RE: [External] : Re: small bug related to deferred_segment_creation parameter

From: Mark W. Farnham <mwf_at_rsiz.com>
Date: Fri, 23 Jul 2021 07:58:26 -0400
Message-ID: <067401d77fba$0c3e67e0$24bb37a0$_at_rsiz.com>

It seems likely that “create” fixes in place the “deferred” mode at a different point in the code than maintenance. That’s a complete guess since I don’t get to see the code, but it is compatible with “create” correctly not creating “deferred” segments in the cases where it might have to create an out of line segment.

Possibly it would be easier to create a “drop empty segments” maintenance that to insert the correction to deferred segments. Since at least logically Oracle could know range partition key references refer to a single partition by name instead of stats more easily, that might be better for the optimizer using partition stats. And, of course, clearing up clutter in the dictionary as well as the possibly designed large initial segment sizes. Gargantuan segments for Monday through Friday and no segments at all for Saturday and Sunday is still a common pattern, even in the days of globalization.

So this message is mostly to the Oracle lurkers: Consider a drop empty segments between <low partition key reference> and <high partition key reference> as a fix up if the maintenance code is too different from the create code to pattern match the “bug except for restriction documentation” behavior.

Quite often such restrictions are because resources could not be prioritized rather than it being arcane to implement without the restriction.

Since this behavior has been around, documented, for a long time, if you change it a lot of folks will be asking for the “cleanup” code anyway.

Good luck.

Thanks Gerald and JL for pointing out it has “always” been this way, ending the mystery.

mwf

From: oracle-l-bounce_at_freelists.org [mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Jonathan Lewis Sent: Friday, July 23, 2021 4:38 AM
To: list, oracle
Subject: Re: [External] : Re: small bug related to deferred_segment_creation parameter

I know that Gerald Venzi quoted the 21c reference manual, and going so far forward to "retro-document" a detail can look like a cop-out; but in this case you can find exactly the same text in the 11gR2 reference manual - search for "Deferred segment creation" at the URL:

https://docs.oracle.com/cd/E11882_01/server.112/e41084/statements_3001.htm#CJABFJEA

Regards

Jonathan Lewis

On Fri, 23 Jul 2021 at 07:45, ahmed.fikri_at_t-online.de <ahmed.fikri_at_t-online.de> wrote:

Hi all,

My impression is that every time Oracle detects a bug it doesn't seem easy to resolve, add two lines of documentation that says this is a restriction. I've seen some examples of this on this forum. We even have conflicting documentation, one saying this is a limitation, the other saying it's a feature, and so on.

But that's just my impression, and doesn't mean it is true.

Many thanks to Jonathan and Andy.

Best regards

Ahmed

-----Original-Nachricht-----

Betreff: Re: [External] : Re: small bug related to deferred_segment_creation parameter

Datum: 2021-07-23T02:44:53+0200

Von: "Gerald Venzl" <gerald.venzl_at_oracle.com>

An: "ahmed.fikri_at_t-online.de" <ahmed.fikri_at_t-online.de>

Hi all,

It is reproducible not only in 19.11 but will also be in 21c as this is not a bug but a documented restriction (see alter_table_partitioning section) <https://docs.oracle.com/en/database/oracle/oracle-database/21/sqlrf/ALTER-TABLE.html> :

Deferred segment creation is not supported for partition maintenance operations that create new segments on tables with LOB columns; segments will always be created for the involved (sub)partitions.

Furthermore, you can see that this restriction holds true for all partition maintenance operations and not only the SPLIT operation.

Nope this helps!

Thx,

On Jul 22, 2021, at 15:14, Jonathan Lewis <jlewisoracle_at_gmail.com> wrote:

It looks like you missed the point in my second paragraph:

>> However, it may be a little more interesting to point out that this example does

>> the same thing in 19.11.0.0 - and still creates the segments even if you include

>> the "segment creation deferred" option with each of the listed partitions in the split.

Regards

Jonathan Lewis

On Thu, 22 Jul 2021 at 20:23, ahmed.fikri_at_t-online.de <ahmed.fikri_at_t-online.de> wrote:

in the current project I'm working on, 12.1 is still in use. Maybe someone can test the script in 12.2 or 19.11. In the meantime, I'll try to remove some dust from my private 12.2 and 19 instances and see if we have the same behavior.

Best regards

Ahmed

-----Original-Nachricht-----

Betreff: Re: small bug related to deferred_segment_creation parameter

Datum: 2021-07-22T20:14:53+0200

Von: "Jonathan Lewis" <jlewisoracle_at_gmail.com>

An: "ahmed.fikri_at_t-online.de" <ahmed.fikri_at_t-online.de>

I'm not surprised that you didn't get a response in less than 3 hours - especially since you were talking about 11.2.0.4 and 12.1.0.2.

However, it may be a little more interesting to point out that this example does the same thing in 19.11.0.0 - and still creates the segments even if you include the "segment creation deferred" option with each of the listed partitions in the split.

Regards

Jonathan Lewis

On Thu, 22 Jul 2021 at 12:53, ahmed.fikri_at_t-online.de <ahmed.fikri_at_t-online.de> wrote:

Hi everyone,

it seems that the deferred_segment_creation parameter does not work properly when splitting a table partition that contains a clob field.

If the partition is empty and has no segment, splitting it creates two segments for the old and new partitions.

I tested this using this block in 11.2.0.4 and 12.1.0.2 (also attached and run with a non dba user):

DECLARE
  PROCEDURE ex(cmd VARCHAR2) IS BEGIN dbms_output.put_line(cmd); EXECUTE IMMEDIATE cmd; END;   PROCEDURE ex_ignore_error(cmd VARCHAR2) IS BEGIN ex(cmd); EXCEPTION WHEN OTHERS THEN NULL; END;   PROCEDURE split_part(tbl VARCHAR2,p_part_val VARCHAR2) IS BEGIN ex('alter table '||tbl||' split partition maxvalue at (to_date('''||p_part_val||''',''yyyymmdd''))   into (partition part_'||p_part_val||' tablespace users, partition maxvalue tablespace users)') ;END; BEGIN
    ex('alter session set deferred_segment_creation = true'); ex_ignore_error('drop table tbl_test purge');   ex_ignore_error('drop table tbl_test2 purge'); /* create range-partitioned table without clob field */   ex('create table tbl_test(id number, part_key date) tablespace users partition by range(part_key) (partition maxvalue values less than (maxvalue))'); /* create range-partitoned table having a clob field */   ex('create table tbl_test2(id number, part_key date, clob_col clob) tablespace users partition by range(part_key) (partition maxvalue values less than (maxvalue))'); /* the table is empty. Splitting the maxvalue partition for the table without clob */   split_part('tbl_test','20200101');
/* until here no segments is generated for tbl_test and tbl_test2 tables spltting the maxvalue partition for the table having clob field */   split_part('tbl_test2','20200101');
-- two segments are created for the table having clob field

/* SELECT * FROM user_segments WHERE segment_name IN ('TBL_TEST','TBL_TEST2') TBL_TEST2 PART_20200101 TABLE PARTITION ASSM USERS TBL_TEST2 MAXVALUE TABLE PARTITION ASSM USERS */

END; Is this already known? Any Idea?
For me, this behavior is annoying and is considered a bug.

Best regards

Ahmed

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Received on Fri Jul 23 2021 - 13:58:26 CEST

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