pbxt-discuss team mailing list archive

[Paul McCullagh <paul.mccullagh@xxxxxxxxxxxxx>]

Hi Brian,

As you know, most engines that need a Table object use the one  
connected to the Cursor object ("table" below):

class Cursor :public memory::SqlAlloc
{
protected:
  TableShare *table_share;   /* The table definition */
  Table *table;               /* The current open table */
  ....

Here the engine will follow the "table" pointer to the "field" array,  
where it uses the offsets of the data in a record, in order to copy  
data in and out of the record.

PBXT also uses information like length and type of the fields, and the  
collation sequence if the column has one.

The main difference between PBXT and other engines, is that it needs  
this information during recovery (the field information on the Table  
object), and PBXT has background threads (that do not have a Cursor),  
which need this information as well.

This means PBXT needs to create it own Table objects for these threads.

On May 14, 2010, at 6:12 PM, Brian Aker wrote:

> Hi!
>
> What are you using Table for? The table definition should have  
> everything you need in it for defining the table. "Table" in drizzle  
> is just a collection of caches used by a Cursor. TableShare, beyond  
> its uses in locking, is just an interface to the information found  
> in the table definition.
>
> Cheers,
> 	-Brian
>
> On May 14, 2010, at 12:38 AM, Paul McCullagh wrote:
>
> > Hi Brian,
> >
> > As far as I know, getTableDefinition() just returns the protobuf of  
> > the table.
> >
> > What I need is a "class Table" (table.h) reference.
> >
> > This structure is usually one per thread (i.e. some methods use  
> > instance variables).
> >
> > However, I share one pointer between all threads, because I don't  
> > use any of the methods that address instance variables.
> >
> > (Of course it would make more sense to move the methods that  
> > independent of instance variables to TableShare).
> >
> > On May 11, 2010, at 2:43 AM, Brian Aker wrote:
> >
> > > Hi!
> > >
> > > If you know the table name you can do a getTableDefinition() for  
> > > the table and get back the definition. Do you need anything beyond  
> > > that?
> > >
> > > Cheers,
> > > 	-Brian
> > >
> > > On May 10, 2010, at 1:20 AM, Paul McCullagh wrote:
> > >
> > > > Yup, This is a C hack, so I agree it should be done differently  
> > > > for Drizzle.
> > > >
> > > > my_close_table() is only used to close "tables" opened by  
> > > > my_open_table().
> > > >
> > > > In my_open_table() we have:
> > > >
> > > > 	if (!(buffer = (char *) xt_malloc(self, size)))
> > > > 		return NULL;
> > > > 	table = (TABLE *) buffer;
> > > > 	buffer += sizeof(TABLE);
> > > > 	share = (TABLE_SHARE *) buffer;
> > > >
> > > > So things have been setup so that:
> > > >
> > > > share == (TABLE_SHARE *) ((char *) table + sizeof(TABLE))
> > > >
> > > > To get rid of this hack, we need to look at the purpose of this  
> > > > code:
> > > >
> > > > PBXT requires a reference (or a copy of) the internal MySQL/ 
> > > > Drizzle data dictionary (i.e. the structure that is created when  
> > > > a .frm file is loaded).
> > > >
> > > > This is required for 2 purposes:
> > > >
> > > > 1. To determine the internal MySQL row and key structures.
> > > > 2. PBXT uses reference to MySQL collation sequence based  
> > > > comparison routines.
> > > >
> > > > MySQL/Drizzle supply this reference when opening a table handler.  
> > > > However, for PBXT this is not good enough, because there are  
> > > > background threads (like the recovery thread) that need the  
> > > > information as well.
> > > >
> > > > Ideally Drizzle would supply a set of callback services. One of  
> > > > these would be to get a reference to Drizzle data dictionary for  
> > > > a table, and another would be a function to release the reference.
> > > >
> > > > Then I could remove the hack for the Drizzle case entirely.
> > > >
> > > > On May 9, 2010, at 1:24 AM, Brian Aker wrote:
> > > >
> > > > > Hi!
> > > > >
> > > > > This code:
> > > > >
> > > > > static void my_close_table(TABLE *table)
> > > > > {
> > > > > #ifdef DRIZZLED
> > > > >    TABLE_SHARE     *share;
> > > > >
> > > > >    share = (TABLE_SHARE *) ((char *) table + sizeof(TABLE));
> > > > >    share->free_table_share();
> > > > > #else
> > > > >    delete_table(table, true);  // TODO: Q, why did Stewart  
> > > > > remove this?
> > > > > #endif
> > > > >    xt_free_ns(table);
> > > > > }
> > > > >
> > > > >
> > > > > This is not going to go well in Drizzle, since we the assumption  
> > > > > that the memory allocation will be aligned this way is wrong. In  
> > > > > my local tree I am going to drop the free_table_share().
> > > > >
> > > > > I'm not really sure what to do though. I believe what you will  
> > > > > be wanting to do is call delete on table->s, though if what you  
> > > > > are using is an TableInstance then it should clean up just fine  
> > > > > on its own.
> > > > >
> > > > > Is there something I can do to make this code a bit more  
> > > > > straightforward for you?
> > > > >
> > > > > Cheers,
> > > > > 	-Brian
> > > > >
> > > > >
> > > > >
> > > > >
> > > > >
> > > > >
> > > > > _______________________________________________
> > > > > Mailing list: https://launchpad.net/~pbxt-discuss
> > > > > Post to     : pbxt-discuss@xxxxxxxxxxxxxxxxxxx
> > > > > Unsubscribe : https://launchpad.net/~pbxt-discuss
> > > > > More help   : https://help.launchpad.net/ListHelp
> > > >
> > > >
> > > >
> > > > --
> > > > Paul McCullagh
> > > > PrimeBase Technologies
> > > > www.primebase.org
> > > > www.blobstreaming.org
> > > > pbxt.blogspot.com
> >
> >
> >
> > --
> > Paul McCullagh
> > PrimeBase Technologies
> > www.primebase.org
> > www.blobstreaming.org
> > pbxt.blogspot.com



--
Paul McCullagh
PrimeBase Technologies
www.primebase.org
www.blobstreaming.org
pbxt.blogspot.com