<!-- doc/src/sgml/storage.sgml -->

<chapter id="storage">

<title>Database Physical Storage</title>

<para>
This chapter provides an overview of the physical storage format used by
<productname>PostgreSQL</productname> databases.
</para>

<sect1 id="storage-file-layout">

<title>Database File Layout</title>

<para>
This section describes the storage format at the level of files and
directories.
</para>

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

<para>
The <varname>PGDATA</> directory contains several subdirectories and control
files, as shown in <xref linkend="pgdata-contents-table">.  In addition to
these required items, the cluster configuration files
<filename>postgresql.conf</filename>, <filename>pg_hba.conf</filename>, and
<filename>pg_ident.conf</filename> are traditionally stored in
<varname>PGDATA</> (although in <productname>PostgreSQL</productname> 8.0 and
later, it is possible to place them elsewhere).
</para>

<table tocentry="1" id="pgdata-contents-table">
<title>Contents of <varname>PGDATA</></title>
<tgroup cols="2">
<thead>
<row>
<entry>
Item
</entry>
<entry>Description</entry>
</row>
</thead>

<tbody>

<row>
 <entry><filename>PG_VERSION</></entry>
 <entry>A file containing the major version number of <productname>PostgreSQL</productname></entry>
</row>

<row>
 <entry><filename>base</></entry>
 <entry>Subdirectory containing per-database subdirectories</entry>
</row>

<row>
 <entry><filename>global</></entry>
 <entry>Subdirectory containing cluster-wide tables, such as
 <structname>pg_database</></entry>
</row>

<row>
 <entry><filename>pg_clog</></entry>
 <entry>Subdirectory containing transaction commit status data</entry>
</row>

<row>
 <entry><filename>pg_multixact</></entry>
 <entry>Subdirectory containing multitransaction status data
  (used for shared row locks)</entry>
</row>

<row>
 <entry><filename>pg_notify</></entry>
 <entry>Subdirectory containing LISTEN/NOTIFY status data</entry>
</row>

<row>
 <entry><filename>pg_serial</></entry>
 <entry>Subdirectory containing information about committed serializable transactions</entry>
</row>

<row>
 <entry><filename>pg_snapshots</></entry>
 <entry>Subdirectory containing exported snapshots</entry>
</row>

<row>
 <entry><filename>pg_stat_tmp</></entry>
 <entry>Subdirectory containing temporary files for the statistics
  subsystem</entry>
</row>

<row>
 <entry><filename>pg_subtrans</></entry>
 <entry>Subdirectory containing subtransaction status data</entry>
</row>

<row>
 <entry><filename>pg_tblspc</></entry>
 <entry>Subdirectory containing symbolic links to tablespaces</entry>
</row>

<row>
 <entry><filename>pg_twophase</></entry>
 <entry>Subdirectory containing state files for prepared transactions</entry>
</row>

<row>
 <entry><filename>pg_xlog</></entry>
 <entry>Subdirectory containing WAL (Write Ahead Log) files</entry>
</row>

<row>
 <entry><filename>postmaster.opts</></entry>
 <entry>A file recording the command-line options the server was
last started with</entry>
</row>

<row>
 <entry><filename>postmaster.pid</></entry>
 <entry>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 <literal>*</>, or empty if
  not listening on TCP),
  and shared memory segment ID
  (this file is not present after server shutdown)</entry>
</row>

</tbody>
</tgroup>
</table>

<para>
For each database in the cluster there is a subdirectory within
<varname>PGDATA</><filename>/base</>, named after the database's OID in
<structname>pg_database</>.  This subdirectory is the default location
for the database's files; in particular, its system catalogs are stored
there.
</para>

<para>
Each table and index is stored in a separate file.  For ordinary relations,
these files are named after the table or index's <firstterm>filenode</> number,
which can be found in <structname>pg_class</>.<structfield>relfilenode</>. But
for temporary relations, the file name is of the form
<literal>t<replaceable>BBB</>_<replaceable>FFF</></>, where <replaceable>BBB</>
is the backend ID of the backend which created the file, and <replaceable>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 <firstterm>free space map</> (see <xref
linkend="storage-fsm">), 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 <literal>_fsm</>.  Tables also have a
<firstterm>visibility map</>, stored in a fork with the suffix <literal>_vm</>,
to track which pages are known to have no dead tuples.  The visibility map is
described further in <xref linkend="storage-vm">.  Unlogged tables and indexes
have a third fork, known as the initialization fork, which is stored in a fork
with the suffix <literal>_init</literal> (see <xref linkend="storage-init">).
</para>

