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diff --git a/doc/src/sgml/html/indexes-index-only-scans.html b/doc/src/sgml/html/indexes-index-only-scans.html new file mode 100644 index 0000000..bd4f06b --- /dev/null +++ b/doc/src/sgml/html/indexes-index-only-scans.html @@ -0,0 +1,209 @@ +<?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>11.9. Index-Only Scans and Covering Indexes</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-partial.html" title="11.8. Partial Indexes" /><link rel="next" href="indexes-opclass.html" title="11.10. Operator Classes and Operator Families" /></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">11.9. Index-Only Scans and Covering Indexes</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="indexes-partial.html" title="11.8. Partial 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 13.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="indexes-opclass.html" title="11.10. Operator Classes and Operator Families">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="INDEXES-INDEX-ONLY-SCANS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">11.9. Index-Only Scans and Covering Indexes</h2></div></div></div><a id="id-1.5.10.12.2" class="indexterm"></a><a id="id-1.5.10.12.3" class="indexterm"></a><a id="id-1.5.10.12.4" class="indexterm"></a><a id="id-1.5.10.12.5" class="indexterm"></a><p> + All indexes in <span class="productname">PostgreSQL</span> + are <em class="firstterm">secondary</em> indexes, meaning that each index is + stored separately from the table's main data area (which is called the + table's <em class="firstterm">heap</em> + in <span class="productname">PostgreSQL</span> terminology). This means that + in an ordinary index scan, each row retrieval requires fetching data from + both the index and the heap. Furthermore, while the index entries that + match a given indexable <code class="literal">WHERE</code> condition are usually + close together in the index, the table rows they reference might be + anywhere in the heap. The heap-access portion of an index scan thus + involves a lot of random access into the heap, which can be slow, + particularly on traditional rotating media. (As described in + <a class="xref" href="indexes-bitmap-scans.html" title="11.5. Combining Multiple Indexes">Section 11.5</a>, bitmap scans try to alleviate + this cost by doing the heap accesses in sorted order, but that only goes + so far.) + </p><p> + To solve this performance problem, <span class="productname">PostgreSQL</span> + supports <em class="firstterm">index-only scans</em>, which can answer + queries from an index alone without any heap access. The basic idea is + to return values directly out of each index entry instead of consulting + the associated heap entry. There are two fundamental restrictions on + when this method can be used: + + </p><div class="orderedlist"><ol class="orderedlist" type="1"><li class="listitem"><p> + The index type must support index-only scans. B-tree indexes always + do. GiST and SP-GiST indexes support index-only scans for some + operator classes but not others. Other index types have no support. + The underlying requirement is that the index must physically store, or + else be able to reconstruct, the original data value for each index + entry. As a counterexample, GIN indexes cannot support index-only + scans because each index entry typically holds only part of the + original data value. + </p></li><li class="listitem"><p> + The query must reference only columns stored in the index. For + example, given an index on columns <code class="literal">x</code> + and <code class="literal">y</code> of a table that also has a + column <code class="literal">z</code>, these queries could use index-only scans: +</p><pre class="programlisting"> +SELECT x, y FROM tab WHERE x = 'key'; +SELECT x FROM tab WHERE x = 'key' AND y < 42; +</pre><p> + but these queries could not: +</p><pre class="programlisting"> +SELECT x, z FROM tab WHERE x = 'key'; +SELECT x FROM tab WHERE x = 'key' AND z < 42; +</pre><p> + (Expression indexes and partial indexes complicate this rule, + as discussed below.) + </p></li></ol></div><p> + </p><p> + If these two fundamental requirements are met, then all the data values + required by the query are available from the index, so an index-only scan + is physically possible. But there is an additional requirement for any + table scan in <span class="productname">PostgreSQL</span>: it must verify that + each retrieved row be <span class="quote">“<span class="quote">visible</span>”</span> to the query's MVCC + snapshot, as discussed in <a class="xref" href="mvcc.html" title="Chapter 