Re: WIP: Access method extendability

Hackers,

there is next revision of patches providing access method extendability.
Now it's based on another patch which reworks access method interface.
http://www.postgresql.org/message-id/CAPpHfdsXwZmojm6Dx+TJnpYk27kT4o7Ri6X_4OSWcByu1Rm+VA@mail.gmail.com

Besides access method interface, major change is generic xlog interface.
Now, generic xlog interface is more user friendly. Generic xlog compares
initial and changed versions of page by itself. The only thing it can't do
is to find data moves inside page, because it would be too high overhead.
So in order to get compact WAL records one should use
GenericXLogMemmove(dst, src, size) in order to move data inside page. If
this function wasn't used then WAL records would just not so compact.

In general pattern of generic WAL usage is following.

1) Start using generic WAL: specify relation

GenericXLogStart(index);

2) Register buffers

GenericXLogRegister(0, buffer1, false);
GenericXLogRegister(1, buffer2, true);

first argument is a slot number, second is the buffer, third is flag
indicating new buffer

3) Do changes in the pages. Use GenericXLogMemmove() if needed.

4) Finish using GenericXLogFinish(), or abort using GenericXLogAbort(). In
the case of abort initial state of pages will be reverted.

Generic xlog takes care about critical section, unlogged relation, setting
lsn, making buffer dirty. User code is just simple and clear.

------
Alexander Korotkov
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company