PostgreSQL 8.1.3 Documentation
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# Chapter 50. Database Physical Storage

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

# 50.1. Database File Layout

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

All the data needed for a database cluster is stored 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 postmasters, can exist on the same machine.

The `PGDATA` directory contains several subdirectories and control files, as shown in Table 50-1. 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 in PostgreSQL 8.0 and later, it is possible to keep them elsewhere).

Table 50-1. 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_clog` | Subdirectory containing transaction commit status data
`pg_multixact` | Subdirectory containing multitransaction status data (used for shared row locks)
`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
`postmaster.opts` | A file recording the command-line options the postmaster was last started with
`postmaster.pid` | A lock file recording the current postmaster PID and shared memory segment ID (not present after postmaster 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, named after the table or index's filenode number, which can be found in `pg_class`.`relfilenode`.

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.

When a table or index exceeds 1Gb, 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. The contents of tables and indexes are discussed further in Section 50.3.

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 50.2 for more information.

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 (as specified in its `CREATE TABLESPACE` command). The symbolic link is named after the tablespace's OID. Inside the physical tablespace directory there is a subdirectory for each database that has elements in the tablespace, named after the database's OID. Tables within that directory follow 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`.

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