {
// This inferred filter was not pushed down as far as possible. All joins
// apart from the root will have to be rebuilt. We have to do this check
// here because we set the op for this edge to the join to which the filter
// *would* have been pushed down if it existed, so the applicable check will
// always succeed for that join.
jb.rebuildAllJoins = true
}

// Construct the edge.
var1 := jb.f.ConstructVariable(col1)
var2 := jb.f.ConstructVariable(col2)
Expand Down Expand Up @@ -754,12 +848,6 @@ func (jb *JoinOrderBuilder) addToGroup(
) {
if len(selectFilters) > 0 {
joinExpr := jb.memoize(op, left, right, on, nil)
if joinExpr.FirstExpr() == grp.FirstExpr() {
// In rare cases, the select filters may be redundant. In this case,
// adding a select to the group with the redundant filters would create a
// memo cycle (see #80901).
return
}
selectExpr := &memo.SelectExpr{
Input: joinExpr,
Filters: selectFilters,
Expand Down Expand Up @@ -904,6 +992,15 @@ func (jb *JoinOrderBuilder) addBaseRelation(rel memo.RelExpr) {
jb.plans[relSet] = rel
}

// joinIsRedundant returns true if a join between the two sets of base relations
// was already present in the original join tree. If so, enumerating this join
// would be redundant, so it should be skipped.
func (jb *JoinOrderBuilder) joinIsRedundant(e *edge, s1, s2 vertexSet) bool {
// The join is never redundant when rebuildAllJoins is true, because
// rebuildAllJoins indicates we don't want to reuse the original joins.
return !jb.rebuildAllJoins && e.op.leftVertexes == s1 && e.op.rightVertexes == s2
}

// checkSize panics if the number of relations is greater than or equal to
// MaxReorderJoinsLimit. checkSize should be called before a vertex is added to
// the join graph.
Expand Down Expand Up @@ -1353,13 +1450,6 @@ func (e *edge) checkRules(s1, s2 vertexSet) bool {
return true
}

// joinIsRedundant returns true if a join between the two sets of base relations
// was already present in the original join tree. If so, enumerating this join
// would be redundant, so it should be skipped.
func (e *edge) joinIsRedundant(s1, s2 vertexSet) bool {
return e.op.leftVertexes == s1 && e.op.rightVertexes == s2
}

// commute returns true if the given join operator type is commutable.
func commute(op opt.Operator) bool {
return op == opt.InnerJoinOp || op == opt.FullJoinOp
Expand Down
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