<caution>
<para>
Note that while a table's filenode often matches its OID, this is
<emphasis>not</> necessarily the case; some operations, like
<command>TRUNCATE</>, <command>REINDEX</>, <command>CLUSTER</> and some forms
of <command>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 <structname>pg_class</> itself,
<structname>pg_class</>.<structfield>relfilenode</> contains zero.  The
actual filenode number of these catalogs is stored in a lower-level data
structure, and can be obtained using the <function>pg_relation_filenode()</>
function.
</para>
</caution>

<para>
When a table or index exceeds 1 GB, it is divided into gigabyte-sized
<firstterm>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 <option>--with-segsize</option>
when building <productname>PostgreSQL</>.)
In principle, free space map and visibility map forks could require multiple
segments as well, though this is unlikely to happen in practice.
</para>

<para>
A table that has columns with potentially large entries will have an
associated <firstterm>TOAST</> table, which is used for out-of-line storage of
field values that are too large to keep in the table rows proper.
<structname>pg_class</>.<structfield>reltoastrelid</> links from a table to
its <acronym>TOAST</> table, if any.
See <xref linkend="storage-toast"> for more information.
</para>

<para>
The contents of tables and indexes are discussed further in
<xref linkend="storage-page-layout">.
</para>

<para>
Tablespaces make the scenario more complicated.  Each user-defined tablespace
has a symbolic link inside the <varname>PGDATA</><filename>/pg_tblspc</>
directory, which points to the physical tablespace directory (i.e., the
location specified in the tablespace's <command>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 <productname>PostgreSQL</>
server version, such as <literal>PG_9.0_201008051</>.  (The reason for using
this subdirectory is so that successive versions of the database can use
the same <command>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 <literal>pg_default</> tablespace is not accessed through
<filename>pg_tblspc</>, but corresponds to
<varname>PGDATA</><filename>/base</>.  Similarly, the <literal>pg_global</>
tablespace is not accessed through <filename>pg_tblspc</>, but corresponds to
<varname>PGDATA</><filename>/global</>.
</para>

<para>
The <function>pg_relation_filepath()</> function shows the entire path
(relative to <varname>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 &mdash; you may need to append a segment number
and/or <literal>_fsm</> or <literal>_vm</> to find all the files associated
with the relation.
</para>

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

</sect1>

<sect1 id="storage-toast">

<title>TOAST</title>

    <indexterm>
     <primary>TOAST</primary>
    </indexterm>
    <indexterm><primary>sliced bread</><see>TOAST</></indexterm>

<para>
This section provides an overview of <acronym>TOAST</> (The
Oversized-Attribute Storage Technique).
</para>

<para>
<productname>PostgreSQL</productname> uses a fixed page size (commonly
8 kB), and does not allow tuples to span multiple pages.  Therefore,  it is
not possible to store very large field values directly.  To overcome
this limitation, large  field values are compressed and/or broken up into
multiple physical rows. This happens transparently to the user, with only
small impact on most of the backend code.  The technique is affectionately
known as <acronym>TOAST</>  (or <quote>the best thing since sliced bread</>).
</para>

<para>
Only certain data types support <acronym>TOAST</> &mdash; there is no need to
impose the overhead on data types that cannot produce large field values.
To support <acronym>TOAST</>, a data type must have a variable-length
(<firstterm>varlena</>) representation, in which the first 32-bit word of any
stored value contains the total length of the value in bytes (including
itself).  <acronym>TOAST</> does not constrain the rest of the representation.
All the C-level functions supporting a <acronym>TOAST</>-able data type must
be careful to handle <acronym>TOAST</>ed input values.  (This is normally done
by invoking <function>PG_DETOAST_DATUM</> before doing anything with an input
value, but in some cases more efficient approaches are possible.)
</para>

