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The plugin that powers ESQL for Elasticsearch
/*
* Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
* or more contributor license agreements. Licensed under the Elastic License
* 2.0; you may not use this file except in compliance with the Elastic License
* 2.0.
*/
package org.elasticsearch.xpack.esql.optimizer.rules;
import org.elasticsearch.xpack.esql.EsqlIllegalArgumentException;
import org.elasticsearch.xpack.esql.core.expression.Alias;
import org.elasticsearch.xpack.esql.core.expression.Attribute;
import org.elasticsearch.xpack.esql.core.expression.AttributeMap;
import org.elasticsearch.xpack.esql.core.expression.AttributeSet;
import org.elasticsearch.xpack.esql.core.expression.Expression;
import org.elasticsearch.xpack.esql.core.expression.Expressions;
import org.elasticsearch.xpack.esql.core.expression.NamedExpression;
import org.elasticsearch.xpack.esql.core.optimizer.OptimizerRules;
import org.elasticsearch.xpack.esql.core.plan.logical.LogicalPlan;
import org.elasticsearch.xpack.esql.core.plan.logical.UnaryPlan;
import org.elasticsearch.xpack.esql.plan.logical.Aggregate;
import org.elasticsearch.xpack.esql.plan.logical.Project;
import java.util.ArrayList;
import java.util.List;
public final class CombineProjections extends OptimizerRules.OptimizerRule {
public CombineProjections() {
super(OptimizerRules.TransformDirection.UP);
}
@Override
@SuppressWarnings("unchecked")
protected LogicalPlan rule(UnaryPlan plan) {
LogicalPlan child = plan.child();
if (plan instanceof Project project) {
if (child instanceof Project p) {
// eliminate lower project but first replace the aliases in the upper one
project = p.withProjections(combineProjections(project.projections(), p.projections()));
child = project.child();
plan = project;
// don't return the plan since the grandchild (now child) might be an aggregate that could not be folded on the way up
// e.g. stats c = count(x) | project c, c as x | project x
// try to apply the rule again opportunistically as another node might be pushed in (a limit might be pushed in)
}
// check if the projection eliminates certain aggregates
// but be mindful of aliases to existing aggregates that we don't want to duplicate to avoid redundant work
if (child instanceof Aggregate a) {
var aggs = a.aggregates();
var newAggs = projectAggregations(project.projections(), aggs);
// project can be fully removed
if (newAggs != null) {
var newGroups = replacePrunedAliasesUsedInGroupBy(a.groupings(), aggs, newAggs);
plan = new Aggregate(a.source(), a.child(), a.aggregateType(), newGroups, newAggs);
}
}
return plan;
}
// Agg with underlying Project (group by on sub-queries)
if (plan instanceof Aggregate a) {
if (child instanceof Project p) {
var groupings = a.groupings();
List groupingAttrs = new ArrayList<>(a.groupings().size());
for (Expression grouping : groupings) {
if (grouping instanceof Attribute attribute) {
groupingAttrs.add(attribute);
} else {
// After applying ReplaceStatsNestedExpressionWithEval, groupings can only contain attributes.
throw new EsqlIllegalArgumentException("Expected an Attribute, got {}", grouping);
}
}
plan = new Aggregate(
a.source(),
p.child(),
a.aggregateType(),
combineUpperGroupingsAndLowerProjections(groupingAttrs, p.projections()),
combineProjections(a.aggregates(), p.projections())
);
}
}
return plan;
}
// variant of #combineProjections specialized for project followed by agg due to the rewrite rules applied on aggregations
// this method tries to combine the projections by paying attention to:
// - aggregations that are projected away - remove them
// - aliases in the project that point to aggregates - keep them in place (to avoid duplicating the aggs)
private static List projectAggregations(
List upperProjection,
List lowerAggregations
) {
AttributeSet seen = new AttributeSet();
for (NamedExpression upper : upperProjection) {
Expression unwrapped = Alias.unwrap(upper);
// projection contains an inner alias (point to an existing fields inside the projection)
if (seen.contains(unwrapped)) {
return null;
}
seen.add(Expressions.attribute(unwrapped));
}
lowerAggregations = combineProjections(upperProjection, lowerAggregations);
return lowerAggregations;
}
// normally only the upper projections should survive but since the lower list might have aliases definitions
// that might be reused by the upper one, these need to be replaced.
// for example an alias defined in the lower list might be referred in the upper - without replacing it the alias becomes invalid
private static List combineProjections(List upper, List lower) {
// collect named expressions declaration in the lower list
AttributeMap namedExpressions = new AttributeMap<>();
// while also collecting the alias map for resolving the source (f1 = 1, f2 = f1, etc..)
AttributeMap aliases = new AttributeMap<>();
for (NamedExpression ne : lower) {
// record the alias
aliases.put(ne.toAttribute(), Alias.unwrap(ne));
// record named expression as is
if (ne instanceof Alias as) {
Expression child = as.child();
namedExpressions.put(ne.toAttribute(), as.replaceChild(aliases.resolve(child, child)));
}
}
List replaced = new ArrayList<>();
// replace any matching attribute with a lower alias (if there's a match)
// but clean-up non-top aliases at the end
for (NamedExpression ne : upper) {
NamedExpression replacedExp = (NamedExpression) ne.transformUp(Attribute.class, a -> namedExpressions.resolve(a, a));
replaced.add((NamedExpression) trimNonTopLevelAliases(replacedExp));
}
return replaced;
}
private static List combineUpperGroupingsAndLowerProjections(
List upperGroupings,
List lowerProjections
) {
// Collect the alias map for resolving the source (f1 = 1, f2 = f1, etc..)
AttributeMap aliases = new AttributeMap<>();
for (NamedExpression ne : lowerProjections) {
// Projections are just aliases for attributes, so casting is safe.
aliases.put(ne.toAttribute(), (Attribute) Alias.unwrap(ne));
}
// Replace any matching attribute directly with the aliased attribute from the projection.
AttributeSet replaced = new AttributeSet();
for (Attribute attr : upperGroupings) {
// All substitutions happen before; groupings must be attributes at this point.
replaced.add(aliases.resolve(attr, attr));
}
return new ArrayList<>(replaced);
}
/**
* Replace grouping alias previously contained in the aggregations that might have been projected away.
*/
private List replacePrunedAliasesUsedInGroupBy(
List groupings,
List oldAggs,
List newAggs
) {
AttributeMap removedAliases = new AttributeMap<>();
AttributeSet currentAliases = new AttributeSet(Expressions.asAttributes(newAggs));
// record only removed aliases
for (NamedExpression ne : oldAggs) {
if (ne instanceof Alias alias) {
var attr = ne.toAttribute();
if (currentAliases.contains(attr) == false) {
removedAliases.put(attr, alias.child());
}
}
}
if (removedAliases.isEmpty()) {
return groupings;
}
var newGroupings = new ArrayList(groupings.size());
for (Expression group : groupings) {
var transformed = group.transformUp(Attribute.class, a -> removedAliases.resolve(a, a));
if (Expressions.anyMatch(newGroupings, g -> Expressions.equalsAsAttribute(g, transformed)) == false) {
newGroupings.add(transformed);
}
}
return newGroupings;
}
public static Expression trimNonTopLevelAliases(Expression e) {
return e instanceof Alias a ? a.replaceChild(trimAliases(a.child())) : trimAliases(e);
}
private static Expression trimAliases(Expression e) {
return e.transformDown(Alias.class, Alias::child);
}
}
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