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IDS Forum

RE: Cost of ALTER table

Posted By: david@smooth1.co.uk
Date: Thursday, 7 November 2013, at 6:13 p.m.

In Response To: RE: Cost of ALTER table (Fernando Nunes)

And on my blog justdave.info !

On 07 November 2013 at 15:25 Fernando Nunes <domusonline@gmail.com> wrote:
> You CAN register feature requests.... the link is on my blog (RFE site)
> On Nov 7, 2013 9:43 AM, "Habichtsberg, Reinhard" <
> RHabichtsberg@arz-emmendingen.de> wrote:
>
> > Hi all.
> >
> > The slow alters are a general problem, resp. with varchar, lvarchar and
> > boolean. If there are lvarchar or boolean columns in your table even
> > each alter will be a slow alter!
> >
> > One have to be aware of the expected amout of rows and size of a table
> > when determining the column types. With big tables I would avoid to use
> > those types which sqeeze you to slow alter, particular if you have to
> > alter tables frequently. That is, for example, the case in my company
> > because most of our software which uses informix is self-developed. With
> > one or two dba's we have to serve 30 developers. Because we are
> > underlying frequent software changes due to comply with changing legal
> > requirements the need to alter even very big tables of 100 of millions
> > or billions of rows appears rather often. And we cannot afford long
> > downtimes. We are running 7 x 24 h.
> >
> > So, we have a critical glance to all "create and alter tables"
> > statements.
> >
> > In your case I would challenge the need of varchar. Probably I would
> > refuse the alter to varchar and would suggest to alter to char(128).
> >
> > And if anyone reads this who is able to place a feature request: Please
> > make inplace alter possible for changes on varchar and tables which hold
> > columns of type lvarchar and boolean.
> >
> > Kind regards
> >
> > Reinhard Habichtsberg
> > Dienstleistungsverantwortlicher IT-Rechenzentrum
> > Bereich Server-, Speicher- und Datenbanksysteme
> > B-203
> >
> > > -----Original Message-----
> > > From: ids-bounces@iiug.org [mailto:ids-bounces@iiug.org] On Behalf Of
> > FRANK
> > > Sent: Thursday, November 07, 2013 12:37 AM
> > > To: ids@iiug.org
> > > Subject: Re: Cost of ALTER table [31886]
> > >
> > > Thanks Fernando and Keith!
> > >
> > > The original table schema is the following.
> > >
> > > It is modifying from check_sum char(32) to check_sum varchar(128).
> > >
> > > Thanks for the info of double space size needed for this job!
> > >
> > > I think we will mark it unavailable and do a online ALTER for the job.
> > >
> > > Regards,
> > > Frank
> > >
> > > create table "informix".ds_archive01
> > > (
> > >
> > > dataset_name varchar(255,44) not null ,
> > >
> > > archive_site char(10) not null ,
> > >
> > > path_name varchar(250,50),
> > >
> > > archive_dt datetime year to second,
> > >
> > > update_dt datetime year to second,
> > >
> > > hsm_fs char(10),
> > >
> > > check_sum char(32),
> > >
> > > tape_name char(16),
> > >
> > > tape_block integer,
> > >
> > > tape_offset integer,
> > >
> > > checksum_dt datetime year to second
> > > ) in dbdw04 extent size 4096 next size 4096 lock mode row; revoke all
> > on
> > > "informix".ds_archive01 from "public" as "informix";
> > >
> > > create index "informix".ds_arch01_idx1 on "informix".ds_archive01
> > >
> > > (archive_dt) using btree in dbdw04;
> > > create index "informix".ds_arch01_idx2 on "informix".ds_archive01
> > >
> > > (tape_name) using btree in dbdw04;
> > > create unique index "informix".ds_arch01_ipk on
> > "informix".ds_archive01
> > >
> > > (dataset_name) using btree in dbdw05;
> > > alter table "informix".ds_archive01 add constraint primary key
> > >
> > > (dataset_name) constraint "informix".ds_arch01_pk ;
> > >
> > > On Wed, Nov 6, 2013 at 5:17 PM, Fernando Nunes
> > > <domusonline@gmail.com>wrote:
> > >
> > > >
> > > >
> > > >
> > >
> > http://pic.dhe.ibm.com/infocenter/idshelp/v115/topic/com.ibm.perf.doc/id
> > s_prf_335.
> > > htm
> > > >
> > > > Without the whole table schema the answer may not be 100% accurate,
> > > > but it looks like you're on the last situation described in the URL
> > above:
> > > >
> > > > "Modifying the original size or reserve specifications of VARCHAR or
> > > > NVARCHAR columns."
> > > >
> > > > As the name implies, it will be "slow" and will consume at least the
> > > > double of the table size. The alternatives don't look good, but
> > that's
> > > > the reason why "in-place alters" were created. The options I see:
> > > >
> > > > 1- UNLOAD the table, drop/create it and load. This will save space
> > and
> > > > with external tables can be fast. Of course it means data
> > unavailability.
> > > > 2- Create a side table type raw, with the new schema. Copy the data.
> > > > Will not save space, but depending on the data use it may help with
> > > > availability (if the table has historic data that doesn't change you
> > > > can do most of the work "online" and then stop access for the
> > "recent"
> > > > data
> > > > 3- Other weird options like create a new table, rename the existing
> > > > one, create a view with union between both tables and move the data
> > > > "online", maybe with the usage of triggers to capture changes of
> > data
> > > > already copied.... Will not save space, but can give you the most
> > > > availability (and
> > > > work....)
> > > >
> > > > Regards
> > > >
> > > > On Wed, Nov 6, 2013 at 9:40 PM, FRANK <yunyaoqu@gmail.com> wrote:
> > > >
> > > > > Hi, All,
> > > > >
> > > > > IDS11.50 FC8, Redhat Linux 5.
> > > > >
> > > > > Seems the following types of ALTER to enlarge a column width of
> > some
> > > > > BIG tables(100 millions of rows),
> > > > >
> > > > > ALTER TABLE ds_archive01 MODIFY check_sum varchar(128);
> > > > >
> > > > > can incur excessive amount of disk consumption !!
> > > > >
> > > > > Any comments or suggestions?
> > > > >
> > > > > Thanks,
> > > > > Frank
> > > > >
> > > > > --089e0163535606074a04ea88ff3f
> > > > >
> > > > >
> > > > >
> > > > >
> > > >
> > > >
> > >
> > ************************************************************************
> > *******
> > > > > Forum Note: Use "Reply" to post a response in the discussion
> > forum.
> > > > >
> > > > >
> > > >
> > > > --
> > > > Fernando Nunes
> > > > Portugal
> > > >
> > > > http://informix-technology.blogspot.com
> > > > My email works... but I don't check it frequently...
> > > >
> > > > --bcaec51d255275373b04ea898271
> > > >
> > > >
> > > >
> > > >
> > >
> > ************************************************************************
> > *******
> > > > Forum Note: Use "Reply" to post a response in the discussion forum.
> > > >
> > > >
> > >
> > > --047d7b62226c17778304ea8a9fdf
> > >
> > >
> > >
> > ************************************************************************
> > *******
> > > Forum Note: Use "Reply" to post a response in the discussion forum.
> >
> >
> >
> >
> *******************************************************************************
> > Forum Note: Use "Reply" to post a response in the discussion forum.
> >
> >
>
> --047d7b2ed34113ffc504ea97df51
>
>
> *******************************************************************************
> Forum Note: Use "Reply" to post a response in the discussion forum.
>

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