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<chapter id="indexes">
<title id="indexes-title">Indexes</title>
<para>
To remove an index, use the <command>DROP INDEX</command> command.
- Indexes can be added and removed from tables at any time.
+ Indexes can be added to and removed from tables at any time.
</para>
<para>
<sect1 id="indexes-multicolumn">
- <title>Multi-Column Indexes</title>
+ <title>Multicolumn Indexes</title>
<indexterm zone="indexes-multicolumn">
<primary>indexes</primary>
- <secondary>multi-column</secondary>
+ <secondary>multicolumn</secondary>
</indexterm>
<para>
</para>
<para>
- Currently, only the B-tree implementation supports multi-column
+ Currently, only the B-tree implementation supports multicolumn
indexes. Up to 16 columns may be specified. (This limit can be
altered when building <productname>Postgres</productname>; see the
file <filename>pg_config.h</filename>.)
</para>
<para>
- The query optimizer can use a multi-column index for queries that
+ The query optimizer can use a multicolumn index for queries that
involve the first <parameter>n</parameter> consecutive columns in
the index (when used with appropriate operators), up to the total
number of columns specified in the index definition. For example,
</para>
<para>
- Multi-column indexes can only be used if the clauses involving the
+ Multicolumn indexes can only be used if the clauses involving the
indexed columns are joined with <literal>AND</literal>. For instance,
<programlisting>
SELECT name FROM test2 WHERE major = <replaceable>constant</replaceable> OR minor = <replaceable>constant</replaceable>;
</para>
<para>
- Multi-column indexes should be used sparingly. Most of the time,
+ Multicolumn indexes should be used sparingly. Most of the time,
an index on a single column is sufficient and saves space and time.
Indexes with more than three columns are almost certainly
inappropriate.
<productname>PostgreSQL</productname> automatically creates unique
indexes when a table is declared with a unique constraint or a
primary key, on the columns that make up the primary key or unique
- columns (a multi-column index, if appropriate), to enforce that
+ columns (a multicolumn index, if appropriate), to enforce that
constraint. A unique index can be added to a table at any later
- time, to add a unique constraint. (But a primary key cannot be
- added after table creation.)
+ time, to add a unique constraint.
</para>
+
+ <note>
+ <para>
+ The preferred way to add a unique constraint to a table is
+ <literal>ALTER TABLE ... ADD CONSTRAINT</literal>. The use of
+ indexes to enforce unique constraints could be considered an
+ implementation detail that should not be accessed directly.
+ </para>
+ </note>
</sect1>
argument, but they must be table columns, not constants.
Functional indexes are always single-column (namely, the function
result) even if the function uses more than one input field; there
- cannot be multi-column indexes that contain function calls.
+ cannot be multicolumn indexes that contain function calls.
</para>
<tip>
</sect1>
- <sect1 id="partial-index">
- <title id="partial-index-title">Partial Indexes</title>
+ <sect1 id="indexes-partial">
+ <title>Partial Indexes</title>
- <indexterm zone="partial-index">
+ <indexterm zone="indexes-partial">
<primary>indexes</primary>
<secondary>partial</secondary>
</indexterm>
- <note>
- <title>Author</title>
- <para>
- This is from a reply to a question on the email list
- by Paul M. Aoki (<email>aoki@CS.Berkeley.EDU</email>)
- on 1998-08-11.
-<!--
- Paul M. Aoki | University of California at Berkeley
- aoki@CS.Berkeley.EDU | Dept. of EECS, Computer Science Division #1776
- | Berkeley, CA 94720-1776
--->
- </para>
- </note>
+ <para>
+ A <firstterm>partial index</firstterm> is an index built over a
+ subset of a table; the subset is defined by a conditional
+ expression (called the <firstterm>predicate</firstterm> of the
+ partial index).
+ </para>
+
+ <para>
+ A major motivation for partial indexes is to avoid indexing common
+ values. Since a query conditionalized on a common value will not
+ use the index anyway, there is no point in keeping those rows in the
+ index at all. This reduces the size of the index, which will speed
+ up queries that do use the index. It will also speed up many data
+ manipulation operations because the index does not need to be
+ updated in all cases. <xref linkend="indexes-partial-ex1"> shows a
+ possible application of this idea.
+ </para>
+
+ <example id="indexes-partial-ex1">
+ <title>Setting up a Partial Index to Exclude Common Values</title>
+
+ <para>
+ Suppose you are storing web server access logs in a database.
+ Most accesses originate from the IP range of your organization but
+ some are from elsewhere (say, employees on dial-up connections).
+ So you do not want to index the IP range that corresponds to your
+ organization's subnet.
+ </para>
+
+ <para>
+ Assume a table like this:
+<programlisting>
+CREATE TABLE access_log (
+ url varchar,
+ client_ip inet,
+ ...