13. Concurrency Control">Chapter 13</a>. Visibility information + is not stored in index entries, only in heap entries; so at first glance + it would seem that every row retrieval would require a heap access + anyway. And this is indeed the case, if the table row has been modified + recently. However, for seldom-changing data there is a way around this + problem. <span class="productname">PostgreSQL</span> tracks, for each page in + a table's heap, whether all rows stored in that page are old enough to be + visible to all current and future transactions. This information is + stored in a bit in the table's <em class="firstterm">visibility map</em>. An + index-only scan, after finding a candidate index entry, checks the + visibility map bit for the corresponding heap page. If it's set, the row + is known visible and so the data can be returned with no further work. + If it's not set, the heap entry must be visited to find out whether it's + visible, so no performance advantage is gained over a standard index + scan. Even in the successful case, this approach trades visibility map + accesses for heap accesses; but since the visibility map is four orders + of magnitude smaller than the heap it describes, far less physical I/O is + needed to access it. In most situations the visibility map remains + cached in memory all the time. + </p><p> + In short, while an index-only scan is possible given the two fundamental + requirements, it will be a win only if a significant fraction of the + table's heap pages have their all-visible map bits set. But tables in + which a large fraction of the rows are unchanging are common enough to + make this type of scan very useful in practice. + </p><p> + <a id="id-1.5.10.12.10.1" class="indexterm"></a> + To make effective use of the index-only scan feature, you might choose to + create a <em class="firstterm">covering index</em>, which is an index + specifically designed to include the columns needed by a particular + type of query that you run frequently. Since queries typically need to + retrieve more columns than just the ones they search + on, <span class="productname">PostgreSQL</span> allows you to create an index + in which some columns are just <span class="quote">“<span class="quote">payload</span>”</span> and are not part + of the search key. This is done by adding an <code class="literal">INCLUDE</code> + clause listing the extra columns. For example, if you commonly run + queries like +</p><pre class="programlisting"> +SELECT y FROM tab WHERE x = 'key'; +</pre><p> + the traditional approach to speeding up such queries would be to create + an index on <code class="literal">x</code> only. However, an index defined as +</p><pre class="programlisting"> +CREATE INDEX tab_x_y ON tab(x) INCLUDE (y); +</pre><p> + could handle these queries as index-only scans, + because <code class="literal">y</code> can be obtained from the index without + visiting the heap. + </p><p> + Because column <code class="literal">y</code> is not part of the index's search + key, it does not have to be of a data type that the index can handle; + it's merely stored in the index and is not interpreted by the index + machinery. Also, if the index is a unique index, that is +</p><pre class="programlisting"> +CREATE UNIQUE INDEX tab_x_y ON tab(x) INCLUDE (y); +</pre><p> + the uniqueness condition applies to just column <code class="literal">x</code>, + not to the combination of <code class="literal">x</code> and <code class="literal">y</code>. + (An <code class="literal">INCLUDE</code> clause can also be written + in <code class="literal">UNIQUE</code> and <code class="literal">PRIMARY KEY</code> + constraints, providing alternative syntax for setting up an index like + this.) + </p><p> + It's wise to be conservative about adding non-key payload columns to an + index, especially wide columns. If an index tuple exceeds the + maximum size allowed for the index type, data insertion will fail. + In any case, non-key columns duplicate data from the index's table + and bloat the size of the index, thus potentially slowing searches. + And remember that there is little point in including payload columns in an + index unless the table changes slowly enough that an index-only scan is + likely to not need to access the heap. If the heap tuple must be visited + anyway, it costs nothing more to get the column's value from there. + Other restrictions are that expressions are not currently supported as + included columns, and that only B-tree and GiST indexes currently support + included columns. + </p><p> + Before <span class="productname">PostgreSQL</span> had + the <code class="literal">INCLUDE</code> feature, people sometimes made covering + indexes by writing the payload columns as ordinary index columns, + that is writing +</p><pre class="programlisting"> +CREATE INDEX tab_x_y ON tab(x, y); +</pre><p> + even though they had no intention of ever using <code class="literal">y</code> as + part of a <code class="literal">WHERE</code> clause. This works fine as long as + the extra columns are trailing columns; making them be leading columns is + unwise for the reasons explained in <a class="xref" href="indexes-multicolumn.html" title="11.3. Multicolumn Indexes">Section 11.3</a>. + However, this method doesn't support the case where you want the index to + enforce uniqueness on the key column(s). + </p><p> + <em class="firstterm">Suffix truncation</em> always removes non-key + columns from upper B-Tree levels. As payload columns, they are + never used to guide index scans. The truncation process also + removes one or more trailing key column(s) when the remaining + prefix of key column(s) happens to be sufficient to describe tuples + on the lowest B-Tree level. In practice, covering indexes without + an <code class="literal">INCLUDE</code> clause often avoid storing columns + that are effectively payload in the upper levels. However, + explicitly defining payload columns as non-key columns + <span class="emphasis"><em>reliably</em></span> keeps the tuples in upper levels + small. + </p><p> + In principle, index-only scans can be used with expression indexes. + For example, given an index on <code class="literal">f(x)</code> + where <code class="literal">x</code> is a table column, it should be possible to + execute +</p><pre class="programlisting"> +SELECT f(x) FROM tab WHERE f(x) < 1; +</pre><p> + as an index-only scan; and this is very attractive + if <code class="literal">f()</code> is an expensive-to-compute function. + However, <span class="productname">PostgreSQL</span>'s planner is currently not + very smart about such cases. It considers a query to be potentially + executable by index-only scan only when all <span class="emphasis"><em>columns</em></span> + needed by the query are available from the index. In this + example, <code class="literal">x</code> is not needed except in the + context <code class="literal">f(x)</code>, but the planner does not notice that and + concludes that an index-only scan is not possible. If an index-only scan + seems sufficiently worthwhile, this can be worked around by + adding <code class="literal">x</code> as an included column, for example +</p><pre class="programlisting"> +CREATE INDEX tab_f_x ON tab (f(x)) INCLUDE (x); +</pre><p> + An additional caveat, if the goal is to avoid + recalculating <code class="literal">f(x)</code>, is that the planner won't + necessarily match uses of <code class="literal">f(x)</code> that aren't in + indexable <code class="literal">WHERE</code> clauses to the index column. It will + usually get this right in simple queries such as shown above, but not in + queries that involve joins. These deficiencies may be remedied in future + versions of <span class="productname">PostgreSQL</span>. + </p><p> + Partial indexes also have interesting interactions with index-only scans. + Consider the partial index shown in <a class="xref" href="indexes-partial.html#INDEXES-PARTIAL-EX3" title="Example 11.3. Setting up a Partial Unique Index">Example 11.3</a>: +</p><pre class="programlisting"> +CREATE UNIQUE INDEX tests_success_constraint ON tests (subject, target) + WHERE success; +</pre><p> + In principle, we could do an index-only scan on this index to satisfy a + query like +</p><pre class="programlisting"> +SELECT target FROM tests WHERE subject = 'some-subject' AND success; +</pre><p> + But there's a problem: the <code class="literal">WHERE</code> clause refers + to <code class="literal">success</code> which is not available as a result column + of the index. Nonetheless, an index-only scan is possible because the + plan does not need to recheck that part of the <code class="literal">WHERE</code> + clause at run time: all entries found in the index necessarily + have <code class="literal">success = true</code> so this need not be explicitly + checked in the plan. <span class="productname">PostgreSQL</span> versions 9.6 + and later will recognize such cases and allow index-only scans to be + generated, but older versions will not. + </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-partial.html" title="11.8. Partial 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-opclass.html" title="11.10. 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