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diff --git a/doc/src/sgml/html/indexes-intro.html b/doc/src/sgml/html/indexes-intro.html new file mode 100644 index 0000000..c800606 --- /dev/null +++ b/doc/src/sgml/html/indexes-intro.html @@ -0,0 +1,75 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!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 Vsnapshot" /><link rel="prev" href="indexes.html" title="Chapter 11. Indexes" /><link rel="next" href="indexes-types.html" title="11.2. Index Types" /></head><body id="docContent" class="container-fluid col-10"><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 14.5 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 xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navfooter"><hr></hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="indexes.html" title="Chapter 11. Indexes">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="indexes.html" title="Chapter 11. Indexes">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="indexes-types.html" title="11.2. Index Types">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" title="PostgreSQL 14.5 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 11.2. Index Types</td></tr></table></div></body></html>
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