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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>34.2. 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 V1.79.1" /><link rel="prev" href="lo-intro.html" title="34.1. Introduction" /><link rel="next" href="lo-interfaces.html" title="34.3. 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">34.2. Implementation Features</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="lo-intro.html" title="34.1. Introduction">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="largeobjects.html" title="Chapter 34. Large Objects">Up</a></td><th width="60%" align="center">Chapter 34. Large Objects</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 12.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="lo-interfaces.html" title="34.3. 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">34.2. 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.
- </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="lo-intro.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="largeobjects.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="lo-interfaces.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">34.1. Introduction </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 34.3. Client Interfaces</td></tr></table></div></body></html>
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