Support LEFT OUTER JOIN

This change introduces support for LEFT OUTER JOIN.

To this end we introduce a dedicated QGM box (`OuterJoinExpression`) to
represent one outer join. It is strictly binary (unlike the SELECT box)
and carries the join type (LEFT/RIGHT/FULL), the ON-clause predicates,
and a reference to the “preserved” and the “null-supplying” quantifier.
During planning, an exploration rule `RewriteOuterJoinRule` eliminates
the outer join box by rewriting it into two nested select boxes. The
preserved (left) side is connected to the outer `SelectExpression`
through a normal FOREACH quantifier. The null-supplying (right) side is
wrapped in an inner `SelectExpression` that carries the ON predicates
and is connected through a FOREACH quantifier with `nullOnEmpty` set to
true. This rewrite happens early during planning so that existing rules
(predicate push-down, join ordering, implementation) take care of
implementing the join and optimizing it without further modification.

Key changes:

* `OuterJoinExpression` represents the OUTER JOIN in the QGM.
* `QueryVisitor` parses the LEFT OUTER JOIN syntax and constructs
  the logical `OuterJoinExpression` via `wrapOperandsForOuterJoin()`.
* `RewriteOuterJoinRule` rewrites the `OuterJoinExpression` into nested
  `SelectExpression` boxes.
* Also extend `SelectMergeRule` to exclude `OuterJoinExpression` from
  merge candidates. This prevents WHERE predicates from being absorbed
  into the join and interfering with outer join semantics.
* Include `OuterJoinExpression` in `ExpressionCountProperty.selectCount`
  so that branches containing outer joins are counted correctly when
  evaluating merge candidates.

Testing:
* `join-tests-outer.yamsql` integration tests.

Author: robert-brunel

fdb-record-layer-core/src/main/java/com/apple/foundationdb/record/query/plan/cascades/Ordering.java

@@ -3,7 +3,7 @@
  *
  * This source file is part of the FoundationDB open source project
  *
- * Copyright 2015-2021 Apple Inc. and the FoundationDB project authors
+ * Copyright 2015-2026 Apple Inc. and the FoundationDB project authors
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -505,6 +505,22 @@ public Ordering pullUp(@Nonnull final Value value, @Nonnull EvaluationContext ev
         return Ordering.ofOrderingSet(bindingMapBuilder.build(), mappedOrderingSet, isDistinct());
     }
 
+    /**
+     * Pulls up this ordering from a child quantifier's column space into the parent expression's result-value
+     * space. This is the common pattern used by join implementation rules to translate a child plan's ordering
+     * into the join output's ordering.
+     *
+     * @param resultValue the parent expression's result value
+     * @param childAlias the alias of the child quantifier whose ordering is being pulled up
+     * @return the translated ordering in the parent's column space
+     */
+    @Nonnull
+    public Ordering pullUpForJoin(@Nonnull final Value resultValue, @Nonnull final CorrelationIdentifier childAlias) {
+        return pullUp(resultValue, EvaluationContext.empty(),
+                AliasMap.ofAliases(childAlias, Quantifier.current()),
+                resultValue.getCorrelatedTo());
+    }
+
     @Nonnull
     public Ordering pushDown(@Nonnull final Value value, @Nonnull final EvaluationContext evaluationContext,
                              @Nonnull final AliasMap aliasMap, @Nonnull final Set<CorrelationIdentifier> constantAliases) {

fdb-record-layer-core/src/main/java/com/apple/foundationdb/record/query/plan/cascades/PlanningRuleSet.java

@@ -83,6 +83,7 @@
 import com.apple.foundationdb.record.query.plan.cascades.rules.PushTypeFilterBelowFilterRule;
 import com.apple.foundationdb.record.query.plan.cascades.rules.RemoveProjectionRule;
 import com.apple.foundationdb.record.query.plan.cascades.rules.RemoveSortRule;
+import com.apple.foundationdb.record.query.plan.cascades.rules.RewriteOuterJoinRule;
 import com.apple.foundationdb.record.query.plan.cascades.rules.SplitSelectExtractIndependentQuantifiersRule;
 import com.apple.foundationdb.record.query.plan.plans.RecordQueryInParameterJoinPlan;
 import com.apple.foundationdb.record.query.plan.plans.RecordQueryInUnionOnValuesPlan;
@@ -110,7 +111,8 @@ public class PlanningRuleSet extends CascadesRuleSet {
             new SplitSelectExtractIndependentQuantifiersRule(),
             new PullUpNullOnEmptyRule(),
             new PartitionSelectRule(),
-            new PartitionBinarySelectRule()
+            new PartitionBinarySelectRule(),
+            new RewriteOuterJoinRule()
             );
     private static final Set<CascadesRule<? extends RelationalExpression>> MATCHING_RULES = ImmutableSet.of(
             new MatchLeafRule(),