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>
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 <
oracle-l-bounce@xxxxxxxxxxxxx> *On Behalf Of *[External] Andy Sayer
*Sent:* Friday, October 2, 2020 3:09 PM
*To:* ahmed.fikri@xxxxxxxxxxx
*Cc:* list, oracle; 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 <
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>
An: "ORACLE-L" <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
