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IDS Forum

Re: 11.70 data page slot table

Posted By: Art Kagel
Date: Wednesday, 19 November 2014, at 3:14 p.m.

In Response To: RE: 11.70 data page slot table (Andrew Ford)

OK, I think I've figured this out and Madison's comment about not rewriting
the slot table tweaked my memory. Here's what I think happened Andrew:

- When the table was altered adding 4 bytes to every row, 13 rows no longer
fit on the page, but this page was either full or had a deleted row in slot
#1 (most likely).
- If slot 1 was deleted then it's slot entry's offset was zero'd out
indicating an empty slot but the length byte was left alone. Done.
- If slot 1 had a row in it, then that row would have been moved to another
page to free up space for expanding the other 12 rows on the page. The
slow entry would have been replaced with a forwarding pointer. But then the
offset would be non-zero (pointing to the forwarding pointer's location on
the page) and the length would be negative IB. I don't know, though, what
would happen if later the migrate row was subsequently deleted to the
original slot entry. Maybe it's offset just gets zero'd as you found it.

Art

Art S. Kagel, President and Principal Consultant
ASK Database Management
www.askdbmgt.com

Blog: http://informix-myview.blogspot.com/

Disclaimer: Please keep in mind that my own opinions are my own opinions
and do not reflect on the IIUG, nor any other organization with which I am
associated either explicitly, implicitly, or by inference. Neither do
those opinions reflect those of other individuals affiliated with any
entity with which I am affiliated nor those of the entities themselves.

On Wed, Nov 19, 2014 at 3:02 PM, Andrew Ford <andrew@informix-dba.com>
wrote:

> Unless I'm confused about this page being updated (or how slots and data
> pages work), then why would there be 13 slots if only 12 rows fit in a
> page.
> Also, why would slot #3 indicate the row starts at byte 179 (0x00b3 = 179)
> when you'd expect the deleted row in slot #1 to start @ byte 24 right after
> the header and the row in slot #2 to start at byte 179 (row size 155 + 24 =
> 179).
>
> Here is the last 64 bytes of a page that does not have a row delete and has
> been updated after the IPA
>
> 00000000 00000000 c1069b00 26069b00
> 8b059b00 f0049b00 55049b00 ba039b00
> 1f039b00 84029b00 e9019b00 4e019b00
> b3009b00 18009b00 00009700 daff72f6
>
> and some oncheck -pP
>
> addr stamp chksum nslots flag type frptr frcnt next
> prev
> 4:55 -160235558 99b 13 6801 DATA 1884 108
> 1000000 0
>
> slot ptr len flg
>
> 2 24 155 0
>
> 3 179 155 0
>
> 4 334 155 0
>
> 5 489 155 0
>
> 6 644 155 0
>
> 7 799 155 0
>
> 8 954 155 0
>
> 9 1109 155 0
>
> 10 1264 155 0
>
> 11 1419 155 0
>
> 12 1574 155 0
>
> 13 1729 155 0
>
> Andrew
>
> -----Original Message-----
> From: ids-bounces@iiug.org [mailto:ids-bounces@iiug.org] On Behalf Of
> Madison Pruet
> Sent: Wednesday, November 19, 2014 1:47 PM
> To: ids@iiug.org
> Subject: Re: 11.70 data page slot table [34192]
>
> It looks to me like slot 1 and slot 2 are empty....
>
> From: "Andrew Ford" <andrew@informix-dba.com>
> To: ids@iiug.org
> Date: 11/19/2014 01:23 PM
> Subject: 11.70 data page slot table [34190] Sent by: ids-bounces@iiug.org
>
> Can anyone tell me what I'm looking at and why?
>
> 11.70 2K data page, row size is fixed at 155 bytes after in place alter=
> (12
>
> rows per page), previous row size was 151 bytes (13 rows per page), pag= e
> has been updated after in place alter.
>
> last 64 bytes of the 2K data page:
>
> 00000000 00000000 c1069b00 26069b00
>
> 8b059b00 f0049b00 55049b00 ba039b00
>
> 1f039b00 84029b00 e9019b00 4e019b00
>
> b3009b00 00009b00 00009700 bd165df6
>
> Last 4 bytes of the page is the checksum and I would expect the previou= s
> 4
> bytes to be slot #1 in the slot table entry, but it looks like it is
> storing
> the pre IPA size of the row (0x0097 =3D 151).
>
> Preceding 12 sets of 4 bytes are the slot table entries (slot #2 to slo= t
> #13, first row in the page at slot #2 has been deleted)
>
> I guess my question is, when the page was rewritten to finalize the row=
>
> alter why wasn't the slot table rewritten to be just 12 slots vs. keepi= ng
> 13
> slots and storing what appears to be the previous page size in slot #1.=
>
> Thanks,
>
> Andrew
>
> ***********************************************************************=
> ********
>
> Forum Note: Use "Reply" to post a response in the discussion forum.
>
> =
>
>
> ****************************************************************************
> ***
> Forum Note: Use "Reply" to post a response in the discussion forum.
>
>
>
> *******************************************************************************
> Forum Note: Use "Reply" to post a response in the discussion forum.
>
>

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