!1672 exists子查询中存在limit const时可简化忽略

Merge pull request !1672 from 胡正超/subexists

src/gausskernel/optimizer/plan/subselect.cpp

@@ -100,7 +100,7 @@ static Node* convert_testexpr_mutator(Node* node, convert_testexpr_context* cont
 static bool subplan_is_hashable(Plan* plan);
 static bool testexpr_is_hashable(Node* testexpr);
 static bool hash_ok_operator(OpExpr* expr);
-static bool simplify_EXISTS_query(Query* query);
+static bool simplify_EXISTS_query(PlannerInfo* root, Query* query);
 static Query* convert_EXISTS_to_ANY(PlannerInfo* root, Query* subselect, Node** testexpr, List** paramIds);
 static Node* replace_correlation_vars_mutator(Node* node, PlannerInfo* root);
 static Node* process_sublinks_mutator(Node* node, process_sublinks_context* context);
@@ -604,7 +604,7 @@ static Node* make_subplan(
      * If it's an EXISTS subplan, we might be able to simplify it.
      */
     if (subLinkType == EXISTS_SUBLINK)
-        simple_exists = simplify_EXISTS_query(subquery);
+        simple_exists = simplify_EXISTS_query(root, subquery);
 
     /*
      * For an EXISTS subplan, tell lower-level planner to expect that only the
@@ -687,7 +687,7 @@ static Node* make_subplan(
         /* Make a second copy of the original subquery */
         subquery = (Query*)copyObject(orig_subquery);
         /* and re-simplify */
-        simple_exists = simplify_EXISTS_query(subquery);
+        simple_exists = simplify_EXISTS_query(root, subquery);
         if (!simple_exists)
             ereport(ERROR,
                 (errmodule(MOD_OPT),
@@ -2156,7 +2156,7 @@ JoinExpr* convert_EXISTS_sublink_to_join(PlannerInfo* root, SubLink* sublink, bo
      * targetlist, we have to fail, because the pullup operation leaves us
      * with noplace to evaluate the targetlist.
      */
-    if (!simplify_EXISTS_query(subselect))
+    if (!simplify_EXISTS_query(root, subselect))
         return NULL;
 
     /*
@@ -2288,7 +2288,7 @@ JoinExpr* convert_EXISTS_sublink_to_join(PlannerInfo* root, SubLink* sublink, bo
  *
  * Returns TRUE if was able to discard the targetlist, else FALSE.
  */
-static bool simplify_EXISTS_query(Query* query)
+static bool simplify_EXISTS_query(PlannerInfo *root, Query* query)
 {
     /*
      * We don't try to simplify at all if the query uses set operations,
@@ -2300,9 +2300,42 @@ static bool simplify_EXISTS_query(Query* query)
      */
     if (query->commandType != CMD_SELECT || query->setOperations || query->hasAggs || query->groupingSets ||
         query->hasWindowFuncs || query->hasModifyingCTE || query->havingQual || query->limitOffset ||
-        query->limitCount || query->rowMarks)
+        query->rowMarks)
         return false;
 
+    /*
+     * LIMIT with a constant positive (or NULL) value doesn't affect the
+     * semantics of EXISTS, so let's ignore such clauses.  This is worth doing
+     * because people accustomed to certain other DBMSes may be in the habit
+     * of writing EXISTS(SELECT ... LIMIT 1) as an optimization.  If there's a
+     * LIMIT with anything else as argument, though, we can't simplify.
+     */
+    if (query->limitCount) {
+        /*
+         * The LIMIT clause has not yet been through eval_const_expressions,
+         * so we have to apply that here.  It might seem like this is a waste
+         * of cycles, since the only case plausibly worth worrying about is
+         * "LIMIT 1" ... but what we'll actually see is "LIMIT int8(1::int4)",
+         * so we have to fold constants or we're not going to recognize it.
+         */
+        Node*  node = eval_const_expressions(root, query->limitCount);
+        Const* limit = NULL;
+
+        /* Might as well update the query if we simplified the clause. */
+        query->limitCount = node;
+
+        if (!IsA(node, Const))
+            return false;
+
+        limit = (Const*)node;
+        Assert(limit->consttype == INT8OID);
+        if (!limit->constisnull && DatumGetInt64(limit->constvalue) <= 0)
+            return false;
+
+        /* Whether or not the targetlist is safe, we can drop the LIMIT. */
+        query->limitCount = NULL;
+    }
+
     /*
      * Mustn't throw away the targetlist if it contains set-returning
      * functions; those could affect whether zero rows are returned!

src/test/regress/expected/subselect_part2.out

@@ -431,6 +431,48 @@ WHERE 1=0);
         1
 (6 rows)
 
+--
+-- Check EXISTS simplification with LIMIT
+--
+explain (costs off)
+select * from int4_tbl o where exists
+  (select 1 from int4_tbl i where i.f1=o.f1 limit null);
+                QUERY PLAN                
+------------------------------------------
+ Hash Join
+   Hash Cond: (o.f1 = i.f1)
+   ->  Seq Scan on int4_tbl o
+   ->  Hash
+         ->  HashAggregate
+               Group By Key: i.f1
+               ->  Seq Scan on int4_tbl i
+(7 rows)
+
+explain (costs off)
+select * from int4_tbl o where not exists
+  (select 1 from int4_tbl i where i.f1=o.f1 limit 1);
+             QUERY PLAN             
+------------------------------------
+ Hash Anti Join
+   Hash Cond: (o.f1 = i.f1)
+   ->  Seq Scan on int4_tbl o
+   ->  Hash
+         ->  Seq Scan on int4_tbl i
+(5 rows)
+
+explain (costs off)
+select * from int4_tbl o where exists
+  (select 1 from int4_tbl i where i.f1=o.f1 limit 0);
+              QUERY PLAN              
+--------------------------------------
+ Seq Scan on int4_tbl o
+   Filter: (SubPlan 1)
+   SubPlan 1
+     ->  Limit
+           ->  Seq Scan on int4_tbl i
+                 Filter: (f1 = o.f1)
+(6 rows)
+
 drop table if exists location_type cascade;
 drop table if exists item_inventory_plan cascade;
 NOTICE:  drop cascades to view usview03

src/test/regress/sql/subselect_part2.sql

@@ -338,6 +338,19 @@ SELECT 'z'
 FROM location_type
 WHERE 1=0);
 
+--
+-- Check EXISTS simplification with LIMIT
+--
+explain (costs off)
+select * from int4_tbl o where exists
+  (select 1 from int4_tbl i where i.f1=o.f1 limit null);
+explain (costs off)
+select * from int4_tbl o where not exists
+  (select 1 from int4_tbl i where i.f1=o.f1 limit 1);
+explain (costs off)
+select * from int4_tbl o where exists
+  (select 1 from int4_tbl i where i.f1=o.f1 limit 0);
+
 
 drop table if exists location_type cascade;
 drop table if exists item_inventory_plan cascade;