PostgreSQL

This section describes the storage format at the level of files and directories.

Traditionally, the configuration and data files used by a database cluster are stored together within the cluster’s data directory, commonly referred to as PGDATA (after the name of the environment variable that can be used to define it). A common location for PGDATA is /var/lib/pgsql/data. Multiple clusters, managed by different server instances, can exist on the same machine.

The PGDATA directory contains several subdirectories and control files, as shown in Table 73.1. In addition to these required items, the cluster configuration files postgresql.conf, pg_hba.conf, and pg_ident.conf are traditionally stored in PGDATA, although it is possible to place them elsewhere.

Table 73.1. Contents of PGDATA

For each database in the cluster there is a subdirectory within PGDATA`/base, named after the database’s OID in `pg_database. This subdirectory is the default location for the database’s files; in particular, its system catalogs are stored there.

Note that the following sections describe the behavior of the builtin heap table access method, and the builtin index access methods. Due to the extensible nature of PostgreSQL, other access methods might work differently.

Each table and index is stored in a separate file. For ordinary relations, these files are named after the table or index’s filenode number, which can be found in pg_class.relfilenode. But for temporary relations, the file name is of the form t`BBB_FFF, where BBB is the backend ID of the backend which created the file, and FFF is the filenode number. In either case, in addition to the main file (a/k/a main fork), each table and index has a free space map (see Section 73.3), which stores information about free space available in the relation. The free space map is stored in a file named with the filenode number plus the suffix `_fsm. Tables also have a visibility map, stored in a fork with the suffix _vm, to track which pages are known to have no dead tuples. The visibility map is described further in Section 73.4. Unlogged tables and indexes have a third fork, known as the initialization fork, which is stored in a fork with the suffix _init (see Section 73.5).

Note that while a table’s filenode often matches its OID, this is not necessarily the case; some operations, like TRUNCATE, REINDEX, CLUSTER and some forms of ALTER TABLE, can change the filenode while preserving the OID. Avoid assuming that filenode and table OID are the same. Also, for certain system catalogs including pg_class itself, pg_class.relfilenode contains zero. The actual filenode number of these catalogs is stored in a lower-level data structure, and can be obtained using the pg_relation_filenode() function.

When a table or index exceeds 1 GB, it is divided into gigabyte-sized segments. The first segment’s file name is the same as the filenode; subsequent segments are named filenode.1, filenode.2, etc. This arrangement avoids problems on platforms that have file size limitations. (Actually, 1 GB is just the default segment size. The segment size can be adjusted using the configuration option --with-segsize when building PostgreSQL.) In principle, free space map and visibility map forks could require multiple segments as well, though this is unlikely to happen in practice.

A table that has columns with potentially large entries will have an associated TOAST table, which is used for out-of-line storage of field values that are too large to keep in the table rows proper. pg_class.reltoastrelid links from a table to its TOAST table, if any. See Section 73.2 for more information.

The contents of tables and indexes are discussed further in Section 73.6.

Tablespaces make the scenario more complicated. Each user-defined tablespace has a symbolic link inside the PGDATA/pg_tblspc directory, which points to the physical tablespace directory (i.e., the location specified in the tablespace’s CREATE TABLESPACE command). This symbolic link is named after the tablespace’s OID. Inside the physical tablespace directory there is a subdirectory with a name that depends on the PostgreSQL server version, such as PG_9.0_201008051. (The reason for using this subdirectory is so that successive versions of the database can use the same CREATE TABLESPACE location value without conflicts.) Within the version-specific subdirectory, there is a subdirectory for each database that has elements in the tablespace, named after the database’s OID. Tables and indexes are stored within that directory, using the filenode naming scheme. The pg_default tablespace is not accessed through pg_tblspc, but corresponds to PGDATA/base. Similarly, the pg_global tablespace is not accessed through pg_tblspc, but corresponds to PGDATA`/global`.

The pg_relation_filepath() function shows the entire path (relative to PGDATA) of any relation. It is often useful as a substitute for remembering many of the above rules. But keep in mind that this function just gives the name of the first segment of the main fork of the relation — you may need to append a segment number and/or _fsm, _vm, or _init to find all the files associated with the relation.

Temporary files (for operations such as sorting more data than can fit in memory) are created within PGDATA`/base/pgsql_tmp, or within a `pgsql_tmp subdirectory of a tablespace directory if a tablespace other than pg_default is specified for them. The name of a temporary file has the form pgsql_tmp`PPP.NNN, where PPP is the PID of the owning backend and NNN` distinguishes different temporary files of that backend.

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