Andy
Yes look like is an option if we are doing work online and despite take more
time but need not require downtime. In our case multiple DDL are running to
existing environment due to Application upgrade and so all work has to be done
with downtime. So challenge is reduce time of DML operations on big tables
containing few billions rows.
TxSanjay
On Wednesday, August 26, 2020, 11:20:55 AM EDT, Andy Sayer
<andysayer@xxxxxxxxx> wrote:
It does sound like a virtual column could be the ideal solution. But if data
needs to be physically stored or cannot be calculated deterministically at any
point in time then Connor has a great demo of using dbms_redefinition to create
a new table online with a function to map the new column. There’s obviously
some overhead with context switching but it may be far better than some of the
obstacles you might be facing at the moment:
https://connor-mcdonald.com/2016/11/16/performing-a-large-correlated-update/ ;
(and you might be able to help it with pragma udf in the right circumstances).
Obviously, how helpful this is depends where the work is currently going and
how online this needs to be.
Thanks,Andrew
On Wed, 26 Aug 2020 at 16:00, Jonathan Lewis <jlewisoracle@xxxxxxxxx> wrote:
Is that 3-4 billion rows each, or total ?
I would be a little suspicious of an update which populates a new column with a
value derived from existing columns. What options might you have for declaring
a virtual column instead - which you could index if needed.
Be extremely cautious about calculating space requirements - if you're updating
every row on old data might you find that you're causing a significant fraction
of the rows in each block to migrate, and there's a peculiarity of bulk row
migration that can effectively "waste" 25% of the space in every block that
becomes the target of a migrated row.
This effects can be MUCH work when the table is compress (even for OLTP) since
the update has to decompress the row before updating and then only
"re-compresses" intermittently as the block becomes full. The CPU cost can be
horrendous and you still have the problem of migration if the addition means
the original rows can no longer fit in the block.
If it is necessary to add the column you may want to review "alter table move
online" can do in the latest versions (in case you can make it add the column
as you move) or review the options for dbms_redefinition - maybe running
several redefinitions concurrently rather than trying to do any parallel update
to any single table.
RegardsJonathan Lewis
