<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <HTML ><HEAD ><TITLE >Implementation Features</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK REV="MADE" HREF="mailto:pgsql-docs@postgresql.org"><LINK REL="HOME" TITLE="PostgreSQL 9.6.4 Documentation" HREF="index.html"><LINK REL="UP" TITLE="Large Objects" HREF="largeobjects.html"><LINK REL="PREVIOUS" TITLE="Introduction" HREF="lo-intro.html"><LINK REL="NEXT" TITLE="Client Interfaces" HREF="lo-interfaces.html"><LINK REL="STYLESHEET" TYPE="text/css" HREF="stylesheet.css"><META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=ISO-8859-1"><META NAME="creation" CONTENT="2017-08-11T02:27:18"></HEAD ><BODY CLASS="SECT1" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="4" ALIGN="center" VALIGN="bottom" ><A HREF="index.html" >PostgreSQL 9.6.4 Documentation</A ></TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A TITLE="Introduction" HREF="lo-intro.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A HREF="largeobjects.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="60%" ALIGN="center" VALIGN="bottom" >Chapter 33. Large Objects</TD ><TD WIDTH="20%" ALIGN="right" VALIGN="top" ><A TITLE="Client Interfaces" HREF="lo-interfaces.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="SECT1" ><H1 CLASS="SECT1" ><A NAME="LO-IMPLEMENTATION" >33.2. Implementation Features</A ></H1 ><P > The large object implementation breaks large objects up into <SPAN CLASS="QUOTE" >"chunks"</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" >"sparsely allocated"</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 HREF="sql-grant.html" >GRANT</A > and <A HREF="sql-revoke.html" >REVOKE</A >. <TT CLASS="LITERAL" >SELECT</TT > privileges are required to read a large object, and <TT CLASS="LITERAL" >UPDATE</TT > 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 HREF="runtime-config-compatible.html#GUC-LO-COMPAT-PRIVILEGES" >lo_compat_privileges</A > run-time parameter. </P ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="lo-intro.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="index.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="lo-interfaces.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Introduction</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="largeobjects.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Client Interfaces</TD ></TR ></TABLE ></DIV ></BODY ></HTML >