<para>
<acronym>TOAST</> usurps two bits of the varlena length word (the high-order
bits on big-endian machines, the low-order bits on little-endian machines),
thereby limiting the logical size of any value of a <acronym>TOAST</>-able
data type to 1 GB (2<superscript>30</> - 1 bytes).  When both bits are zero,
the value is an ordinary un-<acronym>TOAST</>ed value of the data type, and
the remaining bits of the length word give the total datum size (including
length word) in bytes.  When the highest-order or lowest-order bit is set,
the value has only a single-byte header instead of the normal four-byte
header, and the remaining bits give the total datum size (including length
byte) in bytes.  As a special case, if the remaining bits are all zero
(which would be impossible for a self-inclusive length), the value is a
pointer to out-of-line data stored in a separate TOAST table.  (The size of
a TOAST pointer is given in the second byte of the datum.)
Values with single-byte headers aren't aligned on any particular
boundary, either.  Lastly, when the highest-order or lowest-order bit is
clear but the adjacent bit is set, the content of the datum has been
compressed and must be decompressed before use.  In this case the remaining
bits of the length word give the total size of the compressed datum, not the
original data.  Note that compression is also possible for out-of-line data
but the varlena header does not tell whether it has occurred &mdash;
the content of the TOAST pointer tells that, instead.
</para>

<para>
If any of the columns of a table are <acronym>TOAST</>-able, the table will
have an associated <acronym>TOAST</> table, whose OID is stored in the table's
<structname>pg_class</>.<structfield>reltoastrelid</> entry.  Out-of-line
<acronym>TOAST</>ed values are kept in the <acronym>TOAST</> table, as
described in more detail below.
</para>

<para>
The compression technique used is a fairly simple and very fast member
of the LZ family of compression techniques.  See
<filename>src/backend/utils/adt/pg_lzcompress.c</> for the details.
</para>

<para>
Out-of-line values are divided (after compression if used) into chunks of at
most <symbol>TOAST_MAX_CHUNK_SIZE</> bytes (by default this value is chosen
so that four chunk rows will fit on a page, making it about 2000 bytes).
Each chunk is stored
as a separate row in the <acronym>TOAST</> table for the owning table.  Every
<acronym>TOAST</> table has the columns <structfield>chunk_id</> (an OID
identifying the particular <acronym>TOAST</>ed value),
<structfield>chunk_seq</> (a sequence number for the chunk within its value),
and <structfield>chunk_data</> (the actual data of the chunk).  A unique index
on <structfield>chunk_id</> and <structfield>chunk_seq</> provides fast
retrieval of the values.  A pointer datum representing an out-of-line
<acronym>TOAST</>ed value therefore needs to store the OID of the
<acronym>TOAST</> table in which to look and the OID of the specific value
(its <structfield>chunk_id</>).  For convenience, pointer datums also store the
logical datum size (original uncompressed data length) and actual stored size
(different if compression was applied).  Allowing for the varlena header bytes,
the total size of a <acronym>TOAST</> pointer datum is therefore 18 bytes
regardless of the actual size of the represented value.
</para>

<para>
The <acronym>TOAST</> code is triggered only
when a row value to be stored in a table is wider than
<symbol>TOAST_TUPLE_THRESHOLD</> bytes (normally 2 kB).
The <acronym>TOAST</> code will compress and/or move
field values out-of-line until the row value is shorter than
<symbol>TOAST_TUPLE_TARGET</> bytes (also normally 2 kB)
or no more gains can be had.  During an UPDATE
operation, values of unchanged fields are normally preserved as-is; so an
UPDATE of a row with out-of-line values incurs no <acronym>TOAST</> costs if
none of the out-of-line values change.
</para>

<para>
The <acronym>TOAST</> code recognizes four different strategies for storing
<acronym>TOAST</>-able columns:

