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How many extents does a BLOB need?

Hi, could somebody please explain why the following statement consumes
six extents:

CREATE TABLE T1 (C1 INT, C2 BLOB(1K)) IN TS1

I see one extent for the extent map and one extend for data pages. How
many extents does this BLOB thing need?

Thanks in advance.

Regards,
Markus
Nov 12 '05 #1
2 1839
The other four extents are used by the tblspc itself.
One for the con tainer tag, one for header data, one for space map, one
for header object data. These four will always appear in each container.
From then on, each object will be created with two extents; one for the
extent map and one pre-allocated empty for the data to come in. As rows
grow, more extents are allocated and identified in the extent map which
will then grow new ones as needed.

HTH, Pierre.

Markus Neifer wrote:
Hi, could somebody please explain why the following statement consumes
six extents:

CREATE TABLE T1 (C1 INT, C2 BLOB(1K)) IN TS1

I see one extent for the extent map and one extend for data pages. How
many extents does this BLOB thing need?

Thanks in advance.

Regards,
Markus


--
Pierre Saint-Jacques - Reply to: sesconsjunk at attglobaljunk dot com
Reconstruct address: Remove the two junk and replace at and dot by
their symbols.
IBM DB2 Cerified Solutions Expert - Administration
SES Consultants Inc.

Nov 12 '05 #2
Hi Pierre, thank you for your reply. I know about the tablespace
overhead and was thinking about extents consumed by T1 only. Maybe
some more information will clarify this. I create TS1 with

CREATE TABLESPACE TS1
MANAGED BY DATABASE USING (FILE 'TS1_CONT' 120)
EXTENTSIZE 10

and LIST TABLESPACES SHOW DETAIL says total pages 120, usable pages
110 and used pages 30. So far, so good. Now i create a simple table

CREATE TABLE T2 (C1 INT, C2 CHAR(250)) IN TS1

and LIST TABLESPACES SHOW DETAIL says used pages 50. That's ok because
each object (table, index, LOB) needs 2 extents. However, after
creating the following table

CREATE TABLE T1 (C1 INT, C2 BLOB(1K)) IN TS1

LIST TABLESPACES SHOW DETAIL says used pages 110. I see one extent for
the extent map and one extend for data pages. I thought the BLOB would
need 2 extents, as any other object. But if i sum it up, i get only 4
extents, not 6.

What am i missing?

Markus

Pierre Saint-Jacques <se*****@attglobal.net> wrote in message news:<41**************@attglobal.net>...
The other four extents are used by the tblspc itself.
One for the con tainer tag, one for header data, one for space map, one
for header object data. These four will always appear in each container.
From then on, each object will be created with two extents; one for the
extent map and one pre-allocated empty for the data to come in. As rows
grow, more extents are allocated and identified in the extent map which
will then grow new ones as needed.

HTH, Pierre.

Nov 12 '05 #3

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