add simplify subquery with dual exists rule

src/sql/rewrite/ob_transform_simplify_subquery.cpp

@@ -1270,6 +1270,13 @@ int ObTransformSimplifySubquery::eliminate_subquery(ObDMLStmt *stmt,
         }
         LOG_TRACE("finish to eliminate subquery", K(can_be_eliminated), K(ret));
 
+      } else if (OB_FAIL(empty_table_subquery_can_be_eliminated_in_exists(expr, can_be_eliminated))) {
+        LOG_WARN("failed to check empty table subquery can be eliminate", K(ret));
+      } else if (can_be_eliminated) {
+        if (OB_FAIL(do_trans_empty_table_subquery_as_expr(expr, trans_happened))) {
+          LOG_WARN("failed to do trans empty table subquery as expr", K(ret));
+        }
+        LOG_TRACE("finish to eliminate empty table subquery", K(ret));
       } else if (!can_be_eliminated) {
         if (OB_FAIL(simplify_select_items(stmt, expr->get_expr_type(), subquery, false, trans_happened))) {
           LOG_WARN("Simplify select items in EXISTS fails", K(ret));
@@ -2224,6 +2231,8 @@ int ObTransformSimplifySubquery::check_stmt_can_trans_as_exists(ObSelectStmt *st
   } else if (stmt->has_group_by() ||
              stmt->has_having()) {
     // do nothing
+  } else if (0 == stmt->get_from_item_size()) {
+    is_valid = true;
   } else {
     ObArenaAllocator alloc;
     EqualSets &equal_sets = ctx_->equal_sets_;
@@ -2425,4 +2434,111 @@ int ObTransformSimplifySubquery::try_trans_any_all_as_exists(
     }
   }
   return ret;
-}
+}
+
+int ObTransformSimplifySubquery::empty_table_subquery_can_be_eliminated_in_exists(ObRawExpr *expr,
+                                                                                  bool &is_valid)
+{
+  int ret = OB_SUCCESS;
+  ObQueryRefRawExpr* query_ref = NULL;
+  ObSelectStmt* ref_stmt = NULL;
+  bool contain_rownum = false;
+  bool has_limit = false;
+  is_valid = false;
+  if (OB_ISNULL(expr)) {
+    ret = OB_ERR_UNEXPECTED;
+    LOG_WARN("get unexpected null", K(ret));
+  } else if (T_OP_EXISTS != expr->get_expr_type() &&
+             T_OP_NOT_EXISTS != expr->get_expr_type()) {
+    is_valid = false;
+  } else if (OB_UNLIKELY(expr->get_param_count() != 1) ||
+             OB_ISNULL(expr->get_param_expr(0)) ||
+             OB_UNLIKELY(!expr->get_param_expr(0)->is_query_ref_expr())) {
+    ret = OB_ERR_UNEXPECTED;
+    LOG_WARN("get unexpected exists/not exists expr", K(ret));
+  } else if (OB_FALSE_IT(query_ref = static_cast<ObQueryRefRawExpr*>(expr->get_param_expr(0)))) {
+  } else if (OB_ISNULL(ref_stmt = query_ref->get_ref_stmt())) {
+    ret = OB_ERR_UNEXPECTED;
+    LOG_WARN("get unexpected ref stmt", K(ret));
+  } else if (OB_FAIL(ref_stmt->has_rownum(contain_rownum))) {
+    LOG_WARN("failed to check ref stmt has rownum", K(ret));
+  } else if (OB_FAIL(check_limit(expr->get_expr_type(), ref_stmt, has_limit))) {
+    LOG_WARN("failed to check limit", K(ret));
+  } else if (contain_rownum || has_limit) {
+    is_valid = false;
+  } else if (query_ref->get_ref_count() > 1) {
+    is_valid = false;
+  } else if (0 != ref_stmt->get_from_item_size() ||
+             0 == ref_stmt->get_condition_size() ||
+             ref_stmt->is_set_stmt() ||
+             ref_stmt->is_contains_assignment() ||
+             ref_stmt->has_group_by() ||
+             ref_stmt->has_having() ||
+             ref_stmt->is_hierarchical_query() ||
+             ref_stmt->has_sequence() ||
+             ref_stmt->has_window_function()) {
+    is_valid = false;
+  } else {
+    is_valid = true;