+);
+</programlisting>
+ </para>
+
+ <para>
+ To create a partial index that suits our example, use a command
+ such as this:
+<programlisting>
+CREATE INDEX access_log_client_ip_ix ON access_log (client_ip)
+ WHERE NOT (client_ip > inet '192.168.100.0' AND client_ip < inet '192.168.100.255');
+</programlisting>
+ </para>
<para>
- A <firstterm>partial index</firstterm>
- is an index built over a subset of a table; the subset is defined by
- a predicate. <productname>Postgres</productname>
- supports partial indexes with arbitrary
- predicates. I believe IBM's <productname>DB2</productname>
- for AS/400 supports partial indexes
- using single-clause predicates.
+ A typical query that can use this index would be:
+<programlisting>
+SELECT * FROM access_log WHERE url = '/index.html' AND client_ip = inet '212.78.10.32';
+</programlisting>
+ A query that cannot use this index is:
+<programlisting>
+SELECT * FROM access_log WHERE client_ip = inet '192.168.100.23';
+</programlisting>
</para>
<para>
- The main motivation for partial indexes is this:
- if all of the queries you ask that can
- profitably use an index fall into a certain range, why build an index
- over the whole table and suffer the associated space/time costs?
+ Observe that this kind of partial index requires that the common
+ values be actively tracked. If the distribution of values is
+ inherent (due to the nature of the application) and static (does
+ not change), this is not difficult, but if the common values are
+ merely due to the coincidental data load this can require a lot of
+ maintenance work.
+ </para>
+ </example>
+
+ <para>
+ Another possibility is to exclude values from the index that the
+ typical query workload is not interested in; this is shown in <xref
+ linkend="indexes-partial-ex2">. This results in the same
+ advantages as listed above, but it prevents the
+ <quote>uninteresting</quote> values from being accessed via an
+ index at all, even if an index scan might be profitable in that
+ case. Obviously, setting up partial indexes for this kind of
+ scenario will require a lot of care and experimentation.
+ </para>
- (There are other reasons too; see
- <xref endterm="STON89b" linkend="STON89b-full"> for details.)
+ <example id="indexes-partial-ex2">
+ <title>Setting up a Partial Index to Exclude Uninteresting Values</title>
+
+ <para>
+ If you have a table that contains both billed and unbilled orders
+ where the unbilled orders take up a small fraction of the total
+ table and yet that is an often used section, you can improve
+ performance by creating an index on just that portion. The
+ command the create the index would look like this:
+<programlisting>
+CREATE INDEX orders_unbilled_index ON orders (order_nr)
+ WHERE billed is not true;
+</programlisting>
</para>
<para>
- The machinery to build, update and query partial indexes isn't too
- bad. The hairy parts are index selection (which indexes do I build?)
- and query optimization (which indexes do I use?); i.e., the parts
- that involve deciding what predicate(s) match the workload/query in
- some useful way. For those who are into database theory, the problems
- are basically analogous to the corresponding materialized view
- problems, albeit with different cost parameters and formulas. These
- are, in the general case, hard problems for the standard ordinal
- <acronym>SQL</acronym>
- types; they're super-hard problems with black-box extension types,
- because the selectivity estimation technology is so crude.
+ A possible query to use this index would be
+<programlisting>
+SELECT * FROM orders WHERE billed is not true AND order_nr < 10000;
+</programlisting>
+ However, the index can also be used in queries that do not involve
+ <structfield>order_nr</> at all, e.g.,
+<programlisting>
+SELECT * FROM orders WHERE billed is not true AND amount > 5000.00;
+</programlisting>
+ This is not as efficient as a partial index on the
+ <structfield>amount</> column would be, since the system has to
+ scan the entire index in any case.
</para>
<para>
- Check <xref endterm="STON89b" linkend="STON89b-full">,
- <xref endterm="OLSON93" linkend="OLSON93-full">,
- and
- <xref endterm="SESHADRI95" linkend="SESHADRI95-full">
- for more information.
+ Note that this query cannot use this index:
+<programlisting>
+SELECT * FROM orders WHERE order_nr = 3501;
+</programlisting>
+ The order 3501 may be among the billed or among the unbilled
+ orders.
</para>
- </sect1>
- </chapter>
+ </example>
+
+ <para>
+ <xref linkend="indexes-partial-ex2"> also illustrates that the
+ indexed column and the column used in the predicate do not need to
+ match. <productname>PostgreSQL</productname> supports partial
+ indexes with arbitrary predicates, as long as only columns of the
+ table being indexed are involved. However, keep in mind that the
+ predicate must actually match the condition used in the query that
+ is supposed to benefit from the index.
+ <productname>PostgreSQL</productname> does not have a sophisticated
+ theorem prover that can recognize mathematically equivalent
+ predicates that are written in different forms. (Not
+ only is such a general theorem prover extremely difficult to
+ create, it would probably be too slow to be of any real use.)
