diff --git a/x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/optimizer/OptimizerRulesTest.java b/x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/optimizer/OptimizerRulesTest.java
new file mode 100644
index 0000000000000..6e275598fb07f
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+++ b/x-pack/plugin/esql/src/test/java/org/elasticsearch/xpack/esql/optimizer/OptimizerRulesTest.java
@@ -0,0 +1,146 @@
+/*
+ * 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;
+
+import org.elasticsearch.test.ESTestCase;
+import org.elasticsearch.xpack.esql.evaluator.predicate.operator.comparison.Equals;
+import org.elasticsearch.xpack.esql.evaluator.predicate.operator.comparison.LessThan;
+import org.elasticsearch.xpack.esql.expression.predicate.operator.comparison.In;
+import org.elasticsearch.xpack.ql.expression.Expression;
+import org.elasticsearch.xpack.ql.expression.FieldAttribute;
+import org.elasticsearch.xpack.ql.expression.Literal;
+import org.elasticsearch.xpack.ql.expression.predicate.logical.And;
+import org.elasticsearch.xpack.ql.expression.predicate.logical.Or;
+import org.elasticsearch.xpack.ql.plan.logical.Filter;
+import org.elasticsearch.xpack.ql.plan.logical.LogicalPlan;
+import org.elasticsearch.xpack.ql.tree.Source;
+import org.elasticsearch.xpack.ql.type.DataTypes;
+
+import java.util.List;
+
+import static java.util.Arrays.asList;
+import static org.elasticsearch.xpack.ql.TestUtils.getFieldAttribute;
+import static org.elasticsearch.xpack.ql.TestUtils.relation;
+import static org.elasticsearch.xpack.ql.tree.Source.EMPTY;
+import static org.hamcrest.Matchers.contains;
+
+public class OptimizerRulesTest extends ESTestCase {
+    private static final Literal ONE = new Literal(Source.EMPTY, 1, DataTypes.INTEGER);
+    private static final Literal TWO = new Literal(Source.EMPTY, 2, DataTypes.INTEGER);
+    private static final Literal THREE = new Literal(Source.EMPTY, 3, DataTypes.INTEGER);
+
+    private static Equals equalsOf(Expression left, Expression right) {
+        return new Equals(EMPTY, left, right, null);
+    }
+
+    private static LessThan lessThanOf(Expression left, Expression right) {
+        return new LessThan(EMPTY, left, right, null);
+    }
+
+    //
+    // CombineDisjunction in Equals
+    //
+    public void testTwoEqualsWithOr() {
+        FieldAttribute fa = getFieldAttribute("a");
+
+        Or or = new Or(EMPTY, equalsOf(fa, ONE), equalsOf(fa, TWO));
+        Expression e = new OptimizerRules.CombineDisjunctionsToIn().rule(or);
+        assertEquals(In.class, e.getClass());
+        In in = (In) e;
+        assertEquals(fa, in.value());
+        assertThat(in.list(), contains(ONE, TWO));
+    }
+
+    public void testTwoEqualsWithSameValue() {
+        FieldAttribute fa = getFieldAttribute("a");
+
+        Or or = new Or(EMPTY, equalsOf(fa, ONE), equalsOf(fa, ONE));
+        Expression e = new OptimizerRules.CombineDisjunctionsToIn().rule(or);
+        assertEquals(Equals.class, e.getClass());
+        Equals eq = (Equals) e;
+        assertEquals(fa, eq.left());
+        assertEquals(ONE, eq.right());
+    }
+
+    public void testOneEqualsOneIn() {
+        FieldAttribute fa = getFieldAttribute("a");
+
+        Or or = new Or(EMPTY, equalsOf(fa, ONE), new In(EMPTY, fa, List.of(TWO)));
+        Expression e = new OptimizerRules.CombineDisjunctionsToIn().rule(or);
+        assertEquals(In.class, e.getClass());
+        In in = (In) e;
+        assertEquals(fa, in.value());
+        assertThat(in.list(), contains(ONE, TWO));
+    }
+
+    public void testOneEqualsOneInWithSameValue() {
+        FieldAttribute fa = getFieldAttribute("a");
+
+        Or or = new Or(EMPTY, equalsOf(fa, ONE), new In(EMPTY, fa, asList(ONE, TWO)));
+        Expression e = new OptimizerRules.CombineDisjunctionsToIn().rule(or);
+        assertEquals(In.class, e.getClass());
+        In in = (In) e;
+        assertEquals(fa, in.value());
+        assertThat(in.list(), contains(ONE, TWO));
+    }
+
+    public void testSingleValueInToEquals() {
+        FieldAttribute fa = getFieldAttribute("a");
+
+        Equals equals = equalsOf(fa, ONE);
+        Or or = new Or(EMPTY, equals, new In(EMPTY, fa, List.of(ONE)));
+        Expression e = new OptimizerRules.CombineDisjunctionsToIn().rule(or);
+        assertEquals(equals, e);
+    }
+
+    public void testEqualsBehindAnd() {
+        FieldAttribute fa = getFieldAttribute("a");
+
+        And and = new And(EMPTY, equalsOf(fa, ONE), equalsOf(fa, TWO));
+        Filter dummy = new Filter(EMPTY, relation(), and);
+        LogicalPlan transformed = new OptimizerRules.CombineDisjunctionsToIn().apply(dummy);
+        assertSame(dummy, transformed);
+        assertEquals(and, ((Filter) transformed).condition());
+    }
+
+    public void testTwoEqualsDifferentFields() {
+        FieldAttribute fieldOne = getFieldAttribute("ONE");
+        FieldAttribute fieldTwo = getFieldAttribute("TWO");
+
+        Or or = new Or(EMPTY, equalsOf(fieldOne, ONE), equalsOf(fieldTwo, TWO));
+        Expression e = new OptimizerRules.CombineDisjunctionsToIn().rule(or);
+        assertEquals(or, e);
+    }
+
+    public void testMultipleIn() {
+        FieldAttribute fa = getFieldAttribute("a");
+
+        Or firstOr = new Or(EMPTY, new In(EMPTY, fa, List.of(ONE)), new In(EMPTY, fa, List.of(TWO)));
+        Or secondOr = new Or(EMPTY, firstOr, new In(EMPTY, fa, List.of(THREE)));
+        Expression e = new OptimizerRules.CombineDisjunctionsToIn().rule(secondOr);
+        assertEquals(In.class, e.getClass());
+        In in = (In) e;
+        assertEquals(fa, in.value());
+        assertThat(in.list(), contains(ONE, TWO, THREE));
+    }
+
+    public void testOrWithNonCombinableExpressions() {
+        FieldAttribute fa = getFieldAttribute("a");
+
+        Or firstOr = new Or(EMPTY, new In(EMPTY, fa, List.of(ONE)), lessThanOf(fa, TWO));
+        Or secondOr = new Or(EMPTY, firstOr, new In(EMPTY, fa, List.of(THREE)));
+        Expression e = new OptimizerRules.CombineDisjunctionsToIn().rule(secondOr);
+        assertEquals(Or.class, e.getClass());
+        Or or = (Or) e;
+        assertEquals(or.left(), firstOr.right());
+        assertEquals(In.class, or.right().getClass());
+        In in = (In) or.right();
+        assertEquals(fa, in.value());
+        assertThat(in.list(), contains(ONE, THREE));
+    }
+}