adjust locate expr behaviors when null is used as the third param

src/share/ob_compatibility_control_feature_def.h

@@ -33,4 +33,9 @@ DEF_COMPAT_CONTROL_FEATURE(VAR_NAME_LENGTH,
     "MySQL will limit the length of user-defined variable names to within 64 characters",
     MOCK_CLUSTER_VERSION_4_2_3_0, CLUSTER_VERSION_4_3_0_0,
     CLUSTER_VERSION_4_3_2_0)
+
+DEF_COMPAT_CONTROL_FEATURE(FUNC_LOCATE_NULL,
+    "The result of REPLACE('x', 'abc', null) is different in MySQL 5.7 and 8.0",
+    MOCK_CLUSTER_VERSION_4_2_5_0, CLUSTER_VERSION_4_3_0_0,
+    CLUSTER_VERSION_4_3_3_0)
 #endif

src/sql/engine/expr/ob_expr_locate.cpp

@@ -61,5 +61,12 @@ int ObExprLocate::calc_result_typeN(ObExprResType &type,
   return ret;
 }
 
+DEF_SET_LOCAL_SESSION_VARS(ObExprLocate, raw_expr) {
+  int ret = OB_SUCCESS;
+  SET_LOCAL_SYSVAR_CAPACITY(1);
+  EXPR_ADD_LOCAL_SYSVAR(share::SYS_VAR_OB_COMPATIBILITY_VERSION);
+  return ret;
+}
+
 } //namespace sql
 } //namespace oceanbase

src/sql/engine/expr/ob_expr_locate.h

@@ -27,6 +27,9 @@ public:
                                 ObExprResType *types_array,
                                 int64_t param_num,
                                 common::ObExprTypeCtx &type_ctx) const;
+
+  DECLARE_SET_LOCAL_SESSION_VARS;
+
 private:
   static const int8_t PARAM_NUM_TWO = 2;
   static const int8_t PARAM_NUM_THREE = 3;

src/sql/engine/expr/ob_expr_operator.cpp

@@ -5912,13 +5912,36 @@ int ObLocationExprOperator::calc_(const ObExpr &expr, const ObExpr &sub_arg,
   if (OB_SUCC(ret) && !has_result && 3 == expr.arg_cnt_) {
     if (OB_FAIL(expr.args_[2]->eval(ctx, pos))) {
       LOG_WARN("eval arg 2 failed", K(ret));
-    } else if (pos->is_null()) {
-      res_datum.set_int(0);
-      has_result = true;
-    } else {
+    } else if (!pos->is_null()) {
       // TODO: 验证MySQL下uint64超过int64值域范围，隐式cast的结果
       //
       pos_int = pos->get_int();
+    } else if (lib::is_oracle_mode()) {
+      res_datum.set_int(0);
+      has_result = true;
+    } else {
+      has_result = true;
+      ObSolidifiedVarsGetter helper(expr, ctx, ctx.exec_ctx_.get_my_session());
+      const ObSQLSessionInfo *session = ctx.exec_ctx_.get_my_session();
+      uint64_t compat_version = 0;
+      ObCompatType compat_type = COMPAT_MYSQL57;
+      bool is_enable = false;
+      if (OB_ISNULL(session)) {
+        ret = OB_ERR_UNEXPECTED;
+        LOG_WARN("session info is null", K(ret));
+      } else if (OB_FAIL(helper.get_compat_version(compat_version))) {
+        LOG_WARN("failed to get compat version", K(ret));
+      } else if (OB_FAIL(ObCompatControl::check_feature_enable(compat_version,
+                                                               ObCompatFeatureType::FUNC_LOCATE_NULL,
+                                                               is_enable))) {
+        LOG_WARN("failed to check feature enable", K(ret));
+      } else if (OB_FAIL(session->get_compatibility_control(compat_type))) {
+        LOG_WARN("failed to get compat type", K(ret));
+      } else if (is_enable && COMPAT_MYSQL8 == compat_type) {
+        res_datum.set_null();
+      } else {
+        res_datum.set_int(0);
+      }
     }
   }