[improvement](fe) Rewrite nullif to if expression

fe/fe-core/src/main/java/org/apache/doris/nereids/parser/LogicalPlanBuilder.java

@@ -3493,6 +3493,15 @@ public Expression visitFunctionCallExpression(DorisParser.FunctionCallExpression
     private Expression processUnboundFunction(ParserRuleContext ctx, String dbName, String functionName,
             boolean isDistinct, List<Expression> params,
             WindowSpecContext windowContext, IdentifierContext hintContext) {
+        if (dbName == null && "nullif".equalsIgnoreCase(functionName) && !isDistinct
+                && windowContext == null && hintContext == null && params.size() == 2) {
+            Expression first = params.get(0);
+            Expression second = params.get(1);
+            return new UnboundFunction("if", ImmutableList.of(
+                    new EqualTo(first, second),
+                    NullLiteral.INSTANCE,
+                    first));
+        }
         List<UnboundStar> unboundStars = ExpressionUtils.collectAll(params, UnboundStar.class::isInstance);
         if (!unboundStars.isEmpty()) {
             if (dbName != null

fe/fe-core/src/main/java/org/apache/doris/nereids/rules/expression/rules/SimplifyConditionalFunction.java

@@ -108,8 +108,9 @@ private static Expression rewriteNvl(ExpressionMatchingContext<Nvl> ctx) {
 
     /*
     * nullif(null, R) => Null
-    * nullif(L, null) => Null
+    * nullif(L, null) => nullable(L)
     * nullif(null, null) => Null
+    * nullif(L, R) => if(L = R, null, L)
      */
     private static Expression rewriteNullIf(ExpressionMatchingContext<NullIf> ctx) {
         NullIf nullIf = ctx.expr;
@@ -120,7 +121,7 @@ private static Expression rewriteNullIf(ExpressionMatchingContext<NullIf> ctx) {
                     nullIf, new Nullable(nullIf.child(0)), ctx.rewriteContext
             );
         } else {
-            return nullIf;
+            return NullIfToIf.rewrite(nullIf, ctx.rewriteContext);
         }
     }
 }

fe/fe-core/src/test/java/org/apache/doris/nereids/rules/expression/rules/NullIfToIfTest.java

@@ -0,0 +1,44 @@
+// 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.doris.nereids.rules.expression.rules;
+
+import org.apache.doris.nereids.rules.expression.ExpressionRewriteTestHelper;
+import org.apache.doris.nereids.rules.expression.ExpressionRuleExecutor;
+import org.apache.doris.nereids.trees.expressions.EqualTo;
+import org.apache.doris.nereids.trees.expressions.SlotReference;
+import org.apache.doris.nereids.trees.expressions.functions.scalar.If;
+import org.apache.doris.nereids.trees.expressions.functions.scalar.NullIf;
+import org.apache.doris.nereids.trees.expressions.literal.NullLiteral;
+import org.apache.doris.nereids.types.StringType;
+
+import com.google.common.collect.ImmutableList;
+import org.junit.jupiter.api.Test;
+
+class NullIfToIfTest extends ExpressionRewriteTestHelper {
+
+    @Test
+    void testRewriteConstructedNullIfToIf() {
+        executor = new ExpressionRuleExecutor(ImmutableList.of(NullIfToIf.INSTANCE));
+
+        SlotReference first = new SlotReference("a", StringType.INSTANCE, true);
+        SlotReference second = new SlotReference("b", StringType.INSTANCE, true);
+        assertRewrite(
+                new NullIf(first, second),
+                new If(new EqualTo(first, second), new NullLiteral(StringType.INSTANCE), first));
+    }
+}

fe/fe-core/src/test/java/org/apache/doris/nereids/rules/expression/rules/PushIntoCaseWhenBranchTest.java

