Preserve unconstrained placeholders in AndPredicate#and()

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

@@ -187,9 +187,11 @@ public static QueryPredicate and(@Nonnull final Collection<? extends QueryPredic
 
     @Nonnull
     public static QueryPredicate and(@Nonnull final Collection<? extends QueryPredicate> conjuncts, final boolean isAtomic) {
+        // Keep only conjuncts that aren’t tautologies; except for placeholders (where tautology means "no range
+        // constraint"). All placeholders must be retained for the index-matching machinery to work correctly.
         final var filteredConjuncts =
                 conjuncts.stream()
-                        .filter(queryPredicate -> !queryPredicate.isTautology())
+                        .filter(queryPredicate -> (queryPredicate instanceof Placeholder) || !queryPredicate.isTautology())
                         .collect(ImmutableList.toImmutableList());
 
         if (filteredConjuncts.isEmpty()) {

fdb-record-layer-core/src/test/java/com/apple/foundationdb/record/query/plan/cascades/predicates/QueryPredicateTest.java

@@ -28,6 +28,7 @@
 import com.apple.foundationdb.record.provider.foundationdb.FDBRecordStoreBase;
 import com.apple.foundationdb.record.query.expressions.Comparisons;
 import com.apple.foundationdb.record.query.plan.cascades.AliasMap;
+import com.apple.foundationdb.record.query.plan.cascades.CorrelationIdentifier;
 import com.apple.foundationdb.record.query.plan.cascades.Quantifier;
 import com.apple.foundationdb.record.query.plan.cascades.predicates.simplification.DefaultQueryPredicateRuleSet;
 import com.apple.foundationdb.record.query.plan.cascades.predicates.simplification.QueryPredicateWithCnfRuleSet;
@@ -58,6 +59,7 @@
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertSame;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
 /**
@@ -391,4 +393,31 @@ void simplifyPredicateTestRemovalRedundantDeepSubPredicates() {
                         QueryPredicateWithDnfRuleSet.ofSimplificationRules()).get();
         assertEquals(expectedSimplifiedPredicate, simplifiedPredicate);
     }
+
+    /**
+     * Tests that {@link AndPredicate#and} retains placeholders.
+     *
+     * <p>A {@link Placeholder} with no range constraints reports {@code isTautology() == true} because it accepts every
+     * value. The tautology filter in {@code and()} must not drop such placeholders, as they are necessary for correct
+     * index matching.
+     */
+    @Test
+    void andRetainsPlaceholders() {
+        final var alias = CorrelationIdentifier.of("p0");
+        final var value = LiteralValue.ofScalar(42);
+        final Placeholder placeholder = Placeholder.newInstanceWithoutRanges(value, alias);
+
+        // Sanity-check that the placeholder indeed claims to be a tautology.
+        assertTrue(placeholder.isTautology());
+
+        // A conjunction of only an unconstrained placeholder is simplified to just that (not collapsed to TRUE).
+        final QueryPredicate pred1 = AndPredicate.and(List.of(placeholder));
+        assertSame(placeholder, pred1);
+
+        // Mixing a placeholder with a real predicate. The resulting `AndPredicate` must contain the placeholder.
+        final var pred2 = new ValuePredicate(value, new Comparisons.SimpleComparison(Comparisons.Type.EQUALS, 42));
+        final QueryPredicate pred3 = AndPredicate.and(List.of(placeholder, pred2));
+        assertTrue(pred3 instanceof AndPredicate);
+        assertTrue(((AndPredicate)pred3).getChildren().contains(placeholder));
+    }
 }