SQL: Fix issue with folding of CASE/IIF (#49449)

Add extra checks to prevent ConstantFolding rule to try to fold
the CASE/IIF functions early before the SimplifyCase rule gets applied.

Fixes: #49387

x-pack/plugin/sql/src/main/java/org/elasticsearch/xpack/sql/expression/predicate/conditional/Case.java

@@ -116,12 +116,21 @@ public class Case extends ConditionalFunction {
 
     /**
      * All foldable conditions that fold to FALSE should have
-     * been removed by the {@link Optimizer}.
+     * been removed by the {@link Optimizer}#SimplifyCase.
      */
     @Override
     public boolean foldable() {
-        return (conditions.isEmpty() && elseResult.foldable()) ||
-            (conditions.size() == 1 && conditions.get(0).condition().foldable() && conditions.get(0).result().foldable());
+        if (conditions.isEmpty() && elseResult.foldable()) {
+            return true;
+        }
+        if (conditions.size() == 1 && conditions.get(0).condition().foldable()) {
+            if (conditions.get(0).condition().fold() == Boolean.TRUE) {
+                return conditions().get(0).result().foldable();
+            } else {
+                return elseResult().foldable();
+            }
+        }
+        return false;
     }
 
     @Override

x-pack/plugin/sql/src/test/java/org/elasticsearch/xpack/sql/optimizer/OptimizerTests.java

@@ -671,6 +671,8 @@ public class OptimizerTests extends ESTestCase {
             new IfConditional(EMPTY, new Equals(EMPTY, TWO, ONE), Literal.of(EMPTY, "bar2")),
             new IfConditional(EMPTY, new GreaterThan(EMPTY, getFieldAttribute(), ONE), Literal.of(EMPTY, "foo2")),
             Literal.of(EMPTY, "default")));
+        assertFalse(c.foldable());
+
         Expression e = new SimplifyCase().rule(c);
         assertEquals(Case.class, e.getClass());
         c = (Case) e;
@@ -696,13 +698,15 @@ public class OptimizerTests extends ESTestCase {
         // ELSE 'default'
         // END
 
-        SimplifyCase rule = new SimplifyCase();
         Case c = new Case(EMPTY, Arrays.asList(
             new IfConditional(EMPTY, new Equals(EMPTY, getFieldAttribute(), ONE), Literal.of(EMPTY, "foo1")),
             new IfConditional(EMPTY, new Equals(EMPTY, ONE, ONE), Literal.of(EMPTY, "bar1")),
             new IfConditional(EMPTY, new Equals(EMPTY, TWO, ONE), Literal.of(EMPTY, "bar2")),
             new IfConditional(EMPTY, new GreaterThan(EMPTY, getFieldAttribute(), ONE), Literal.of(EMPTY, "foo2")),
             Literal.of(EMPTY, "default")));
+        assertFalse(c.foldable());
+
+        SimplifyCase rule = new SimplifyCase();
         Expression e = rule.rule(c);
         assertEquals(Case.class, e.getClass());
         c = (Case) e;
@@ -713,7 +717,7 @@ public class OptimizerTests extends ESTestCase {
         assertEquals(TypeResolution.TYPE_RESOLVED, c.typeResolved());
     }
 
-    public void testSimplifyCaseConditionsFoldCompletely() {
+    public void testSimplifyCaseConditionsFoldCompletely_FoldableElse() {
         // CASE WHEN 1 = 2 THEN 'foo1'
         //      WHEN 1 = 1 THEN 'foo2'
         // ELSE 'default'
@@ -723,11 +727,13 @@ public class OptimizerTests extends ESTestCase {
         //
         // 'foo2'
 
-        SimplifyCase rule = new SimplifyCase();
         Case c = new Case(EMPTY, Arrays.asList(
             new IfConditional(EMPTY, new Equals(EMPTY, ONE, TWO), Literal.of(EMPTY, "foo1")),
             new IfConditional(EMPTY, new Equals(EMPTY, ONE, ONE), Literal.of(EMPTY, "foo2")),
             Literal.of(EMPTY, "default")));
+        assertFalse(c.foldable());
+
+        SimplifyCase rule = new SimplifyCase();
         Expression e = rule.rule(c);
         assertEquals(Case.class, e.getClass());
         c = (Case) e;
@@ -738,9 +744,34 @@ public class OptimizerTests extends ESTestCase {
         assertEquals(TypeResolution.TYPE_RESOLVED, c.typeResolved());
     }
 
