## Chapter 65. Database Physical Storage

This chapter provides an overview of the physical storage format used by PostgreSQL databases.

## 65.1. Database File Layout

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 65.1. 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 65.1. Contents of `PGDATA`

Item | Description
--- | ---
`PG_VERSION` | A file containing the major version number of PostgreSQL
`base` | Subdirectory containing per-database subdirectories
`global` | Subdirectory containing cluster-wide tables, such as `pg_database`
`pg_commit_ts` | Subdirectory containing transaction commit timestamp data
`pg_clog` | Subdirectory containing transaction commit status data
`pg_dynshmem` | Subdirectory containing files used by the dynamic shared memory subsystem
`pg_logical` | Subdirectory containing status data for logical decoding
`pg_multixact` | Subdirectory containing multitransaction status data (used for shared row locks)
`pg_notify` | Subdirectory containing LISTEN/NOTIFY status data
`pg_replslot` | Subdirectory containing replication slot data
`pg_serial` | Subdirectory containing information about committed serializable transactions
`pg_snapshots` | Subdirectory containing exported snapshots
`pg_stat` | Subdirectory containing permanent files for the statistics subsystem
`pg_stat_tmp` | Subdirectory containing temporary files for the statistics subsystem
`pg_subtrans` | Subdirectory containing subtransaction status data
`pg_tblspc` | Subdirectory containing symbolic links to tablespaces
`pg_twophase` | Subdirectory containing state files for prepared transactions
`pg_xlog` | Subdirectory containing WAL (Write Ahead Log) files
`postgresql.auto.conf` | A file used for storing configuration parameters that are set by `ALTER SYSTEM`
`postmaster.opts` | A file recording the command-line options the server was last started with
`postmaster.pid` | A lock file recording the current postmaster process ID (PID), cluster data directory path, postmaster start timestamp, port number, Unix-domain socket directory path (empty on Windows), first valid listen_address (IP address or `*`, or empty if not listening on TCP), and shared memory segment ID (this file is not present after server shutdown)

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.

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 65.3), 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 65.4. 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 65.5).

### Caution

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 65.2 for more information.

The contents of tables and indexes are discussed further in Section 65.6.

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.