On Dec 17, 2009, at 4:12 AM, Stewart Smith wrote:
On Wed, Dec 16, 2009 at 11:07:52AM +0100, Paul McCullagh wrote:
I see it working as a 2 step process.
step 1 is pass a list of changes to the engine and it returns a list
of what it can't do. (this could then be used for EXPLAIN ALTER
TABLE).
If the list of what it can't do is not empty, we do a copying alter
table.
Yup, but actually I see only 2 possibilities here:
1. The engine can handle ALL actions internally.
2. We use the (default) copy alter table method.
I don't see why we would need a combination of 1 and 2.
Probably wouldn't... although I guess in some situations it *could* be
faster... maybe... but i'm pretty sure we don't need to worry about it
yet :)
EXPLAIN ALTER TABLE is more of a tool for working out what can (and
cannot) be done online.
Yes, my comments does not change the fact that EXPLAIN ALTER TABLE
would be a very useful tool.
If fact, it really is essential for DBA's trying to make a decision
about how to implement a schema change on a production system.
For example if an ALTER TABLE calls for 2 actions: ADD COLUMN and
ADD INDEX, and the engine can only do ADD INDEX internally, then it
would make no sense to use method 1 for ADD INDEX and method 2 for
ADD COLUMN.
In fact, if an ALTER TABLE adds 2 indexes it may be better to for
the engine to pick method 2 (which does all changes at once), if its
implementation of ADD INDEX can only add one index at a time.
of course, it can depend on if you want a lock to be held or not.
Sometimes you won't care if it takes 10 times as long, as long as
you're not blocking users during it.
Other situations may call for "just get it done quickly".
Which is why we have ALTER TABLE, ALTER TABLE ONLINE and ALTER TABLE
OFFLINE to do "best effort", "fail if cannot be done online" and
"force an offline copying alter table" (all introduced for NDB).
I see. Makes sense! :)
So ALTER TABLE ONLINE/OFFLINE is exactly what will help an engine to
decide what method to use, if there are multiple possibilities, cool.
--
Paul McCullagh
PrimeBase Technologies
www.primebase.org
www.blobstreaming.org
pbxt.blogspot.com
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