On Dec 15, 2009, at 11:08 PM, Stewart Smith wrote:
On Tue, Dec 15, 2009 at 09:16:42AM -0800, Brian Aker wrote:
On Dec 14, 2009, at 9:27 PM, Stewart Smith wrote:
You can go from actions to SQL easily.
You can also use the actions to simply write code that does the
alter
on non-sql systems (e.g. an applier that just uses Embedded InnoDB)
SQL statements are interpreted by an engine. One engine may need to
do a rebuild of a full table, another may not.
If an engine can just "add" a column, then the action for it may
just be "add column". If another engine needs to copy/rename/delete
then it will have a different set of actions.
We need to be descriptive about the change, but the actual
mechanics are up to the individual engine.
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.
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.
This would be applied the same way on a slave, as we have the original
set of actions in the replication stream.
Yes, I know what you mean. You mean the same actions (ADD COLUMN, ADD
INDEX) are executed on the slave, but they may use a different method
(1 instead of 2, or 2 instead of 1).
--
Stewart Smith
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Paul McCullagh
PrimeBase Technologies
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