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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>11.1. 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 V1.79.1" /><link rel="prev" href="indexes.html" title="Chapter 11. Indexes" /><link rel="next" href="indexes-types.html" title="11.2. Index Types" /></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">11.1. Introduction</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="indexes.html" title="Chapter 11. Indexes">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="indexes.html" title="Chapter 11. Indexes">Up</a></td><th width="60%" align="center">Chapter 11. Indexes</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="indexes-types.html" title="11.2. Index Types">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="INDEXES-INTRO"><div class="titlepage"><div><div><h2 class="title" style="clear: both">11.1. Introduction</h2></div></div></div><p>
- Suppose we have a table similar to this:
- </p><pre class="programlisting">
- CREATE TABLE test1 (
- id integer,
- content varchar
- );
- </pre><p>
- and the application issues many queries of the form:
- </p><pre class="programlisting">
- SELECT content FROM test1 WHERE id = <em class="replaceable"><code>constant</code></em>;
- </pre><p>
- With no advance preparation, the system would have to scan the entire
- <code class="structname">test1</code> table, row by row, to find all
- matching entries. If there are many rows in
- <code class="structname">test1</code> and only a few rows (perhaps zero
- or one) that would be returned by such a query, this is clearly an
- inefficient method. But if the system has been instructed to maintain an
- index on the <code class="structfield">id</code> column, it can use a more
- efficient method for locating matching rows. For instance, it
- might only have to walk a few levels deep into a search tree.
- </p><p>
- A similar approach is used in most non-fiction books: terms and
- concepts that are frequently looked up by readers are collected in
- an alphabetic index at the end of the book. The interested reader
- can scan the index relatively quickly and flip to the appropriate
- page(s), rather than having to read the entire book to find the
- material of interest. Just as it is the task of the author to
- anticipate the items that readers are likely to look up,
- it is the task of the database programmer to foresee which indexes
- will be useful.
- </p><p>
- The following command can be used to create an index on the
- <code class="structfield">id</code> column, as discussed:
- </p><pre class="programlisting">
- CREATE INDEX test1_id_index ON test1 (id);
- </pre><p>
- The name <code class="structname">test1_id_index</code> can be chosen
- freely, but you should pick something that enables you to remember
- later what the index was for.
- </p><p>
- To remove an index, use the <code class="command">DROP INDEX</code> command.
- Indexes can be added to and removed from tables at any time.
- </p><p>
- Once an index is created, no further intervention is required: the
- system will update the index when the table is modified, and it will
- use the index in queries when it thinks doing so would be more efficient
- than a sequential table scan. But you might have to run the
- <code class="command">ANALYZE</code> command regularly to update
- statistics to allow the query planner to make educated decisions.
- See <a class="xref" href="performance-tips.html" title="Chapter 14. Performance Tips">Chapter 14</a> for information about
- how to find out whether an index is used and when and why the
- planner might choose <span class="emphasis"><em>not</em></span> to use an index.
- </p><p>
- Indexes can also benefit <code class="command">UPDATE</code> and
- <code class="command">DELETE</code> commands with search conditions.
- Indexes can moreover be used in join searches. Thus,
- an index defined on a column that is part of a join condition can
- also significantly speed up queries with joins.
- </p><p>
- Creating an index on a large table can take a long time. By default,
- <span class="productname">PostgreSQL</span> allows reads (<code class="command">SELECT</code> statements) to occur
- on the table in parallel with index creation, but writes (<code class="command">INSERT</code>,
- <code class="command">UPDATE</code>, <code class="command">DELETE</code>) are blocked until the index build is finished.
- In production environments this is often unacceptable.
- It is possible to allow writes to occur in parallel with index
- creation, but there are several caveats to be aware of —
- for more information see <a class="xref" href="sql-createindex.html#SQL-CREATEINDEX-CONCURRENTLY" title="Building Indexes Concurrently">Building Indexes Concurrently</a>.
- </p><p>
- After an index is created, the system has to keep it synchronized with the
- table. This adds overhead to data manipulation operations.
- Therefore indexes that are seldom or never used in queries
- should be removed.
- </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="indexes.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="indexes.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="indexes-types.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 11. Indexes </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 11.2. Index Types</td></tr></table></div></body></html>
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