On Mon, Dec 14, 2009 at 07:34:50PM -0500, Jay Pipes wrote: > Right, but this is NOT what Stewart has proposed for the AlterTable > statement. Stewart (and Stewart, correct me if I'm wrong) would > like to send the *actions* that the master executed for the Alter > Table. I am opposed to this (see link above for the ML post with my > reasons why)
Replicating a before and after image requires engines or database systems parsing this to know *everything* in the table definition and correctly look at *everything* and compare it to work out if it can do the alter operation (especially think about online alter). As soon as we introduce a new thing into the table structure, all of these systems instantly and silently break. For example: We add a STORAGE property to columns. and storage could be NULL.. i.e. don't store anything and return a default value. So if you "ALTER TABLE t1 COLUMN a STORAGE=NULL;" on a master, column a will always come back with the default value. If the slave now reads the ALTER, it only knows the old definition of table, examines everything, does it's work (which could be a no-op, or there could be other things in the ALTER TABLE statement). You now have a different master and slave. You will never work out why.... until you go and check the proto definition version... the source code to the slave... and file for Chapter 11 because it all took too long to save your business. If we had a series of modifications, we'd easily be able to detect this in the slave: we'd have an invalid transformation! We could error out! It also creates much fewer opportunities for those implementing replication slaves to get the code wrong. Comparing two tables and generating the list of operations to run in your engine is fraught with lots of code and potential bugs (check out the hoops NDB jumps through to avoid rewriting ALTER table in the MySQL server to be sane). Now... shipping before and after images as well could be useful... perhaps slave should error out if start definition is different (and if start definition is the same but the same sequences of actions results in a different final proto then something is certainly wrong). Passing the statement down means that it becomes impossible to replicate to non-sql systems without a SQL parser for ALTER statements (which, no doubt, will also be buggy). -- Stewart Smith _______________________________________________ Mailing list: https://launchpad.net/~drizzle-discuss Post to : [email protected] Unsubscribe : https://launchpad.net/~drizzle-discuss More help : https://help.launchpad.net/ListHelp
