What is the alternative for 'for loop' ?

kalyan vedagiri · Jul 7, 2021 8:14AM

i want to update the table by using a decrementor and row id as below.

my_line_count := 0;

select count(*) into my_line_count from emp where empno = in_empno

For line_rec in ( select rowid from emp where empno = in_empno order by countline desc)

loop

update emp set countline = my_line_count where ROWID = line_rec.ROWID;

my_line_count = my_line_count -1;

end loop;

This procedure will be called for suppose 314 times, by another procedure( based on count in emp table i:e for each row);

Before data is like

select countline from emp where empno = in_empno ;

countline

1

-- so on

Expected result

select countline from emp where empno = in_empno ; (For exapmle there are 314 rows)

countline

314

313

312

311

.

3

2

1

Is there any way to perform above logic without loop?

BluShadow · Jul 7, 2021 8:34AM

Please provide some example data and the expected results of the logic you want so we can understand better what you are trying to do.

At first sight it looks like you want to just do a MERGE statement to update data based on some determined values, and that would be better than a query followed by a row by row loop, as it would keep everything as a single transaction and avoid conflict with other sessions that may (in your case) update the data inbetween you querying and updating each row.

Without data though, it's hard to determine exactly what you're trying to do.

kalyan vedagiri · Jul 7, 2021 8:40AM

https://community.oracle.com/tech/developers/discussion/comment/16801338#Comment_16801338

I have added more details now to the question. Here we are updating values using a decrementor. hence we are using loop. So if we have more rows like in millions, the iterations will be more , hence i want to avoid this loop for performance issues. But to update row by row without loop , is this possible?

BluShadow · Jul 7, 2021 8:45AM

I think you are trying to re-number the "countline" values for an employee in sequential order. Something like this perhaps...

SQL> create table myemp as
  2    select 1 as empno, 2 as countline from dual union all
  3    select 1, 4 from dual union all
  4    select 1, 7 from dual union all
  5    select 1, 9 from dual union all
  6    select 2, 1 from dual union all
  7    select 2, 2 from dual union all
  8    select 2, 8 from dual
  9  /

Table created.

SQL> -- counting the records for an emp and then assigning the line count-1
SQL> -- in descending order from the last countline is the same as
SQL> -- just re-numbering all the countlines in ascending order from 1 upwards
SQL> merge into myemp t
  2  using (select rowid as rid
  3               ,empno
  4               ,row_number() over (partition by empno order by countline) as new_countline
  5         from   myemp
  6        ) s
  7  on (t.rowid = s.rid)
  8  when matched then
  9    update set countline = s.new_countline
 10  /

7 rows merged.

SQL> select *
  2  from   myemp
  3  /

     EMPNO  COUNTLINE
---------- ----------
         1          1
         1          2
         1          3
         1          4
         2          1
         2          2
         2          3

7 rows selected.

In this case, I've re-numbered for all the employee records, but you could limit that to a known employee number if you want.

kalyan vedagiri · Jul 7, 2021 10:24AM

https://community.oracle.com/tech/developers/discussion/comment/16801342#Comment_16801342

Thank you , i will try with this logic and see if i get the expected result.

kalyan vedagiri · Jul 8, 2021 5:37AM

https://community.oracle.com/tech/developers/discussion/comment/16801342#Comment_16801342

I have tried this with my data set but it took total 102 minutes to process. Previously when 'for loop' was used it took 72 minutes only. Looks like merge statement is taking more time to process than for loop. Is there any other way to improve performance?

BluShadow · Jul 8, 2021 7:59AM

Processing data in a single SQL statement, with MERGE should typically be a lot faster than row-by-row updates using multiple SQL Updates that switch between the PL engine and the SQL engine.

You haven't shown what you've tried and we don't have your actual data.

Are you doing the merge across all the data in one go, or are you calling it for each of the employee numbers one after the other?

Are there other people using the data? i.e. sessions that could be causing (b)locks on it?

How much data is there? What indexes does it have?

As a simple comparison, a difference in timing between using a merge and row-by-row updates...

SQL> drop table myemp purge
  2  /

Table dropped.

