I assumed that it would be done in parallel. Agreed that there
was no sense doing it serially.
Liz
From: [External] ahmed.fikri at t-online.de <ahmed.fikri@xxxxxxxxxxx>
Sent: Monday, October 5, 2020 4:35 PM
To: gogala.mladen@xxxxxxxxx; Reen, Elizabeth [ICG-IT]; Ashoke Mandal
Cc: andysayer@xxxxxxxxx; list, oracle
Subject: AW: Fastest way to count exact number of rows in a very large table
If you are just interested in getting the exact number of rows in a large
partitioned table, then the fasted way (in my opinion) is to use PL/SQL.
Looping the partition serially will not help (this is more slower than select
count(*) from huge_table).You have to create a pipelined function and use the
parallel_enable feature.
Pseudo code( give only the Idea, if you adjust it you can reduce the time
significantly):
create or replace package pkg_test as
type xxx IS RECORD(column_value VARCHAR2(200));
type t_parallel_test_ref_cursor IS REF CURSOR RETURN xxx;
function fu_count(p_cursor t_parallel_test_ref_cursor);
end;
/
create or replace package body pkg_test as
function fu_count_part(p_part_nameVARCHAR2, p_dop NUMBER) RETURN NUMBER IS
v_count number := 0;
BEGIN
-- you can try here to use bind variable to avoid hard parsing (just make
sure that the partition pruning works
EXECUTE IMMEDIATE 'select /*+ look how to use parallel hint */ count(*) from
huge_table partition('||p_part_name||')' into v_count;
RETURN v_count;
EXCEPTION
when others then
return 0;
END fu_count_part;
function fu_count(p_cursor t_parallel_test_ref_cursor) RETURN t2_list PIPELINED
PARALLEL_ENABLE(PARTITION p_cursor BY HASH(column_value)) IS
ret t2;
x VARCHAR2(33);
BEGIN
LOOP
FETCH p_cursor INTO x;
exit when p_cursor%NOTFOUND;
ret.id<https://urldefense.com/v3/__http:/ret.id__;!!Jkho33Y!w1JYGGiqBHQ42f2zBi1qLYakaWuRcTJMLEK2e97Hiw8tye54siYMHHQONRltggJkdQ$>
:= fu_count_part(x);
PIPE ROW(ret);
END LOOP;
END fu_count;
end pkg_test
And then this will return the total count:
SELECT count(*) FROM pkg_test.fu_count(CURSOR(SELECT /*+ parallel(t 16) */
partition_name COLUMN_VALUE FROM table_hold_all_partitions_name t),16);
If you really want to cut the query time to less than 10 minutes, just try
getting the code above working.
Best regards
Ahmed
-----Original-Nachricht-----
Betreff: Re: Fastest way to count exact number of rows in a very large table
Datum: 2020-10-05T21:58:55+0200
Von: "Mladen Gogala" <gogala.mladen@xxxxxxxxx<mailto:gogala.mladen@xxxxxxxxx>>
An: "elizabeth.reen@xxxxxxxx<mailto:elizabeth.reen@xxxxxxxx>"
<elizabeth.reen@xxxxxxxx<mailto:elizabeth.reen@xxxxxxxx>>, "Ashoke Mandal"
<ramukam1983@xxxxxxxxx<mailto:ramukam1983@xxxxxxxxx>>
You can also try the following:
SELECT /*+ PARALLEL(16) */ 100000*count(*) FROM TABLE SAMPLE(0.001);
That would give almost correct count of rows in the table. Accuracy would be
similar to SELECT NUM_ROWS from USER_TABLES WHERE TABLE_NAME=<TABLE_NAME>;
BTW, has anyone played with APPROX_FOR_COUNT_DISTINCT parameter in 19c?
Regards
On Mon, 2020-10-05 at 19:40 +0000, Reen, Elizabeth wrote:
Since it is partitioned, why don’t you just count each partition
separately? Have you run stats on the old partitions? Assuming that prior
years do not change, you should be able to get a count there. If you do an
import, then you can get the number of rows from the log. A transportable
tablespace, will not mount if there is something wrong.
Liz
Liz Reen
CPB Database Administration
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From:
[gmail.com<https://urldefense.com/v3/__http:/gmail.com__;!!Jkho33Y!w1JYGGiqBHQ42f2zBi1qLYakaWuRcTJMLEK2e97Hiw8tye54siYMHHQONRlaOO1SKg$>]
Ashoke Mandal <ramukam1983@xxxxxxxxx<mailto:ramukam1983@xxxxxxxxx>>
Sent: Monday, October 5, 2020 3:14 PM
To: Reen, Elizabeth [ICG-IT]
Cc: andysayer@xxxxxxxxx<mailto:andysayer@xxxxxxxxx>;
ahmed.fikri@xxxxxxxxxxx<mailto:ahmed.fikri@xxxxxxxxxxx>; list, oracle
Subject: Re: Fastest way to count exact number of rows in a very large table
Hello Ahmed/Andy/Jackson/Mark/Gogala/Liz, Thanks for your response to my
posting. Even though my question was how to improve the query time of a query
to check row count in a big table, you have also brought up many good points
related to cross platform migration.
