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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>11.9. 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 Vsnapshot" /><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><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 11.5 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 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). Also, explicitly marking
non-searchable columns as <code class="literal">INCLUDE</code> columns makes the
index slightly smaller, because such columns need not be stored in upper
B-tree levels.
</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.
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