mirror of https://github.com/postgres/postgres
a Postgres database. Update page.sgml to match 8.0 tuple header layout.REL8_0_STABLE
Tom Lane
21 years ago
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$PostgreSQL: pgsql/doc/src/sgml/filelayout.sgml,v 1.1 2004/11/12 21:50:53 tgl Exp $ |
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--> |
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<chapter id="file-layout"> |
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<title>Database File Layout</title> |
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<abstract> |
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<para> |
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A description of the database physical storage layout. |
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</para> |
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</abstract> |
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<para> |
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This section provides an overview of the physical format used by |
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<productname>PostgreSQL</productname> databases. |
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</para> |
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<para> |
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All the data needed for a database cluster is stored within the cluster's data |
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directory, commonly referred to as <varname>PGDATA</> (after the name of the |
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environment variable that can be used to define it). A common location for |
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<varname>PGDATA</> is <filename>/var/lib/pgsql/data</>. Multiple clusters, |
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managed by different postmasters, can exist on the same machine. |
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</para> |
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<para> |
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The <varname>PGDATA</> directory contains several subdirectories and control |
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files, as shown in <xref linkend="pgdata-contents-table">. In addition to |
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these required items, the cluster configuration files |
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<filename>postgresql.conf</filename>, <filename>pg_hba.conf</filename>, and |
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<filename>pg_ident.conf</filename> are traditionally stored in |
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<varname>PGDATA</> (although beginning in |
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<productname>PostgreSQL</productname> 8.0 it is possible to keep them |
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elsewhere). |
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</para> |
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<table tocentry="1" id="pgdata-contents-table"> |
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<title>Contents of <varname>PGDATA</></title> |
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<tgroup cols="2"> |
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<thead> |
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<row> |
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<entry> |
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Item |
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</entry> |
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<entry>Description</entry> |
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</row> |
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</thead> |
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<tbody> |
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<row> |
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<entry><filename>PG_VERSION</></entry> |
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<entry>A file containing the major version number of <productname>PostgreSQL</productname></entry> |
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</row> |
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<row> |
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<entry><filename>base</></entry> |
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<entry>Subdirectory containing per-database subdirectories</entry> |
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</row> |
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<row> |
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<entry><filename>global</></entry> |
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<entry>Subdirectory containing cluster-wide tables, such as |
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<structname>pg_database</></entry> |
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</row> |
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<row> |
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<entry><filename>pg_clog</></entry> |
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<entry>Subdirectory containing transaction commit status data</entry> |
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</row> |
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<row> |
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<entry><filename>pg_subtrans</></entry> |
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<entry>Subdirectory containing subtransaction status data</entry> |
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</row> |
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<row> |
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<entry><filename>pg_tblspc</></entry> |
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<entry>Subdirectory containing symbolic links to tablespaces</entry> |
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</row> |
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<row> |
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<entry><filename>pg_xlog</></entry> |
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<entry>Subdirectory containing WAL (Write Ahead Log) files</entry> |
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</row> |
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<row> |
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<entry><filename>postmaster.opts</></entry> |
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<entry>A file recording the command-line options the postmaster was |
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last started with</entry> |
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</row> |
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<row> |
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<entry><filename>postmaster.pid</></entry> |
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<entry>A lock file recording the current postmaster PID and shared memory |
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segment ID (not present after postmaster shutdown)</entry> |
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</row> |
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</tbody> |
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</tgroup> |
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</table> |
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<para> |
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For each database in the cluster there is a subdirectory within |
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<varname>PGDATA</><filename>/base</>, named after the database's OID in |
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<structname>pg_database</>. This subdirectory is the default location |
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for the database's files; in particular, its system catalogs are stored |
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there. |
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</para> |
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<para> |
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Each table and index is stored in a separate file, named after the table |
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or index's <firstterm>filenode</> number, which can be found in |
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<structname>pg_class</>.<structfield>relfilenode</>. |
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</para> |
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<caution> |
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<para> |
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Note that while a table's filenode often matches its OID, this is |
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<emphasis>not</> necessarily the case; some operations, like |
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<command>TRUNCATE</>, <command>REINDEX</>, <command>CLUSTER</> and some forms |
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of <command>ALTER TABLE</>, can change the filenode while preserving the OID. |
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Avoid assuming that filenode and table OID are the same. |
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</para> |
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</caution> |
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<para> |
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When a table or index exceeds 1Gb, it is divided into gigabyte-sized |
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<firstterm>segments</>. The first segment's file name is the same as the |
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filenode; subsequent segments are named filenode.1, filenode.2, etc. |
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This arrangement avoids problems on platforms that have file size limitations. |
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The contents of tables and indexes are discussed further in |
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<xref linkend="page">. |
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</para> |
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<para> |
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A table that has columns with potentially large entries will have an |
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associated <firstterm>TOAST</> table, which is used for out-of-line storage of |
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field values that are too large to keep in the table rows proper. |
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<structname>pg_class</>.<structfield>reltoastrelid</> links from a table to |
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its TOAST table, if any. |
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</para> |
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<para> |
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Tablespaces make the scenario more complicated. Each non-default tablespace |
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has a symbolic link inside the <varname>PGDATA</><filename>/pg_tblspc</> |
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directory, which points to the physical tablespace directory (as specified in |
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its <command>CREATE TABLESPACE</> command). The symbolic link is named after |
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the tablespace's OID. Inside the physical tablespace directory there is |
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a subdirectory for each database that has elements in the tablespace, named |
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after the database's OID. Tables within that directory follow the filenode |
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naming scheme. The <literal>pg_default</> tablespace is not accessed through |
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<filename>pg_tblspc</>, but corresponds to |
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<varname>PGDATA</><filename>/base</>. Similarly, the <literal>pg_global</> |
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tablespace is not accessed through <filename>pg_tblspc</>, but corresponds to |
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<varname>PGDATA</><filename>/global</>. |
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</para> |
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</chapter> |
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