   <itemizedlist>
    <listitem>
     <para>
      <literal>PLAIN</literal> prevents either compression or
      out-of-line storage; furthermore it disables use of single-byte headers
      for varlena types.
      This is the only possible strategy for
      columns of non-<acronym>TOAST</>-able data types.
     </para>
    </listitem>
    <listitem>
     <para>
      <literal>EXTENDED</literal> allows both compression and out-of-line
      storage.  This is the default for most <acronym>TOAST</>-able data types.
      Compression will be attempted first, then out-of-line storage if
      the row is still too big.
     </para>
    </listitem>
    <listitem>
     <para>
      <literal>EXTERNAL</literal> allows out-of-line storage but not
      compression.  Use of <literal>EXTERNAL</literal> will
      make substring operations on wide <type>text</type> and
      <type>bytea</type> columns faster (at the penalty of increased storage
      space) because these operations are optimized to fetch only the
      required parts of the out-of-line value when it is not compressed.
     </para>
    </listitem>
    <listitem>
     <para>
      <literal>MAIN</literal> allows compression but not out-of-line
      storage.  (Actually, out-of-line storage will still be performed
      for such columns, but only as a last resort when there is no other
      way to make the row small enough to fit on a page.)
     </para>
    </listitem>
   </itemizedlist>

Each <acronym>TOAST</>-able data type specifies a default strategy for columns
of that data type, but the strategy for a given table column can be altered
with <command>ALTER TABLE SET STORAGE</>.
</para>

<para>
This scheme has a number of advantages compared to a more straightforward
approach such as allowing row values to span pages.  Assuming that queries are
usually qualified by comparisons against relatively small key values, most of
the work of the executor will be done using the main row entry. The big values
of <acronym>TOAST</>ed attributes will only be pulled out (if selected at all)
at the time the result set is sent to the client. Thus, the main table is much
smaller and more of its rows fit in the shared buffer cache than would be the
case without any out-of-line storage. Sort sets shrink also, and sorts will
more often be done entirely in memory. A little test showed that a table
containing typical HTML pages and their URLs was stored in about half of the
raw data size including the <acronym>TOAST</> table, and that the main table
contained only about 10% of the entire data (the URLs and some small HTML
pages). There was no run time difference compared to an un-<acronym>TOAST</>ed
comparison table, in which all the HTML pages were cut down to 7 kB to fit.
</para>

</sect1>

<sect1 id="storage-fsm">

<title>Free Space Map</title>

<indexterm>
 <primary>Free Space Map</primary>
</indexterm>
<indexterm><primary>FSM</><see>Free Space Map</></indexterm>

<para>
Each heap and index relation, except for hash indexes, has a Free Space Map
(FSM) to keep track of available space in the relation. It's stored
alongside the main relation data in a separate relation fork, named after the
filenode number of the relation, plus a <literal>_fsm</> suffix. For example,
if the filenode of a relation is 12345, the FSM is stored in a file called
<filename>12345_fsm</>, in the same directory as the main relation file.
</para>

<para>
The Free Space Map is organized as a tree of <acronym>FSM</> pages. The
bottom level <acronym>FSM</> pages store the free space available on each
heap (or index) page, using one byte to represent each such page. The upper
levels aggregate information from the lower levels.
</para>

<para>
Within each <acronym>FSM</> page is a binary tree, stored in an array with
one byte per node. Each leaf node represents a heap page, or a lower level
<acronym>FSM</> page. In each non-leaf node, the higher of its children's
values is stored. The maximum value in the leaf nodes is therefore stored
at the root.
</para>

<para>
See <filename>src/backend/storage/freespace/README</> for more details on
how the <acronym>FSM</> is structured, and how it's updated and searched.
The <xref linkend="pgfreespacemap"> module
can be used to examine the information stored in free space maps.
</para>

</sect1>

<sect1 id="storage-vm">

<title>Visibility Map</title>

<indexterm>
 <primary>Visibility Map</primary>
</indexterm>
<indexterm><primary>VM</><see>Visibility Map</></indexterm>

<para>
Each heap relation has a Visibility Map
(VM) to keep track of which pages contain only tuples that are known to be
visible to all active transactions. It's stored
alongside the main relation data in a separate relation fork, named after the
filenode number of the relation, plus a <literal>_vm</> suffix. For example,
if the filenode of a relation is 12345, the VM is stored in a file called
<filename>12345_vm</>, in the same directory as the main relation file.
Note that indexes do not have VMs.
</para>