-    public void testSimplifyIif_ConditionTrue() {
+    public void testSimplifyCaseConditionsFoldCompletely_NonFoldableElse() {
+        // CASE WHEN 1 = 2 THEN 'foo1'
+        // ELSE myField
+        // END
+        //
+        // ==>
+        //
+        // myField (non-foldable)
+
+        Case c = new Case(EMPTY, Arrays.asList(
+                new IfConditional(EMPTY, new Equals(EMPTY, ONE, TWO), Literal.of(EMPTY, "foo1")),
+                getFieldAttribute("myField")));
+        assertFalse(c.foldable());
+
+        SimplifyCase rule = new SimplifyCase();
+        Expression e = rule.rule(c);
+        assertEquals(Case.class, e.getClass());
+        c = (Case) e;
+        assertEquals(0, c.conditions().size());
+        assertFalse(c.foldable());
+        assertEquals("myField", Expressions.name(c.elseResult()));
+    }
+
+    public void testSimplifyIif_ConditionTrue_FoldableResult() {
         SimplifyCase rule = new SimplifyCase();
         Iif iif = new Iif(EMPTY, new Equals(EMPTY, ONE, ONE), Literal.of(EMPTY, "foo"), Literal.of(EMPTY, "bar"));
+        assertTrue(iif.foldable());
+
         Expression e = rule.rule(iif);
         assertEquals(Iif.class, e.getClass());
         iif = (Iif) e;
@@ -750,9 +781,26 @@ public class OptimizerTests extends ESTestCase {
         assertEquals(TypeResolution.TYPE_RESOLVED, iif.typeResolved());
     }
 
-    public void testSimplifyIif_ConditionFalse() {
+    public void testSimplifyIif_ConditionTrue_NonFoldableResult() {
+        SimplifyCase rule = new SimplifyCase();
+        Iif iif = new Iif(EMPTY, new Equals(EMPTY, ONE, ONE), getFieldAttribute("myField"), Literal.of(EMPTY, "bar"));
+        assertFalse(iif.foldable());
+
+        Expression e = rule.rule(iif);
+        assertEquals(Iif.class, e.getClass());
+        iif = (Iif) e;
+        assertEquals(1, iif.conditions().size());
+        assertFalse(iif.foldable());
+        assertTrue(iif.conditions().get(0).condition().foldable());
+        assertEquals(Boolean.TRUE, iif.conditions().get(0).condition().fold());
+        assertEquals("myField", Expressions.name(iif.conditions().get(0).result()));
+    }
+
+    public void testSimplifyIif_ConditionFalse_FoldableResult() {
         SimplifyCase rule = new SimplifyCase();
         Iif iif = new Iif(EMPTY, new Equals(EMPTY, ONE, TWO), Literal.of(EMPTY, "foo"), Literal.of(EMPTY, "bar"));
+        assertTrue(iif.foldable());
+
         Expression e = rule.rule(iif);
         assertEquals(Iif.class, e.getClass());
         iif = (Iif) e;
@@ -762,6 +810,19 @@ public class OptimizerTests extends ESTestCase {
         assertEquals(TypeResolution.TYPE_RESOLVED, iif.typeResolved());
     }
 
+    public void testSimplifyIif_ConditionFalse_NonFoldableResult() {
+        SimplifyCase rule = new SimplifyCase();
+        Iif iif = new Iif(EMPTY, new Equals(EMPTY, ONE, TWO), Literal.of(EMPTY, "foo"), getFieldAttribute("myField"));
+        assertFalse(iif.foldable());
+
+        Expression e = rule.rule(iif);
+        assertEquals(Iif.class, e.getClass());
+        iif = (Iif) e;
+        assertEquals(0, iif.conditions().size());
+        assertFalse(iif.foldable());
+        assertEquals("myField", Expressions.name(iif.elseResult()));
+    }
+
     //
     // Logical simplifications
     //