+ </para>
+
+ <para>
+ Finally, a partial index can also be used to override the system's
+ query plan choices. It may occur that data sets with peculiar
+ distributions will cause the system to use an index when it really
+ should not. In that case the index can be set up so that it is not
+ available for the offending query. Normally,
+ <productname>PostgreSQL</> makes reasonable choices about index
+ usage (e.g., it avoids them when retrieving common values, so the
+ earlier example really only saves index size, it is not required to
+ avoid index usage), and grossly incorrect plan choices are cause
+ for a bug report.
+ </para>
+
+ <para>
+ Keep in mind that setting up a partial index indicates that you
+ know at least as much as the query planner knows, in particular you
+ know when an index might be profitable. Forming this knowledge
+ requires experience and understanding of how indexes in
+ <productname>PostgreSQL</> work. In most cases, the advantage of a
+ partial index over a regular index will not be much.
+ </para>
+
+ <para>
+ More information about partial indexes can be found in <xref
+ endterm="STON89b" linkend="STON89b-full">, <xref endterm="OLSON93"
+ linkend="OLSON93-full">, and <xref endterm="SESHADRI95"
+ linkend="SESHADRI95-full">.
+ </para>
+ </sect1>
+
+ <sect1 id="indexes-examine">
+ <title>Examining Index Usage</title>
+
+ <para>
+ Although indexes in <productname>PostgreSQL</> do not need
+ maintenance and tuning, it is still important that it is checked
+ which indexes are actually used by the real-life query workload.
+ Examining index usage is done with the <command>EXPLAIN</> command;
+ its application for this purpose is illustrated in <xref
+ linkend="using-explain">.
+ </para>
+
+ <para>
+ It is difficult to formulate a general procedure for determining
+ which indexes to set up. There are a number of typical cases that
+ have been shown in the examples throughout the previous sections.
+ A good deal of experimentation will be necessary in most cases.
+ The rest of this section gives some tips for that.
+ </para>
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ Always run <command>ANALYZE</command> first. This command
+ collects statistics about the distribution of the values in the
+ table. This information is required to guess the number of rows
+ returned by a query, which is needed by the planner to assign
+ realistic costs to each possible query plan. In absence of any
+ real statistics, some default values are assumed, which are
+ almost certain to be inaccurate. Examining an application's
+ index usage without having run <command>ANALYZE</command> is
+ therefore a lost cause.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ Use real data for experimentation. Using test data for setting
+ up indexes will tell you what indexes you need for the test data,
+ but that is all.
+ </para>
+
+ <para>
+ It is especially fatal to use proportionally reduced data sets.
+ While selecting 1000 out of 100000 rows could be a candidate for
+ an index, selecting 1 out of 100 rows will hardly be, because the
+ 100 rows will probably fit within a single disk page, and there
+ is no plan that can beat sequentially fetching 1 disk page.
+ </para>
+
+ <para>
+ Also be careful when making up test data, which is often
+ unavoidable when the application is not in production use yet.
+ Values that are very similar, completely random, or inserted in
+ sorted order will skew the statistics away from the distribution
+ that real data would have.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ When indexes are not used, it can be useful for testing to force
+ their use. There are run-time parameters that can turn off
+ various plan types (described in the <citetitle>Administrator's
+ Guide</citetitle>). For instance, turning off sequential scans
+ (<varname>enable_seqscan</>) and nested-loop joins
+ (<varname>enable_nestloop</>), which are the most basic plans,
+ will force the system to use a different plan. If the system
+ still chooses a sequential scan or nested-loop join then there is
+ probably a more fundamental problem for why the index is not
+ used, for example, the query condition does not match the index.
+ (What kind of query can use what kind of index is explained in
+ the previous sections.)
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ If forcing index usage does use the index, then there are two
+ possibilities: Either the system is right and using the index is
+ indeed not appropriate, or the cost estimates of the query plans
+ are not reflecting reality. So you should time your query with
+ and without indexes. The <command>EXPLAIN ANALYZE</command>
+ command can be useful here.
+ </para>
+ </listitem>
+
+ <listitem>
+ <para>
+ If it turns out that the cost estimates are wrong, there are,
+ again, two possibilities. The total cost is computed from the
+ per-row costs of each plan node times the selectivity estimate of
+ the plan node. The costs of the plan nodes can be tuned with
+ run-time parameters (described in the <citetitle>Administrator's
+ Guide</citetitle>). An inaccurate selectivity estimate is due to
+ insufficient statistics. It may be possible to help this by
+ tuning the statistics-gathering parameters (see <command>ALTER
+ TABLE</command> reference).
+ </para>
+
+ <para>
+ If you do not succeed in adjusting the costs to be more
+ appropriate, then you may have to do with forcing index usage
+ explicitly. You may also want to contact the
+ <productname>PostgreSQL</> developers to examine the issue.
+ </para>
+ </listitem>
+ </itemizedlist>
+ </sect1>
+</chapter>
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