From: Sergei Golubchik
Date: June 4 2009 2:13pm
Subject: Re: About definition of I_S tables
List-Archive: http://lists.mysql.com/internals/36836
Message-Id: <20090604141346.GA12255@janus.mylan>
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Hi, scut_tang!

On Jun 04, scut_tang wrote:
> >Yes, but I think it would be easier to go just a bit deeper. That is to
> >keep open_table() and open_table_from_share() as is, and implement the
> >function to create a TABLE_SHARE without an frm file. When TABLE_SHARE
> >exists, open_table_from_share() will work normally, no need to modify
> >it.
> >And the simplest way to create a custom TABLE_SHARE, I suppose, is a
> >table discovery - ha_create_table_from_engine() -> ha_discover().
> 
> I browse those function source code today. They drive me crazy.
> What do "Try to discover table from engine", "table exist in handler" and
> "table in engine" mean?  Could you give some example?
> I know MySQL server will reach ha_create_table_from_engine(), when a
> query as "SELECT * FROM Whatever.none". Whatever.none is not exist.
> The function will return a result means table did not exist. In what
> situation, it will return table created ok?
>  
> Could you say something more about ha_discover() in detail?

Sure. "Table discovery" is a mechanism for a server to, well, discover
tables that already exist in the engine.

Originally it was developed for NDB. Indeed, in the cluster environment
there could be many MySQL daemons connected to one cluster. And a table
created from one of these daemons should automatically be visible in all
others. This is implemented via "table discovery" - when a server
doesn't find an frm file for a table it asks the engine "does this table
exists in the engine?", and the engine can answer "yes, here's an frm
file for this table". It's transparent for the end user, it looks
like the table always existed.

But discovery is more useful than just for the Cluster. For example, if
there would be an engine that reads XLS files, with the table discovery
one can just copy an XLS file into MySQL datadir, and start SELECT'ing
from it. Without discovery one would have to do CREATE TABLE with
exactly the same structure as the table in the XLS file has, to create a
matching frm file. With discovery such an frm file can be created
automatically.

Similarly, I suggest that we take use of this feature and allow the
server to "discover" I_S tables automatically.

There are few methods in the handlerton that are used for discovery. The
main is discover(), but there are also find_files() and
table_exists_in_engine().

Regards / Mit vielen Grüßen,
Sergei

-- 
   __  ___     ___ ____  __
  /  |/  /_ __/ __/ __ \/ /   Sergei Golubchik <serg@stripped>
 / /|_/ / // /\ \/ /_/ / /__  Principal Software Engineer/Server Architect
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