Обсуждение: how to read table options during smgropen()
Dear pgsql hackers,
I am developing custom storage for pgsql tables. I am using md* functions and smgrsw[] structure to switch between different magnetic disk access methods.
I want to add some custom options while table created
psql# create table t(...) with (my_option='value');
And thus I want to set "reln->smgr_which" conditionally during smgropen(). If myoption='value' i would use another smgr_which
I am really stuck at this point.
smgr.c:
SMgrRelationsmgropen(RelFileNode rnode, BackendId backend){
...
if ( HasOption(rnode, "my_option","value")){ //<< how to implement this check ?
reln->smgr_which = 1; //new access method
}else{
reln->smgr_which = 0; //old access method
}
...
}
The question is --- can I read table options while the table is identified by "RelFileNode rnode" ??
The only available information is
typedef struct RelFileNode
{
Oid spcNode; /* tablespace */
Oid dbNode; /* database */
Oid relNode; /* relation */
} RelFileNode;
{
Oid spcNode; /* tablespace */
Oid dbNode; /* database */
Oid relNode; /* relation */
} RelFileNode;
But there are no table options available directly from this structure.
What is the best way to implement HasOption(rnode, "my_option","value")
Thank you in advance for any ideas.
Sincerely,
Dmitry R
On 22/02/2024 20:22, Dima Rybakov (Tlt) wrote:
> Dear pgsql hackers,
>
> I am developing custom storage for pgsql tables. I am using md*
> functions and smgrsw[] structure to switch between different magnetic
> disk access methods.
>
> I want to add some custom options while table created
> psql# create table t(...) with (my_option='value');
>
> And thus I want to set "reln->smgr_which" conditionally during
> smgropen(). If myoption='value' i would use another smgr_which
>
> I am really stuck at this point.
>
> smgr.c:
> SMgrRelation
> smgropen(RelFileNode rnode, BackendId backend){
> ...
> if ( HasOption(rnode, "my_option","value")){ //<< how to implement
> this check ?
> reln->smgr_which = 1; //new access method
> }else{
> reln->smgr_which = 0; //old access method
> }
> ...
> }
>
>
> The question is --- can I read table options while the table is
> identified by "RelFileNode rnode" ??
The short answer is that you can not. smgropen() operates at a lower
level, and doesn't have access to the catalogs. smgropen() can be called
by different backends connected to different databases, and even WAL
recovery when the system is not in a consistent state yet.
Take a look at the table AM interface. It sounds like it might be a
better fit for what you're doing.
There have been a few threads here on pgsql-hackers on making the smgr
interface extensible, see
https://www.postgresql.org/message-id/CAEze2WgMySu2suO_TLvFyGY3URa4mAx22WeoEicnK%3DPCNWEMrA%40mail.gmail.com
one recent patch. That thread concluded that it's difficult to make it a
per-tablespace option, let alone per-table.
--
Heikki Linnakangas
Neon (https://neon.tech)