Add support for NullIfExpr in eval_const_expressions

Author: Hou Zhijie <houzj.fnst@cn.fujitsu.com>
Discussion: https://www.postgresql.org/message-id/flat/7ea5ce773bbc4eea9ff1a381acd3b102@G08CNEXMBPEKD05.g08.fujitsu.local

src/backend/optimizer/util/clauses.c

@@ -2497,6 +2497,36 @@ eval_const_expressions_mutator(Node *node,
 				newexpr->location = expr->location;
 				return (Node *) newexpr;
 			}
+		case T_NullIfExpr:
+			{
+				NullIfExpr	   *expr;
+				ListCell	   *arg;
+				bool			has_nonconst_input = false;
+
+				/* Copy the node and const-simplify its arguments */
+				expr = (NullIfExpr *) ece_generic_processing(node);
+
+				/* If either argument is NULL they can't be equal */
+				foreach(arg, expr->args)
+				{
+					if (!IsA(lfirst(arg), Const))
+						has_nonconst_input = true;
+					else if (((Const *) lfirst(arg))->constisnull)
+						return (Node *) linitial(expr->args);
+				}
+
+				/*
+				 * Need to get OID of underlying function before checking if
+				 * the function is OK to evaluate.
+				 */
+				set_opfuncid((OpExpr *) expr);
+
+				if (!has_nonconst_input &&
+					ece_function_is_safe(expr->opfuncid, context))
+					return ece_evaluate_expr(expr);
+
+				return (Node *) expr;
+			}
 		case T_ScalarArrayOpExpr:
 			{
 				ScalarArrayOpExpr *saop;

src/test/regress/expected/case.out

@@ -263,6 +263,31 @@ SELECT *
  4 |   | 2 | -4
 (2 rows)
 
+-- Tests for constant subexpression simplification
+explain (costs off)
+SELECT * FROM CASE_TBL WHERE NULLIF(1, 2) = 2;
+        QUERY PLAN        
+--------------------------
+ Result
+   One-Time Filter: false
+(2 rows)
+
+explain (costs off)
+SELECT * FROM CASE_TBL WHERE NULLIF(1, 1) IS NOT NULL;
+        QUERY PLAN        
+--------------------------
+ Result
+   One-Time Filter: false
+(2 rows)
+
+explain (costs off)
+SELECT * FROM CASE_TBL WHERE NULLIF(1, null) = 2;
+        QUERY PLAN        
+--------------------------
+ Result
+   One-Time Filter: false
+(2 rows)
+
 --
 -- Examples of updates involving tables
 --

src/test/regress/sql/case.sql

@@ -137,6 +137,17 @@ SELECT *
   FROM CASE_TBL a, CASE2_TBL b
   WHERE COALESCE(f,b.i) = 2;
 
+-- Tests for constant subexpression simplification
+
+explain (costs off)
+SELECT * FROM CASE_TBL WHERE NULLIF(1, 2) = 2;
+
+explain (costs off)
+SELECT * FROM CASE_TBL WHERE NULLIF(1, 1) IS NOT NULL;
+
+explain (costs off)
+SELECT * FROM CASE_TBL WHERE NULLIF(1, null) = 2;
+
 --
 -- Examples of updates involving tables
 --