query. We could reduce that by specifying a larger signature length.

In this example, creating the index with <literal>length=200</>

reduced the number of false positives to 55; but it doubled the index size

</para>

<para>

</programlisting>

Although this query runs much faster than with either of the single

indexes, we pay a large penalty in index size. Each of the single-column

more than 8 times the space used by the bloom index.

</para>

</sect2>

segments or <quote>pages</> small enough to be conveniently stored as rows

in <structname>pg_largeobject</structname>.

The amount of data per page is defined to be <symbol>LOBLKSIZE</> (which is currently

</para>

<para>

cache, where performance might degrade. This setting may have no

effect on some platforms. The valid range is between

<literal>0</literal>, which disables controlled writeback, and

<literal>0</> elsewhere. (Non-default values of

<symbol>BLCKSZ</symbol> change the default and maximum.)

This parameter can only be set in the <filename>postgresql.conf</>

than the OS's page cache, where performance might degrade. This

setting may have no effect on some platforms. The valid range is

between <literal>0</literal>, which disables controlled writeback,

Linux, <literal>0</> elsewhere. (Non-default values of

<symbol>BLCKSZ</symbol> change the default and maximum.)

This parameter can only be set in the <filename>postgresql.conf</>

checkpoints. This is a soft limit; WAL size can exceed

<varname>max_wal_size</> under special circumstances, like

under heavy load, a failing <varname>archive_command</>, or a high

Increasing this parameter can increase the amount of time needed for

crash recovery.

This parameter can only be set in the <filename>postgresql.conf</>

always recycled for future use at a checkpoint, rather than removed.

This can be used to ensure that enough WAL space is reserved to

handle spikes in WAL usage, for example when running large batch

This parameter can only be set in the <filename>postgresql.conf</>

file or on the server command line.

</para>

the physical requirement on disk might be less. Very long values are also

stored in background tables so that they do not interfere with rapid

access to shorter column values. In any case, the longest

maximum value that will be allowed for <replaceable>n</> in the data

type declaration is less than that. It wouldn't be useful to

change this because with multibyte character encodings the number of

<term><literal>sqlilongdata</></term>

<listitem>

<para>

</para>

</listitem>

</varlistentry>

<para>

Also check that you have sufficient disk space. You will need about

the installation directory. An empty database cluster takes about

flat text file with the same data would take. If you are going to

run the regression tests you will temporarily need up to an extra

space.

</para>

</sect1>

createlang: language installation failed: ERROR: could not load library "/opt/dbs/pgsql748/lib/plperl.so": Bad address

</screen>

Another example is out of memory errors in the PostgreSQL server

failing.

</para>

and memory model used by the server process. By default, all

binaries built on AIX are 32-bit. This does not depend upon

hardware type or kernel in use. These 32-bit processes are

heap as it shares a single segment with the stack.

</para>

settings to find a configuration that works satisfactorily. This

use of <symbol>LDR_CNTRL</symbol> tells AIX that you want the

server to have <symbol>MAXDATA</symbol> bytes set aside for the

configuration,

<command>ldedit</command> can be used to modify the binaries so

that they default to using the desired heap size. PostgreSQL can

larger than a single database page into a secondary storage area per table.

This makes the large object facility partially obsolete. One

remaining advantage of the large object facility is that it allows values

done efficiently, while most operations on a <acronym>TOAST</acronym>ed

field will read or write the whole value as a unit.

</para>

A <firstterm>label path</firstterm> is a sequence of zero or more

labels separated by dots, for example <literal>L1.L2.L3</>, representing

a path from the root of a hierarchical tree to a particular node. The

preferable. In practice this is not a major limitation; for example,

the longest label path in the DMOZ catalog (<ulink

url="http://www.dmoz.org"></ulink>) is about 240 bytes.

Limit (throttle) the maximum amount of data transferred from server

to client per unit of time. The expected unit is kilobytes per second.

If this option is specified, the value must either be equal to zero

If zero is passed or the option is not specified, no restriction is

imposed on the transfer.

</para>

</term>

<listitem>

<para>

default maximum total size is 2097152

pages. A page is almost always 4096 bytes except in unusual

kernel configurations with <quote>huge pages</quote>

<para>

The shared memory size settings can be changed via the

<screen>

<prompt>$</prompt> <userinput>sysctl -w kernel.shmmax=17179869184</userinput>

<prompt>$</prompt> <userinput>sysctl -w kernel.shmall=4194304</userinput>

</para>

<para>

</para>

<para>

</term>

<listitem>

<para>

segment is allowed. To increase the setting, first change to the

directory <filename>/etc/conf/cf.d</>. To display the current value of

<varname>SHMMAX</>, run:

<listitem>

<para>

On <productname>UnixWare</> 7, the maximum size for shared

To display the current value of <varname>SHMMAX</>, run:

<programlisting>

/etc/conf/bin/idtune -g SHMMAX

<para>

Individual leaf tuples and inner tuples must fit on a single index page

data types, long values can only be supported by methods such as radix

trees, in which each level of the tree includes a prefix that is short

enough to fit on a page, and the final leaf level includes a suffix also

</caution>

<para>

<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.

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

<para>

<productname>PostgreSQL</productname> uses a fixed page size (commonly

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

<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

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 of that byte give the total datum size

(including length byte) in bytes. This alternative supports space-efficient

storage of values shorter than 127 bytes, while still allowing the data type

any particular boundary, whereas values with four-byte headers are aligned on

at least a four-byte boundary; this omission of alignment padding provides

additional space savings that is significant compared to short values.

<para>

The <acronym>TOAST</> management code is triggered only

when a row value to be stored in a table is wider than

The <acronym>TOAST</> code will compress and/or move

field values out-of-line until the row value is shorter than

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

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

</para>

</sect2>

Indirect <acronym>TOAST</> pointers simply point at a non-indirect varlena

value stored somewhere in memory. This case was originally created merely

as a proof of concept, but it is currently used during logical decoding to

all out-of-line field values into the tuple might do). The case is of

limited use since the creator of the pointer datum is entirely responsible

that the referenced data survives for as long as the pointer could exist,

<para>

Every table and index is stored as an array of <firstterm>pages</> of a

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</>

linkend="guc-checkpoint-timeout"> seconds, or if

<xref linkend="guc-max-wal-size"> is about to be exceeded,

whichever comes first.

If no WAL has been written since the previous checkpoint, new checkpoints

will be skipped even if <varname>checkpoint_timeout</> has passed.

(If WAL archiving is being used and you want to put a lower limit on how

<para>

<acronym>WAL</acronym> logs are stored in the directory

<filename>pg_xlog</filename> under the data directory, as a set of

by altering the <option>--with-wal-segsize</> configure option when

building the server). Each segment is divided into pages, normally

configure option). The log record headers are described in

<filename>access/xlogrecord.h</filename>; the record content is dependent

on the type of event that is being logged. Segment files are given