<para>
The visibility map simply stores one bit per heap page. A set bit means
that all tuples on the page are known to be visible to all transactions.
This means that the page does not contain any tuples that need to be vacuumed.
This information can also be used by <firstterm>index-only scans</> to answer
queries using only the index tuple.
</para>

<para>
The map is conservative in the sense that we make sure that whenever a bit is
set, we know the condition is true, but if a bit is not set, it might or
might not be true. Visibility map bits are only set by vacuum, but are
cleared by any data-modifying operations on a page.
</para>

</sect1>

<sect1 id="storage-init">

<title>The Initialization Fork</title>

<indexterm>
 <primary>Initialization Fork</primary>
</indexterm>

<para>
Each unlogged table, and each index on an unlogged table, has an initialization
fork.  The initialization fork is an empty table or index of the appropriate
type.  When an unlogged table must be reset to empty due to a crash, the
initialization fork is copied over the main fork, and any other forks are
erased (they will be recreated automatically as needed).
</para>

</sect1>

<sect1 id="storage-page-layout">

<title>Database Page Layout</title>

<para>
This section provides an overview of the page format used within
<productname>PostgreSQL</productname> tables and indexes.<footnote>
  <para>
    Actually, index access methods need not use this page format.
    All the existing index methods do use this basic format,
    but the data kept on index metapages usually doesn't follow
    the item layout rules.
  </para>
</footnote>
Sequences and <acronym>TOAST</> tables are formatted just like a regular table.
</para>

<para>
In the following explanation, a
<firstterm>byte</firstterm>
is assumed to contain 8 bits.  In addition, the term
<firstterm>item</firstterm>
refers to an individual data value that is stored on a page.  In a table,
an item is a row; in an index, an item is an index entry.
</para>

<para>
Every table and index is stored as an array of <firstterm>pages</> of a
fixed size (usually 8 kB, although a different page size can be selected
when compiling the server).  In a table, all the pages are logically
equivalent, so a particular item (row) can be stored in any page.  In
indexes, the first page is generally reserved as a <firstterm>metapage</>
holding control information, and there can be different types of pages
within the index, depending on the index access method.
</para>

<para>
<xref linkend="page-table"> shows the overall layout of a page.
There are five parts to each page.
</para>

<table tocentry="1" id="page-table">
<title>Overall Page Layout</title>
<titleabbrev>Page Layout</titleabbrev>
<tgroup cols="2">
<thead>
<row>
<entry>
Item
</entry>
<entry>Description</entry>
</row>
</thead>

<tbody>

<row>
 <entry>PageHeaderData</entry>
 <entry>24 bytes long. Contains general information about the page, including
free space pointers.</entry>
</row>

<row>
<entry>ItemIdData</entry>
<entry>Array of (offset,length) pairs pointing to the actual items.
4 bytes per item.</entry>
</row>

<row>
<entry>Free space</entry>
<entry>The unallocated space. New item pointers are allocated from the start
of this area, new items from the end.</entry>
</row>

<row>
<entry>Items</entry>
<entry>The actual items themselves.</entry>
</row>

<row>
<entry>Special space</entry>
<entry>Index access method specific data. Different methods store different
data. Empty in ordinary tables.</entry>
</row>

</tbody>
</tgroup>
</table>

 <para>

  The first 24 bytes of each page consists of a page header
  (PageHeaderData). Its format is detailed in <xref
  linkend="pageheaderdata-table">. The first two fields track the most
  recent WAL entry related to this page. Next is a 2-byte field
  containing flag bits. This is followed by three 2-byte integer fields
  (<structfield>pd_lower</structfield>, <structfield>pd_upper</structfield>,
  and <structfield>pd_special</structfield>). These contain byte offsets
  from the page start to the start
  of unallocated space, to the end of unallocated space, and to the start of
  the special space.
  The next 2 bytes of the page header,
  <structfield>pd_pagesize_version</structfield>, store both the page size
  and a version indicator.  Beginning with
  <productname>PostgreSQL</productname> 8.3 the version number is 4;
  <productname>PostgreSQL</productname> 8.1 and 8.2 used version number 3;
  <productname>PostgreSQL</productname> 8.0 used version number 2;
  <productname>PostgreSQL</productname> 7.3 and 7.4 used version number 1;
  prior releases used version number 0.
  (The basic page layout and header format has not changed in most of these
  versions, but the layout of heap row headers has.)  The page size
  is basically only present as a cross-check; there is no support for having
  more than one page size in an installation.
  The last field is a hint that shows whether pruning the page is likely
  to be profitable: it tracks the oldest un-pruned XMAX on the page.

