diff --git a/sdks/java/extensions/sql/src/main/java/org/apache/beam/sdk/extensions/sql/impl/planner/BeamRuleSets.java b/sdks/java/extensions/sql/src/main/java/org/apache/beam/sdk/extensions/sql/impl/planner/BeamRuleSets.java
index 8382fcb6e382..c29a9bc12489 100644
--- a/sdks/java/extensions/sql/src/main/java/org/apache/beam/sdk/extensions/sql/impl/planner/BeamRuleSets.java
+++ b/sdks/java/extensions/sql/src/main/java/org/apache/beam/sdk/extensions/sql/impl/planner/BeamRuleSets.java
@@ -79,8 +79,15 @@ public class BeamRuleSets {
           CoreRules.FILTER_AGGREGATE_TRANSPOSE,
           // push filter through set operation
           CoreRules.FILTER_SET_OP_TRANSPOSE,
-          // push project through set operation
-          CoreRules.PROJECT_SET_OP_TRANSPOSE,
+          // NOTE: CoreRules.PROJECT_SET_OP_TRANSPOSE is intentionally NOT enabled.
+          // Pushing a projection through a set operation (UNION/INTERSECT/EXCEPT) transposes
+          // it into each branch, where PROJECT_MERGE fuses it with an adjacent projection.
+          // That fusion cancels the deterministic IEEE-754 FP surrogate wrapped around
+          // FLOAT/DOUBLE set-operation columns (collapsing from_bits(bits(x)) back to the
+          // raw DOUBLE), reintroducing the non-deterministic Double/Float key coder that
+          // Beam's CoGroup-based set operators reject ("the keyCoder of a GroupByKey must
+          // be deterministic"). The rule is a pure optimization, so omitting it is
+          // correctness-preserving.
 
           // aggregation and projection rules
           // BeamAggregateProjectMergeRule.INSTANCE,
diff --git a/sdks/java/extensions/sql/src/test/java/org/apache/beam/sdk/extensions/sql/impl/planner/BeamRuleSetsTest.java b/sdks/java/extensions/sql/src/test/java/org/apache/beam/sdk/extensions/sql/impl/planner/BeamRuleSetsTest.java
new file mode 100644
index 000000000000..a6f5c04fd80d
--- /dev/null
+++ b/sdks/java/extensions/sql/src/test/java/org/apache/beam/sdk/extensions/sql/impl/planner/BeamRuleSetsTest.java
@@ -0,0 +1,71 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.beam.sdk.extensions.sql.impl.planner;
+
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
+import java.util.Collection;
+import org.apache.beam.vendor.calcite.v1_40_0.org.apache.calcite.tools.RuleSet;
+import org.junit.Test;
+
+/** Tests for {@link BeamRuleSets}, verifying the composition of the active planner rule set. */
+public class BeamRuleSetsTest {
+
+  @Test
+  public void testProjectSetOpTransposeIsDisabled() {
+    Collection<RuleSet> ruleSets = BeamRuleSets.getRuleSets();
+    boolean foundProjectSetOpTranspose = false;
+    for (RuleSet ruleSet : ruleSets) {
+      for (org.apache.beam.vendor.calcite.v1_40_0.org.apache.calcite.plan.RelOptRule rule :
+          ruleSet) {
+        String ruleName = rule.toString();
+        if (ruleName.contains("PROJECT_SET_OP_TRANSPOSE")
+            || ruleName.contains("ProjectSetOpTranspose")) {
+          foundProjectSetOpTranspose = true;
+          break;
+        }
+      }
+    }
+    assertFalse(
+        "CoreRules.PROJECT_SET_OP_TRANSPOSE should NOT be enabled in the active rule set, "
+            + "as it collapses IEEE-754 FP surrogates around FLOAT/DOUBLE set-operation columns, "
+            + "reintroducing non-deterministic key coders.",
+        foundProjectSetOpTranspose);
+  }
+
+  @Test
+  public void testFilterSetOpTransposeIsEnabled() {
+    Collection<RuleSet> ruleSets = BeamRuleSets.getRuleSets();
+    boolean foundFilterSetOpTranspose = false;
+    for (RuleSet ruleSet : ruleSets) {
+      for (org.apache.beam.vendor.calcite.v1_40_0.org.apache.calcite.plan.RelOptRule rule :
+          ruleSet) {
+        String ruleName = rule.toString();
+        if (ruleName.contains("FILTER_SET_OP_TRANSPOSE")
+            || ruleName.contains("FilterSetOpTranspose")) {
+          foundFilterSetOpTranspose = true;
+          break;
+        }
+      }
+    }
+    assertTrue(
+        "CoreRules.FILTER_SET_OP_TRANSPOSE should be enabled in the active rule set",
+        foundFilterSetOpTranspose);
+  }
+}