@@ -84,9 +84,10 @@ void testPushIntoNullIf() {
         ));
         assertRewriteAfterTypeCoercion("cast(nullif(TA, TB) as bigint)", "if(TA = TB, NULL, cast(TA as bigint))");
         assertRewriteAfterTypeCoercion("cast(nullif(TA, 1) as bigint)", "if(TA = 1, null, cast(TA as bigint))");
-        assertRewriteAfterTypeCoercion("a > nullif(b, c)", "a > nullif(b, c)");
+        assertRewriteAfterTypeCoercion("a > nullif(b, c)", "if(b = c, null, a > b)");
         assertRewriteAfterTypeCoercion("2 > nullif(b, c)", "if(b = c, null, 2 > b)");
-        assertRewriteAfterTypeCoercion("2 > nullif(b + random(1, 10), c)", "2 > nullif(b + random(1, 10), c)");
+        assertRewriteAfterTypeCoercion("2 > nullif(b + random(1, 10), c)",
+                "if(b + random(1, 10) = c, null, 2 > b + random(1, 10))");
         assertRewriteAfterTypeCoercion("2 > nullif(b, c + random(1, 10))", "if(b = c + random(1, 10), null, 2 > b)");
     }
 }

fe/fe-core/src/test/java/org/apache/doris/nereids/rules/expression/rules/SimplifyConditionalFunctionTest.java

@@ -178,6 +178,9 @@ public void testNullIf() {
                 ),
                 new NullLiteral(DateTimeV2Type.of(6))
         );
+
+        // nullif(L, R) -> if(L = R, null, L)
+        assertRewriteAfterTypeCoercion("nullif(a, b)", "if(a = b, null, a)");
     }
 
 }

fe/fe-core/src/test/java/org/apache/doris/nereids/trees/expressions/ExpressionParserTest.java

@@ -17,14 +17,17 @@
 
 package org.apache.doris.nereids.trees.expressions;
 
+import org.apache.doris.nereids.analyzer.UnboundFunction;
 import org.apache.doris.nereids.analyzer.UnboundSlot;
 import org.apache.doris.nereids.exceptions.SyntaxParseException;
 import org.apache.doris.nereids.parser.NereidsParser;
 import org.apache.doris.nereids.parser.ParserTestBase;
+import org.apache.doris.nereids.trees.expressions.literal.NullLiteral;
 import org.apache.doris.nereids.trees.expressions.literal.StringLiteral;
 import org.apache.doris.qe.ConnectContext;
 import org.apache.doris.qe.SqlModeHelper;
 
+import com.google.common.collect.ImmutableList;
 import org.junit.jupiter.api.Test;
 
 public class ExpressionParserTest extends ParserTestBase {
@@ -92,6 +95,15 @@ public void testExprBetweenPredicate() {
                 );
     }
 
+    @Test
+    public void testNullIfRewriteInPlanBuilder() {
+        parseExpression("nullif(a, b)")
+                .assertEquals(new UnboundFunction("if", ImmutableList.of(
+                        new EqualTo(new UnboundSlot("a"), new UnboundSlot("b")),
+                        NullLiteral.INSTANCE,
+                        new UnboundSlot("a"))));
+    }
+
     @Test
     public void testInPredicate() {
         String in = "select * from test1 where d1 in (1, 2, 3)";

regression-test/data/nereids_rules_p0/join_extract_or_from_case_when/join_extract_or_from_case_when.out

@@ -275,7 +275,7 @@ PhysicalResultSink
 -- !nullif_one_side_1 --
 PhysicalResultSink
 --PhysicalProject[t1.a, t2.x]
-----NestedLoopJoin[INNER_JOIN](nullif(a, x) = (a + b))
+----NestedLoopJoin[INNER_JOIN](if((a = x), NULL, a) = (a + b))
 ------PhysicalProject[(a + b) AS `(a + b)`, t1.a]
 --------filter((t1.a = (t1.a + t1.b)))
 ----------PhysicalOlapScan[tbl_join_extract_or_from_case_when_1(t1)]
@@ -285,7 +285,7 @@ PhysicalResultSink
 -- !nullif_two_side_1 --
 PhysicalResultSink
 --PhysicalProject[t1.a, t2.x]
-----hashJoin[INNER_JOIN] hashCondition=((t1.a = expr_(x + y))) otherCondition=((nullif(a, x) = (t2.x + t2.y)))
+----hashJoin[INNER_JOIN] hashCondition=((t1.a = expr_(x + y))) otherCondition=((if((a = x), NULL, a) = (t2.x + t2.y)))
 ------PhysicalProject[t1.a]
 --------PhysicalOlapScan[tbl_join_extract_or_from_case_when_1(t1)]
 ------PhysicalProject[(x + y) AS `expr_(x + y)`, tbl_join_extract_or_from_case_when_2.x, tbl_join_extract_or_from_case_when_2.y]