SQL> create table myemp as
  2  --  select 1 as empno, 2 as countline from dual union all
  3  --  select 1, 4 from dual union all
  4  --  select 1, 7 from dual union all
  5  --  select 1, 9 from dual union all
  6  --  select 2, 1 from dual union all
  7  --  select 2, 2 from dual union all
  8  --  select 2, 8 from dual
  9  -- generate larger set of data for timing comparison
 10  --   1,000 employees with 25 lines each = 25,000 rows
 11    with empno as (select level as empno from dual connect by level <= 1000) -- 10,000 employees
 12        ,lns as (select 1 as countline from dual connect by level <= 25) -- 250 lines per employee
 13    select empno
 14          ,countline
 15    from   empno
 16           cross join lns
 17  /

Table created.

SQL> create index imyemp on myemp(empno, countline)
  2  /

Index created.

SQL> -- counting the records for an emp and then assigning the line count-1
SQL> -- in descending order from the last countline is the same as
SQL> -- just re-numbering all the countlines in ascending order from 1 upwards
SQL> set timing on;
SQL> merge into myemp t
  2  using (select rowid as rid
  3               ,empno
  4               ,row_number() over (partition by empno order by countline) as new_countline
  5         from   myemp
  6        ) s
  7  on (t.rowid = s.rid)
  8  when matched then
  9    update set countline = s.new_countline
 10  /

25000 rows merged.

Elapsed: 00:00:03.47
SQL> set timing off;
SQL> select *
  2  from   myemp
  3  where rownum <= 20 -- just sample
  4  /

     EMPNO  COUNTLINE
---------- ----------
         1          1
         1          2
         1          3
         1          4
         1          5
         1          6
         1          7
         1          8
         1          9
         1         10
         1         11
         1         12
         1         13
         1         14
         1         15
         1         16
         1         17
         1         18
         1         19
         1         20

20 rows selected.

SQL> commit;

Commit complete.

SQL> drop table myemp purge
  2  /

Table dropped.

SQL> create table myemp as
  2    with empno as (select level as empno from dual connect by level <= 1000) -- 10,000 employees
  3        ,lns as (select 1 as countline from dual connect by level <= 25) -- 250 lines per employee
  4    select empno
  5          ,countline
  6    from   empno
  7           cross join lns
  8  /

Table created.

SQL> create index imyemp on myemp(empno, countline)
  2  /

Index created.

SQL> set timing on;
SQL> declare
  2    en number := -1;
  3    c number;
  4    cursor cemp is
  5      select *
  6      from   myemp
  7      order by empno, countline
  8      for update;
  9  begin
 10    for e in cemp
 11    loop
 12      if en != e.empno then
 13        en := e.empno;
 14        c := 1;
 15      else
 16        c := c+1;
 17      end if;
 18      update myemp
 19      set    countline = c
 20      where current of cemp;
 21    end loop;
 22  end;
 23  /

PL/SQL procedure successfully completed.

Elapsed: 00:00:08.93
SQL> set timing off;
SQL> select *
  2  from myemp
  3  where rownum <= 20
  4  /

     EMPNO  COUNTLINE
---------- ----------
         1          1
         1          2
         1          3
         1          4
         1          5
         1          6
         1          7
         1          8
         1          9
         1         10
         1         11
         1         12
         1         13
         1         14
         1         15
         1         16
         1         17
         1         18
         1         19
         1         20

20 rows selected.

SQL> commit;

Commit complete.

So the row by row processing took more than twice as long as the merge (around 8.9 seconds compared to the merge's 3.4 seconds); and my row-by-row processing is probably more efficient than your version as I'm not doing a count of records for each employee, I'm just setting a counter to 1 at the start of each employee number and incrementing that to update the current row of the cursor loop.

This is why you need to show us exactly what you're doing, especially when you make claims that a single MERGE statement is slower than multiple update statements in a loop.

Kalaiarasu.Pandian · Jul 9, 2021 1:34PM

SQL> select a from (select level a from dual connect by level <= &emp_no) order by a desc;

Enter value for emp_no: 12

old 1: select a from (select level a from dual connect by level <= &emp_no) order by a desc

new 1: select a from (select level a from dual connect by level <= 12) order by a desc

A

----------

12

11

10

9

8

7

6

5

4

3

2

1

12 rows selected.

BluShadow · Jul 9, 2021 3:08PM

https://community.oracle.com/tech/developers/discussion/comment/16801659#Comment_16801659

What on Earth is that supposed to achieve? That is nothing like the requirement. It just generates as many rows as the employee id number which is completely unrelated to the rows the employee has on the table.

😲

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