been ruminating on the question (mostly in a 390/v7 context) of
whether, and if so when, a row update becomes an insert/delete.
i assume that there is a threshold on the number of columns of
the table, or perhaps bytes, being updated where the engine just
decides, screw it, i'll just make a new one.
surfed this group and google, but couldn't find anything.
the context: we have some java folk who like to parametize/
generalize/exersize into what they call Data Access Objects, and
end up looping through a database result set based on column
metadata (thus not having to write any table specific code),
resetting all columns. makes for generatable code, but i for one
don't like it.
seems to be the worst possible performance scenario??
Nov 12 '05
16  3874 
Serge Rielau <sr*****@ca.e ye-bee-em.com> wrote in message news:<fB******* *************@t wister01.bloor. is.net.cable.ro gers.com>... Correct and also update to a range-clustered table.
When range partitioning become availbale, that too will have the same
behaviour.
To comment on the question what DB2 really updates in a "real" update.
DB2 actually ahndles teh fact that more elements are mentioned in the
SET clause (or before trigger) than ar eactually changed (SET c1 = c1).
By default these smarts are ON. There is a Db2 registry variable called
ASSUMEUPDATED (or something like that) which can be set to disable the
smarts (to shave some codepath in well behaved high performance
environments).
The requirement to be that smart indeed coems from teh fact that many
Apps don't bother to find out what was updated. They simply update all
the columns.
The savings, btw is not in the writing of the page. It's in the logging.
And DB2 indeed logs the stripe from the first changed column to the last
changed column.
kind of figured that part <G>
this is an LUW facility? or DB2 wide? (my case study is NT, so it
doesn't, for the moment, affect 390).
robert
Cheers
Serge
I have no clue whether DB2 for z/OS has a similar implementation.
Cheers
Serge
> many Apps don't bother to find out what was updated. They simply update all
the columns.
Just wanted to add that there may be a strong reason not to bother.
Let's assume I'm performing an array update, and I've bound 40
parameters. Should I be figuring out what's actually changed and
what's not, and issuing individual dynamic updates for each row, that
would be much slower. Keeping 2^40 prepared updates is also out of
question, is it not ak************@ yahoo.com (AK) wrote in message news:<46******* *************** ****@posting.go ogle.com>... many
Apps don't bother to find out what was updated. They simply update all
the columns.
Just wanted to add that there may be a strong reason not to bother.
Let's assume I'm performing
an array update
real relational databases (even just 1st normal form ones) don't
have arrays. don't you read date, or even celko? <G>
, and I've bound 40 parameters. Should I be figuring out what's actually changed and
what's not, and issuing individual dynamic updates for each row, that
would be much slower. Keeping 2^40 prepared updates is also out of
question, is it not
Real relational systems don't have cleint side logic ;-)
I think SQL is SQL and procedural is procedural.
But those two conecpts must talk with each other.
Wherher the academics like it or not.
And neither Java nor .Net are SQL friendly.
Cheers
Serge
gn*****@rcn.com (robert) wrote in message news:<da******* *************** ***@posting.goo gle.com>... ak************@ yahoo.com (AK) wrote in message news:<46******* *************** ****@posting.go ogle.com>... many
Apps don't bother to find out what was updated. They simply update all
the columns.
Just wanted to add that there may be a strong reason not to bother.
Let's assume I'm performing
an array update
real relational databases (even just 1st normal form ones) don't
have arrays. don't you read date, or even celko? <G>
you probably don't know yet about binding arrays to parameters.
if you wish to learn a little bit, start here: http://www-106.ibm.com/developerwork...04pooloth.html ak************@ yahoo.com (AK) wrote in message news:<46******* *************** ****@posting.go ogle.com>... gn*****@rcn.com (robert) wrote in message news:<da******* *************** ***@posting.goo gle.com>... ak************@ yahoo.com (AK) wrote in message news:<46******* *************** ****@posting.go ogle.com>... > many
> Apps don't bother to find out what was updated. They simply update all
> the columns.
Just wanted to add that there may be a strong reason not to bother.
Let's assume I'm performing an array update
real relational databases (even just 1st normal form ones) don't
have arrays. don't you read date, or even celko? <G>
you probably don't know yet about binding arrays to parameters.
if you wish to learn a little bit, start here:
http://www-106.ibm.com/developerwork...04pooloth.html
it was a pun on array (there was a <G>, after all). we use a framework,
pre-Struts but much like it, and jdbc, and indexed parameters to the
servlets. an interesting article, nonetheless.
<G><G>
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