Re: Table with ~255+ columns

Knowing about lots of Oracle's patents with Håkan Jakobsson in the authors' list, I think it was his invention.

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вс, 6 дек. 2020 г., 15:38 Michael D O'Shea/Woodward Informatics Ltd <
woodwardinformatics_at_strychnine.co.uk>:



>

> > and (possibly) side effects of the *Hakan factor*.


>

> From where does this name come? I did a quick online search, expecting the

> top hit to be Wikipedia and something to do with bitmap indexes and Bloom

> filters, but no, the top hits are Oracle „names“, including Jonathan

> https://jonathanlewis.wordpress.com/2013/05/10/hakan-factor/ explaining

> what it is and side effects *etc*.

>

> Furthermore the search hit just a bit down the page is „Everything about

> Hakan Factor, yes EVERYTHING“,

> http://secondhalforacle.blogspot.com/2013/10/everything-about-hakan-factor.html except

> it seems the origin of the name Hakan.

>

> Can anyone throw any light onto the source of this name?

>

> Mike

>

>

>

> Am 06/12/2020 um 10:26 schrieb Jonathan Lewis <jlewisoracle_at_gmail.com>:

>

> Not made any easier by the fact that it varies with version of Oracle

> (though after 11.2.?.? it may have become consistent across versions -

> there are too many to check.

> And ctas, insert /*+append */, insert, alter table move can introduce

> variations, and then there's the possibility of Exadata specials, and

> (possibly) side effects of the Hakan factor. You just don't know what

> you've missed until someone else gets a result that says that you 've

> missed SOMETHING.

>

> You've reminded me, by the way, of a note that I wrote some time ago that

> included a demonstration of the differences that might appear.

> https://jonathanlewis.wordpress.com/2017/05/19/255-columns-2/

>

>

> There's a parameter in 11.2.0.4 that's relevant:

> name: _rowlen_for_chaining_threshold

> default value: 1000

> description:  maximum rowlen above which rows may be chained across blocks

>

>

>

> Regards

> Jonathan Lewis

>

>

> On Sun, 6 Dec 2020 at 08:49, Lothar Flatz <l.flatz_at_bluewin.ch> wrote:

>

>> Hi,

>>

>> I was studying block dumps for several weeks to find out what is going

>> on. In my test it was like that:

>> On an insert /*+ APPEND */ the db was filling a block bottom  to top(low

>> block address)  and fit as much as possible in one block.

>> On conventional insert it was filling the block top to bottom and was

>> using a new block after a number of row pieces. There seems to be a

>> threshold of some kind.

>> Alter table move is insert /*+ APPEND */, thus it can be used to

>> consolidate the result of a conventional insert.

>>

>> Regards

>>

>> Lothar

>>

>> *Sent:* Wednesday, December 02, 2020 2:44 PM

>> *To:* Oracle L

>> *Subject:* Re: Table with ~255+ columns

>>

>>

>>

>>

>>

>> And while we're on the topic of row chaining because of excessive (> 255)

>> columns, I have seen a table get BIGGER because of a rebuild because when

>> you do the move Oracle tries to keep all the row pieces of a single row in

>> the same block as they are re-created, but this can mean a block gets full

>> prematurely because (e.g.) two rows that were sharing their fragments

>> across a couple of blocks with another couple of rows both end up "filling"

>> a block and denying the available space to any other row because no other

>> row will fit completely in the available free space in their blocks.  LIke:

>> I can fit 5 rows in 4 blocks if I put 1 anda quarter rows per block, but

>> I'm not going to spread a row over 4 blocks on a new insert, I'll put it in

>> its own block.

>>

>>

>>

>> Regards

>>

>> Jonathan Lewis

>>

>>

>>

>>

>>

>


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