sql - Mapped relation in postgresql -


i have been playing postgresql while , 1 caught eye. "mapped relation" in postgresql. according documentation,

when name of on-disk file zero, called "mapped" relation disk file name determined low - level state.

is simple relation doesnt have fixed oid reference with. why created? significance?or similar temp table? can 1 thow light on this?

https://www.postgresql.org/docs/current/static/storage-file-layout.html

also, system catalogs including pg_class itself, pg_class.relfilenode contains zero. actual filenode number of these catalogs stored in lower-level data structure, , can obtained using pg_relation_filenode() function.

t=# select relfilenode pg_class relname = 'pg_class';  relfilenode -------------            0 (1 row)  t=# select pg_relation_filenode('pg_class');  pg_relation_filenode ----------------------                 12712 (1 row)

now little barbarian (yet user friendly) way make sure file:

t=# create table very_special_name(i int); create table t=# checkpoint; --to write disk checkpoint t=# select oid pg_database datname='t';    oid ----------  13805223 (1 row)

so check readable strings:

-bash-4.2$ strings /pg/data/base/13805223/12712  | grep very_special very_special_name

new table name in...


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