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)