+  }
+  return ret;
+}
+
+int ObTransformSimplifySubquery::do_trans_empty_table_subquery_as_expr(ObRawExpr *&expr,
+                                                                       bool &trans_happened)
+{
+  int ret = OB_SUCCESS;
+  ObQueryRefRawExpr* query_ref = NULL;
+  ObSelectStmt* ref_stmt = NULL;
+  bool add_limit_constraint = false;
+  ObSEArray<ObRawExpr*, 4> conditions;
+  ObRawExpr *out_expr = NULL;
+  trans_happened = false;
+  if (OB_ISNULL(expr) ||
+      OB_ISNULL(ctx_) ||
+      OB_ISNULL(ctx_->expr_factory_)) {
+    ret = OB_ERR_UNEXPECTED;
+    LOG_WARN("get unexpected null", K(ret));
+  } else if (OB_UNLIKELY(expr->get_param_count() != 1) ||
+             OB_ISNULL(expr->get_param_expr(0)) ||
+             OB_UNLIKELY(!expr->get_param_expr(0)->is_query_ref_expr())) {
+    ret = OB_ERR_UNEXPECTED;
+    LOG_WARN("get unexpected exists/not exists expr", K(ret));
+  } else if (OB_FALSE_IT(query_ref = static_cast<ObQueryRefRawExpr*>(expr->get_param_expr(0)))) {
+  } else if (OB_ISNULL(ref_stmt = query_ref->get_ref_stmt())) {
+    ret = OB_ERR_UNEXPECTED;
+    LOG_WARN("get unexpected ref stmt", K(ret));
+  } else if (OB_FAIL(need_add_limit_constraint(expr->get_expr_type(), ref_stmt, add_limit_constraint))){
+    LOG_WARN("failed to check limit constraints", K(ret));
+  } else if (add_limit_constraint &&
+            OB_FAIL(ObTransformUtils::add_const_param_constraints(ref_stmt->get_limit_expr(), ctx_))) {
+    LOG_WARN("failed to add const param constraints", K(ret));
+  } else if (OB_FAIL(conditions.assign(ref_stmt->get_condition_exprs()))) {
+    LOG_WARN("failed to assign condition exprs", K(ret));
+  } else if (ObTransformUtils::decorrelate(conditions, query_ref->get_exec_params())) {
+    LOG_WARN("failed to decorrelate condition exprs", K(ret));
+  } else if (OB_FAIL(ObRawExprUtils::build_and_expr(*ctx_->expr_factory_,
+                                                    conditions,
+                                                    out_expr))) {
+    LOG_WARN("failed to formalize expr", K(ret));
+  } else if (OB_ISNULL(out_expr)) {
+    ret = OB_ERR_UNEXPECTED;
+    LOG_WARN("get unexpected null", K(ret));
+  } else if (T_OP_NOT_EXISTS == expr->get_expr_type() &&
+             OB_FAIL(ObRawExprUtils::build_lnnvl_expr(*ctx_->expr_factory_,
+                                                      out_expr,
+                                                      out_expr))) {
+    LOG_WARN("failed to build lnnvl expr", K(ret));
+  } else if (OB_ISNULL(out_expr)) {
+    ret = OB_ERR_UNEXPECTED;
+    LOG_WARN("get unexpected null", K(ret));
+  } else if (OB_FAIL(out_expr->formalize(ctx_->session_info_))) {
+    LOG_WARN("failed to formalize expr", K(ret));
+  } else if (OB_FAIL(out_expr->pull_relation_id())) {
+    LOG_WARN("failed to pull relation id", K(ret));
+  } else {
+    expr = out_expr;
+    trans_happened = true;
+  }
+  return ret;
+}

src/sql/rewrite/ob_transform_simplify_subquery.h

@@ -217,6 +217,10 @@ private:
                                   ObNotNullContext *not_null_cxt,
                                   bool is_bool_expr,
                                   bool &trans_happened);
+  int empty_table_subquery_can_be_eliminated_in_exists(ObRawExpr *expr,
+                                                       bool &is_valid);
+  int do_trans_empty_table_subquery_as_expr(ObRawExpr *&expr,
+                                            bool &trans_happened);
 };
 
 }