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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>35.2. Implementation Features</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="lo-intro.html" title="35.1. Introduction" /><link rel="next" href="lo-interfaces.html" title="35.3. Client Interfaces" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">35.2. Implementation Features</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="lo-intro.html" title="35.1. Introduction">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="largeobjects.html" title="Chapter 35. Large Objects">Up</a></td><th width="60%" align="center">Chapter 35. Large Objects</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 11.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="lo-interfaces.html" title="35.3. Client Interfaces">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="LO-IMPLEMENTATION"><div class="titlepage"><div><div><h2 class="title" style="clear: both">35.2. Implementation Features</h2></div></div></div><p>
The large object implementation breaks large
objects up into <span class="quote">“<span class="quote">chunks</span>”</span> and stores the chunks in
rows in the database. A B-tree index guarantees fast
searches for the correct chunk number when doing random
access reads and writes.
</p><p>
The chunks stored for a large object do not have to be contiguous.
For example, if an application opens a new large object, seeks to offset
1000000, and writes a few bytes there, this does not result in allocation
of 1000000 bytes worth of storage; only of chunks covering the range of
data bytes actually written. A read operation will, however, read out
zeroes for any unallocated locations preceding the last existing chunk.
This corresponds to the common behavior of <span class="quote">“<span class="quote">sparsely allocated</span>”</span>
files in <acronym class="acronym">Unix</acronym> file systems.
</p><p>
As of <span class="productname">PostgreSQL</span> 9.0, large objects have an owner
and a set of access permissions, which can be managed using
<a class="xref" href="sql-grant.html" title="GRANT"><span class="refentrytitle">GRANT</span></a> and
<a class="xref" href="sql-revoke.html" title="REVOKE"><span class="refentrytitle">REVOKE</span></a>.
<code class="literal">SELECT</code> privileges are required to read a large
object, and
<code class="literal">UPDATE</code> privileges are required to write or
truncate it.
Only the large object's owner (or a database superuser) can delete,
comment on, or change the owner of a large object.
To adjust this behavior for compatibility with prior releases, see the
<a class="xref" href="runtime-config-compatible.html#GUC-LO-COMPAT-PRIVILEGES">lo_compat_privileges</a> run-time parameter.
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