Initialization fork
An empty table or index created for an unlogged table
An initialization fork is an empty table or index of the appropriate type created for an unlogged table. If the unlogged table is reinitialized following a crash, the initialization fork is copied over the main fork.
Source code
Code handling reinitialisation of unlogged tables is in: src/backend/storage/file/reinit.c, specifically the function ResetUnloggedRelations().
The latter is called from src/backend/access/transam/xlog.c in StartupXLOG().
Examples
The initialisation fork(s) are created with the suffix _init:
postgres=# CREATE UNLOGGED TABLE foo (id int primary key);
CREATE TABLE
postgres=# SELECT pg_relation_filepath('foo'), pg_relation_filepath('foo_pkey')
pg_relation_filepath | pg_relation_filepath
----------------------+----------------------
base/13261/16499 | base/13261/16502
(1 row)
postgres:data$ ll base/13261/16499*
-rw------- 1 postgres postgres 0 Nov 17 11:01 base/13261/16499
-rw------- 1 postgres postgres 0 Nov 17 11:01 base/13261/16499_init
postgres:data$ ll base/13261/16502*
-rw------- 1 postgres postgres 8192 Nov 17 11:01 base/13261/16502
-rw------- 1 postgres postgres 8192 Nov 17 11:01 base/13261/16502_init
After inserting some data into the table:
postgres:data$ ll base/13261/16499* -rw------- 1 postgres postgres 368640 Nov 17 11:09 base/13261/16499 -rw------- 1 postgres postgres 24576 Nov 17 11:09 base/13261/16499_fsm -rw------- 1 postgres postgres 0 Nov 17 11:01 base/13261/16499_init postgres:data$ ll base/13261/16502* -rw------- 1 postgres postgres 245760 Nov 17 11:09 base/13261/16502 -rw------- 1 postgres postgres 8192 Nov 17 11:01 base/13261/16502_init
References
- PostgreSQL documentation: The Initialization Fork