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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>71.1. Introduction</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="brin.html" title="Chapter 71. BRIN Indexes" /><link rel="next" href="brin-builtin-opclasses.html" title="71.2. Built-in Operator Classes" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">71.1. Introduction</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="brin.html" title="Chapter 71. BRIN Indexes">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="brin.html" title="Chapter 71. BRIN Indexes">Up</a></td><th width="60%" align="center">Chapter 71. BRIN Indexes</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.7 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="brin-builtin-opclasses.html" title="71.2. Built-in Operator Classes">Next</a></td></tr></table><hr /></div><div class="sect1" id="BRIN-INTRO"><div class="titlepage"><div><div><h2 class="title" style="clear: both">71.1. Introduction</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="brin-intro.html#BRIN-OPERATION">71.1.1. Index Maintenance</a></span></dt></dl></div><p>
<acronym class="acronym">BRIN</acronym> stands for Block Range Index.
<acronym class="acronym">BRIN</acronym> is designed for handling very large tables
in which certain columns have some natural correlation with their
physical location within the table.
</p><p>
<acronym class="acronym">BRIN</acronym> works in terms of <em class="firstterm">block ranges</em>
(or <span class="quote">“<span class="quote">page ranges</span>”</span>).
A block range is a group of pages that are physically
adjacent in the table; for each block range, some summary info is stored
by the index.
For example, a table storing a store's sale orders might have
a date column on which each order was placed, and most of the time
the entries for earlier orders will appear earlier in the table as well;
a table storing a ZIP code column might have all codes for a city
grouped together naturally.
</p><p>
<acronym class="acronym">BRIN</acronym> indexes can satisfy queries via regular bitmap
index scans, and will return all tuples in all pages within each range if
the summary info stored by the index is <em class="firstterm">consistent</em> with the
query conditions.
The query executor is in charge of rechecking these tuples and discarding
those that do not match the query conditions — in other words, these
indexes are lossy.
Because a <acronym class="acronym">BRIN</acronym> index is very small, scanning the index
adds little overhead compared to a sequential scan, but may avoid scanning
large parts of the table that are known not to contain matching tuples.
</p><p>
The specific data that a <acronym class="acronym">BRIN</acronym> index will store,
as well as the specific queries that the index will be able to satisfy,
depend on the operator class selected for each column of the index.
Data types having a linear sort order can have operator classes that
store the minimum and maximum value within each block range, for instance;
geometrical types might store the bounding box for all the objects
in the block range.
</p><p>
The size of the block range is determined at index creation time by
the <code class="literal">pages_per_range</code> storage parameter. The number of index
entries will be equal to the size of the relation in pages divided by
the selected value for <code class="literal">pages_per_range</code>. Therefore, the smaller
the number, the larger the index becomes (because of the need to
store more index entries), but at the same time the summary data stored can
be more precise and more data blocks can be skipped during an index scan.
</p><div class="sect2" id="BRIN-OPERATION"><div class="titlepage"><div><div><h3 class="title">71.1.1. Index Maintenance</h3></div></div></div><p>
At the time of creation, all existing heap pages are scanned and a
summary index tuple is created for each range, including the
possibly-incomplete range at the end.
As new pages are filled with data, page ranges that are already
summarized will cause the summary information to be updated with data
from the new tuples.
When a new page is created that does not fall within the last
summarized range, the range that the new page belongs to
does not automatically acquire a summary tuple;
those tuples remain unsummarized until a summarization run is
invoked later, creating the initial summary for that range.
</p><p>
There are several ways to trigger the initial summarization of a page range.
If the table is vacuumed, either manually or by
<a class="link" href="routine-vacuuming.html#AUTOVACUUM" title="25.1.6. The Autovacuum Daemon">autovacuum</a>, all existing unsummarized
page ranges are summarized.
Also, if the index's
<a class="xref" href="sql-createindex.html#INDEX-RELOPTION-AUTOSUMMARIZE">autosummarize</a> parameter is enabled,
which it isn't by default,
whenever autovacuum runs in that database, summarization will occur for all
unsummarized page ranges that have been filled,
regardless of whether the table itself is processed by autovacuum; see below.
</p><p>
Lastly, the following functions can be used:
</p><table border="0" summary="Simple list" class="simplelist"><tr><td>
<code class="function">brin_summarize_new_values(regclass)</code>
which summarizes all unsummarized ranges;
</td></tr><tr><td>
<code class="function">brin_summarize_range(regclass, bigint)</code>
which summarizes only the range containing the given page,
if it is unsummarized.
</td></tr></table><p>
</p><p>
When autosummarization is enabled, a request is sent to
<code class="literal">autovacuum</code> to execute a targeted summarization
for a block range when an insertion is detected for the first item
of the first page of the next block range,
to be fulfilled the next time an autovacuum
worker finishes running in the
same database. If the request queue is full, the request is not recorded
and a message is sent to the server log:
</p><pre class="screen">
LOG: request for BRIN range summarization for index "brin_wi_idx" page 128 was not recorded
</pre><p>
When this happens, the range will remain unsummarized until the next
regular vacuum run on the table, or one of the functions mentioned above
are invoked.
</p><p>
Conversely, a range can be de-summarized using the
<code class="function">brin_desummarize_range(regclass, bigint)</code> function,
which is useful when the index tuple is no longer a very good
representation because the existing values have changed.
See <a class="xref" href="functions-admin.html#FUNCTIONS-ADMIN-INDEX" title="9.27.8. Index Maintenance Functions">Section 9.27.8</a> for details.
</p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="brin.html" title="Chapter 71. BRIN Indexes">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="brin.html" title="Chapter 71. BRIN Indexes">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="brin-builtin-opclasses.html" title="71.2. Built-in Operator Classes">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 71. BRIN Indexes </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.7 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 71.2. Built-in Operator Classes</td></tr></table></div></body></html>
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