From 74f96d9f46b06bc3785dc1bd660e4bb3f3a723ae Mon Sep 17 00:00:00 2001
From: bvolpato <brunocvcunha@gmail.com>
Date: Thu, 3 Sep 2026 13:15:12 -0400
Subject: [PATCH] SEMI JOIN should not duplicate a singleton left row

---
 .../rules/SingleValuesOptimizationRules.java  |  2 ++
 .../apache/calcite/test/RelOptRulesTest.java  | 21 +++++++++++++++++++
 .../apache/calcite/test/RelOptRulesTest.xml   | 19 +++++++++++++++++
 3 files changed, 42 insertions(+)

diff --git a/core/src/main/java/org/apache/calcite/rel/rules/SingleValuesOptimizationRules.java b/core/src/main/java/org/apache/calcite/rel/rules/SingleValuesOptimizationRules.java
index d0e53be44d12..8c134ce1af39 100644
--- a/core/src/main/java/org/apache/calcite/rel/rules/SingleValuesOptimizationRules.java
+++ b/core/src/main/java/org/apache/calcite/rel/rules/SingleValuesOptimizationRules.java
@@ -284,8 +284,10 @@ static Predicate<Join> isJoinTransformable(boolean isLeft) {
           || jn.getJoinType() == JoinRelType.FULL;
 
       if (isLeft) {
+        // A semi join must emit the left row only once, even if several right rows match.
         return jn -> !(jn.getJoinType() == JoinRelType.LEFT
             || jn.getJoinType() == JoinRelType.LEFT_MARK
+            || jn.getJoinType() == JoinRelType.SEMI
             || isFullOrAntiJoin.test(jn));
       } else {
         return jn -> !(jn.getJoinType() == JoinRelType.RIGHT || isFullOrAntiJoin.test(jn));
diff --git a/core/src/test/java/org/apache/calcite/test/RelOptRulesTest.java b/core/src/test/java/org/apache/calcite/test/RelOptRulesTest.java
index 76ff0a831335..822894f0a0ea 100644
--- a/core/src/test/java/org/apache/calcite/test/RelOptRulesTest.java
+++ b/core/src/test/java/org/apache/calcite/test/RelOptRulesTest.java
@@ -5625,6 +5625,27 @@ RelOptFixture checkDynamicFunctions(boolean treatDynamicCallsAsConstant) {
         .check();
   }
 
+  @Test void testSemiJoinWithConstantRowOnLeft() {
+    relFn(builder -> builder
+        .values(new String[]{"l"}, 1)
+        .values(new String[]{"r"}, 1, 1)
+        .semiJoin(builder.equals(builder.field(2, 0, 0), builder.field(2, 1, 0)))
+        .build())
+        .withRule(SingleValuesOptimizationRules.JOIN_LEFT_INSTANCE)
+        .checkUnchanged();
+  }
+
+  @Test void testSemiJoinWithProjectOnConstantRowOnLeft() {
+    relFn(builder -> builder
+        .values(new String[]{"l"}, 1)
+        .projectPlus(builder.call(SqlStdOperatorTable.CURRENT_TIMESTAMP))
+        .values(new String[]{"r"}, 1, 1)
+        .semiJoin(builder.equals(builder.field(2, 0, 0), builder.field(2, 1, 0)))
+        .build())
+        .withRule(SingleValuesOptimizationRules.JOIN_LEFT_PROJECT_INSTANCE)
+        .checkUnchanged();
+  }
+
   @Test void testRightJoinWithConstantRowOnLeft() {
     final String sql = "select e.empno, e.ename, c.*"
         + " from (select 5 as nm) c right join emp e on e.empno = c.nm";
diff --git a/core/src/test/resources/org/apache/calcite/test/RelOptRulesTest.xml b/core/src/test/resources/org/apache/calcite/test/RelOptRulesTest.xml
index 35dc172427d2..4ae5fd499184 100644
--- a/core/src/test/resources/org/apache/calcite/test/RelOptRulesTest.xml
+++ b/core/src/test/resources/org/apache/calcite/test/RelOptRulesTest.xml
@@ -18935,6 +18935,25 @@ LogicalProject(DEPTNO=[$0])
             LogicalTableScan(table=[[CATALOG, SALES, EMP]])
     LogicalProject(ACCTNO=[$0])
       LogicalTableScan(table=[[CATALOG, CUSTOMER, ACCOUNT]])
+]]>
+    </Resource>
+  </TestCase>
+  <TestCase name="testSemiJoinWithConstantRowOnLeft">
+    <Resource name="planBefore">
+      <![CDATA[
+LogicalJoin(condition=[=($0, $1)], joinType=[semi])
+  LogicalValues(tuples=[[{ 1 }]])
+  LogicalValues(tuples=[[{ 1 }, { 1 }]])
+]]>
+    </Resource>
+  </TestCase>
+  <TestCase name="testSemiJoinWithProjectOnConstantRowOnLeft">
+    <Resource name="planBefore">
+      <![CDATA[
+LogicalJoin(condition=[=($0, $2)], joinType=[semi])
+  LogicalProject(l=[$0], $f1=[CURRENT_TIMESTAMP])
+    LogicalValues(tuples=[[{ 1 }]])
+  LogicalValues(tuples=[[{ 1 }, { 1 }]])
 ]]>
     </Resource>
   </TestCase>