Here is some information regarding my table:
We have used transportable tablespace for data migration.
The primary key consists of three columns (UT_ID, UT_SEQ, TEST_DATE).
This table is partitioned by date and has one partition for every month. So, 12
partitions for every calendar year and has data for the last 20 years.
After adding a parallel hint as described below the query time went down from
2.2 hours to 42 min. Let me know if you have any more recommendations to
improve the query time for this select statement.
select /*+ parallel */ to_char(count(*), '999,999,999,999') from test_data;
Ashoke
On Mon, Oct 5, 2020 at 10:40 AM Reen, Elizabeth
<elizabeth.reen@xxxxxxxx<mailto:elizabeth.reen@xxxxxxxx>> wrote:
We just completed such a transition. We kept the Oracle version
the same so we could see the impact of Linux. Transportable tablespaces was
how we did it. We were able to move a 17 terabyte database in under 10 hours.
Liz
From: [External]
oracle-l-bounce@xxxxxxxxxxxxx<mailto:oracle-l-bounce@xxxxxxxxxxxxx>
<oracle-l-bounce@xxxxxxxxxxxxx<mailto:oracle-l-bounce@xxxxxxxxxxxxx>> On Behalf
Of [External] Andy Sayer
Sent: Friday, October 2, 2020 3:09 PM
To: ahmed.fikri@xxxxxxxxxxx<mailto:ahmed.fikri@xxxxxxxxxxx>
Cc: list, oracle; ramukam1983@xxxxxxxxx<mailto:ramukam1983@xxxxxxxxx>
Subject: Re: Fastest way to count exact number of rows in a very large table
Just because a table has the same number of rows, it doesn’t mean it has the
same data. With 108 billion rows, your data is going to be changing quickly, in
order to get accurate counts at the right point in time you’re going to end up
keeping your application offline for a window before and after your migration.
What you need to do is determine where you expect data to go missing and work
out a way to check.
This will depend on how you’re doing your migration, I would suggest you use
Cross-Platform Transportable Tablespaces (Doc Id 371556.1) as that would allow
you to do a physical import and just convert the files to the right endianness.
This starts by making sure all data has been written to your data files (so
they can be read only on the source system). As you’re working with the
physical data files rather than the logical data (rows in tables), the only way
you’re going to loose rows is by corrupting your files. You can check for
corruption using RMAN once you’ve imported the converted files. No need to
count all your rows, and no need to hope that that’s all you need to compare.
Hope that helps,
Andy
On Fri, 2 Oct 2020 at 19:38,
ahmed.fikri@xxxxxxxxxxx<mailto:ahmed.fikri@xxxxxxxxxxx>
<ahmed.fikri@xxxxxxxxxxx<mailto:ahmed.fikri@xxxxxxxxxxx>> wrote:
Hi Ashoke,
could you send the execute plan of the query too? I think there is no general
approach for that, it depends on several factors: whether the table has indexes
(normal/bitmap) and in case the table has indexes the size of the table
compared to the existing index...... But generally parallel processing should
help.
Best regards
Ahmed
-----Original-Nachricht-----
Betreff: Fastest way to count exact number of rows in a very large table
Datum: 2020-10-02T19:45:19+0200
Von: "Ashoke Mandal" <ramukam1983@xxxxxxxxx<mailto:ramukam1983@xxxxxxxxx>>
An: "ORACLE-L" <oracle-l@xxxxxxxxxxxxx<mailto:oracle-l@xxxxxxxxxxxxx>>
Dear All,
I have a table with 108 billion rows and migrating this database from Oracle
11g on Solaris to Oracle 12c on Linux.
After the migration I need to compare the row count of this table in both the
source DB and the destination DB. It takes almost two hours to get the row
count from this table.
SQL> select to_char(count(*), '999,999,999,999') from test_data;
TO_CHAR(COUNT(*)
----------------
108,424,262,144
Elapsed: 02:22:46.18
Could you please suggest some tips to get the row count faster so that it
reduces the cut-over downtime.
Thanks,
Ashoke
--
Mladen Gogala
Database Consultant
Tel: (347) 321-1217