 </para>

 <table tocentry="1" id="pageheaderdata-table">
 <title>PageHeaderData Layout</title>
 <titleabbrev>PageHeaderData Layout</titleabbrev>
 <tgroup cols="4">
 <thead>
  <row>
   <entry>Field</entry>
   <entry>Type</entry>
   <entry>Length</entry>
   <entry>Description</entry>
  </row>
 </thead>
 <tbody>
  <row>
   <entry>pd_lsn</entry>
   <entry>XLogRecPtr</entry>
   <entry>8 bytes</entry>
   <entry>LSN: next byte after last byte of xlog record for last change
   to this page</entry>
  </row>
  <row>
   <entry>pd_tli</entry>
   <entry>uint16</entry>
   <entry>2 bytes</entry>
   <entry>TimeLineID of last change (only its lowest 16 bits)</entry>
  </row>
  <row>
   <entry>pd_flags</entry>
   <entry>uint16</entry>
   <entry>2 bytes</entry>
   <entry>Flag bits</entry>
  </row>
  <row>
   <entry>pd_lower</entry>
   <entry>LocationIndex</entry>
   <entry>2 bytes</entry>
   <entry>Offset to start of free space</entry>
  </row>
  <row>
   <entry>pd_upper</entry>
   <entry>LocationIndex</entry>
   <entry>2 bytes</entry>
   <entry>Offset to end of free space</entry>
  </row>
  <row>
   <entry>pd_special</entry>
   <entry>LocationIndex</entry>
   <entry>2 bytes</entry>
   <entry>Offset to start of special space</entry>
  </row>
  <row>
   <entry>pd_pagesize_version</entry>
   <entry>uint16</entry>
   <entry>2 bytes</entry>
   <entry>Page size and layout version number information</entry>
  </row>
  <row>
   <entry>pd_prune_xid</entry>
   <entry>TransactionId</entry>
   <entry>4 bytes</entry>
   <entry>Oldest unpruned XMAX on page, or zero if none</entry>
  </row>
 </tbody>
 </tgroup>
 </table>

 <para>
  All the details can be found in
  <filename>src/include/storage/bufpage.h</filename>.
 </para>

 <para>

  Following the page header are item identifiers
  (<type>ItemIdData</type>), each requiring four bytes.
  An item identifier contains a byte-offset to
  the start of an item, its length in bytes, and a few attribute bits
  which affect its interpretation.
  New item identifiers are allocated
  as needed from the beginning of the unallocated space.
  The number of item identifiers present can be determined by looking at
  <structfield>pd_lower</>, which is increased to allocate a new identifier.
  Because an item
  identifier is never moved until it is freed, its index can be used on a
  long-term basis to reference an item, even when the item itself is moved
  around on the page to compact free space.  In fact, every pointer to an
  item (<type>ItemPointer</type>, also known as
  <type>CTID</type>) created by
  <productname>PostgreSQL</productname> consists of a page number and the
  index of an item identifier.

 </para>

 <para>

  The items themselves are stored in space allocated backwards from the end
  of unallocated space.  The exact structure varies depending on what the
  table is to contain. Tables and sequences both use a structure named
  <type>HeapTupleHeaderData</type>, described below.

 </para>

 <para>

  The final section is the <quote>special section</quote> which can
 contain anything the access method wishes to store.  For example,
  b-tree indexes store links to the page's left and right siblings,
  as well as some other data relevant to the index structure.
  Ordinary tables do not use a special section at all (indicated by setting
  <structfield>pd_special</> to equal the page size).

