Subject: Two Window aggregate node for logically same over clause

From: "\"Anitha S\"" <[email-masked]>
Date: Thu, 05 Oct 2023 16:35:08 +0530

Hi team,

      We have observed that for logically same over clause two different window aggregate nodes are created in plan.  
The below query contains two window functions. Both Over clause contain the same partition & order clause in it. But in one over clause ordering option is mentioned as ascending but not in another over clause which represents the default option "ascending". 

Sample Query:

 CREATE TEMPORARY TABLE empsalary (

 depname varchar,

 empno bigint,

 salary int,

 enroll_date date

 );

 INSERT INTO empsalary VALUES ('develop', 10, 5200, '2007-08-01'), ('sales', 1, 5000, '2006-10-01'),

 ('personnel', 5, 3500, '2007-12-10'),('sales', 4, 4800, '2007-08-08'),('personnel', 2, 3900, '2006-12-23'),

 ('develop', 7, 4200, '2008-01-01'),('develop', 9, 4500, '2008-01-01'),('sales', 3, 4800, '2007-08-01'),

 ('develop', 8, 6000, '2006-10-01'),('develop', 11, 5200, '2007-08-15');

 explain verbose select rank() over (partition by depname order by salary),  percent_rank() over(partition by depname order by salary asc) from empsalary;

                  QUERY PLAN

 ------------------------------------------------------------------------------------------

     WindowAgg  (cost=10000000074.54..10000000114.66 rows=1070 width=52)

     Output: (rank() OVER (?)), percent_rank() OVER (?), salary, depname

      ->  WindowAgg  (cost=10000000074.54..10000000095.94 rows=1070 width=44)

          Output: salary, depname, rank() OVER (?)

            ->  Sort  (cost=10000000074.54..10000000077.21 rows=1070 width=36)

                Output: salary, depname

                Sort Key: empsalary.depname, empsalary.salary

                   ->  Seq Scan on pg_temp_7.empsalary  (cost=0.00..20.70 rows=1070 width=36)

                       Output: salary, depname 

Ordering option of Sort is represented by enum SortByDir (parsenodes.h). 

List of SortBy is present in WindowDef structure which has info of order by clause in Over clause

 /* Sort ordering options for ORDER BY and CREATE INDEX */ 

 typedef enum SortByDir

 {

  SORTBY_DEFAULT,

  SORTBY_ASC,

  SORTBY_DESC,

  SORTBY_USING /* not allowed in CREATE INDEX ... */

 } SortByDir;  

 typedef struct SortBy

 {

  NodeTag type;

  Node *node; /* expression to sort on */

  SortByDir sortby_dir; /* ASC/DESC/USING/default */

  SortByNulls sortby_nulls; /* NULLS FIRST/LAST */

  List *useOp; /* name of op to use, if SORTBY_USING */

  int location; /* operator location, or -1 if none/unknown */

 } SortBy;  

In transformWindowFuncCall API, Equality check of order clause in window definition failed while comparing SortByDir enum of both over clause i.e SORT_DEFAULT  is not equal to SORT_ASC. Hence two window clause are created in parse tree resulting in the creation of two different window aggregate node.

This check can be modified to form a single window aggregate node for the above results in faster query execution. Is there any reason for creating two different window aggregate node?

Thanks 
Anitha S