 </para>

 <para>

  All table rows are structured in the same way. There is a fixed-size
  header (occupying 23 bytes on most machines), followed by an optional null
  bitmap, an optional object ID field, and the user data. The header is
  detailed
  in <xref linkend="heaptupleheaderdata-table">.  The actual user data
  (columns of the row) begins at the offset indicated by
  <structfield>t_hoff</>, which must always be a multiple of the MAXALIGN
  distance for the platform.
  The null bitmap is
  only present if the <firstterm>HEAP_HASNULL</firstterm> bit is set in
  <structfield>t_infomask</structfield>. If it is present it begins just after
  the fixed header and occupies enough bytes to have one bit per data column
  (that is, <structfield>t_natts</> bits altogether). In this list of bits, a
  1 bit indicates not-null, a 0 bit is a null.  When the bitmap is not
  present, all columns are assumed not-null.
  The object ID is only present if the <firstterm>HEAP_HASOID</firstterm> bit
  is set in <structfield>t_infomask</structfield>.  If present, it appears just
  before the <structfield>t_hoff</> boundary.  Any padding needed to make
  <structfield>t_hoff</> a MAXALIGN multiple will appear between the null
  bitmap and the object ID.  (This in turn ensures that the object ID is
  suitably aligned.)

 </para>

 <table tocentry="1" id="heaptupleheaderdata-table">
 <title>HeapTupleHeaderData Layout</title>
 <titleabbrev>HeapTupleHeaderData Layout</titleabbrev>
 <tgroup cols="4">
 <thead>
  <row>
   <entry>Field</entry>
   <entry>Type</entry>
   <entry>Length</entry>
   <entry>Description</entry>
  </row>
 </thead>
 <tbody>
  <row>
   <entry>t_xmin</entry>
   <entry>TransactionId</entry>
   <entry>4 bytes</entry>
   <entry>insert XID stamp</entry>
  </row>
  <row>
   <entry>t_xmax</entry>
   <entry>TransactionId</entry>
   <entry>4 bytes</entry>
   <entry>delete XID stamp</entry>
  </row>
  <row>
   <entry>t_cid</entry>
   <entry>CommandId</entry>
   <entry>4 bytes</entry>
   <entry>insert and/or delete CID stamp (overlays with t_xvac)</entry>
  </row>
  <row>
   <entry>t_xvac</entry>
   <entry>TransactionId</entry>
   <entry>4 bytes</entry>
   <entry>XID for VACUUM operation moving a row version</entry>
  </row>
  <row>
   <entry>t_ctid</entry>
   <entry>ItemPointerData</entry>
   <entry>6 bytes</entry>
   <entry>current TID of this or newer row version</entry>
  </row>
  <row>
   <entry>t_infomask2</entry>
   <entry>uint16</entry>
   <entry>2 bytes</entry>
   <entry>number of attributes, plus various flag bits</entry>
  </row>
  <row>
   <entry>t_infomask</entry>
   <entry>uint16</entry>
   <entry>2 bytes</entry>
   <entry>various flag bits</entry>
  </row>
  <row>
   <entry>t_hoff</entry>
   <entry>uint8</entry>
   <entry>1 byte</entry>
   <entry>offset to user data</entry>
  </row>
 </tbody>
 </tgroup>
 </table>

 <para>
   All the details can be found in
   <filename>src/include/access/htup.h</filename>.
 </para>

 <para>

  Interpreting the actual data can only be done with information obtained
  from other tables, mostly <structname>pg_attribute</structname>. The
  key values needed to identify field locations are
  <structfield>attlen</structfield> and <structfield>attalign</structfield>.
  There is no way to directly get a
  particular attribute, except when there are only fixed width fields and no
  null values. All this trickery is wrapped up in the functions
  <firstterm>heap_getattr</firstterm>, <firstterm>fastgetattr</firstterm>
  and <firstterm>heap_getsysattr</firstterm>.

 </para>
 <para>

  To read the data you need to examine each attribute in turn. First check
  whether the field is NULL according to the null bitmap. If it is, go to
  the next. Then make sure you have the right alignment.  If the field is a
  fixed width field, then all the bytes are simply placed. If it's a
  variable length field (attlen = -1) then it's a bit more complicated.
  All variable-length data types share the common header structure
  <type>struct varlena</type>, which includes the total length of the stored
  value and some flag bits.  Depending on the flags, the data can be either
  inline or in a <acronym>TOAST</> table;
  it might be compressed, too (see <xref linkend="storage-toast">).

 </para>
</sect1>

</chapter>
