- <chapter id="indices">
- <title id="indices-title">Indices and Keys</title>
+<!-- $Header: /cvsroot/pgsql/doc/src/sgml/indices.sgml,v 1.14 2001/02/20 22:27:56 petere Exp $ -->
+
+<chapter id="indices">
+ <title id="indices-title">Indices</title>
+
+ <para>
+ Indices are a common way to enhance database performance. An index
+ allows the database server to find and retrieve specific rows much
+ faster than it could do without an index. But indices also add
+ overhead to the database system as a whole, so they should be used
+ sensibly.
+ </para>
+
+
+ <sect1 id="indices-intro">
+ <title>Introduction</title>
<para>
- Indexes are commonly used to enhance database
- performance. They should be defined on table columns (or class
- attributes) that are used as qualifications in repetitive queries.
- Inappropriate use will result in slower performance, since update
- and insertion times are increased in the presence of indices.
+ The classical example for the need of an index is if there is a
+ table similar to this:
+<programlisting>
+CREATE TABLE test1 (
+ id integer,
+ content varchar
+);
+</programlisting>
+ and the application requires a lot of queries of the form
+<programlisting>
+SELECT content FROM test1 WHERE id = <replaceable>constant</replaceable>;
+</programlisting>
+ Ordinarily, the system would have to scan the entire
+ <structname>test1</structname> table row by row to find all
+ matching entries. If there are a lot of rows in
+ <structname>test1</structname> and only a few rows (possibly zero
+ or one) returned by the query, then this is clearly an inefficient
+ method. If the system were instructed to maintain an index on the
+ <structfield>id</structfield> column, then it could 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.
</para>
<para>
- Indexes may also be used to enforce uniqueness of a table's primary key.
- When an index is declared UNIQUE, multiple table rows with identical
- index entries won't be allowed.
- For this purpose, the goal is ensuring data consistency, not improving
- performance, so the above caution about inappropriate use doesn't apply.
+ A similar approach is used in most books of non-fiction: 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, and would not have to read the entire book to find the
+ interesting location. As it is the task of the author to
+ anticipate the items that the readers are most likely to look up,
+ it is the task of the database programmer to foresee which indexes
+ would be of advantage.
</para>
<para>
- Two forms of indices may be defined:
+ The following command would be used to create the index on the
+ <structfield>id</structfield> column, as discussed:
+<programlisting>
+CREATE INDEX test1_id_index ON test1 (id);
+</programlisting>
+ The name <structname>test1_id_index</structname> can be chosen
+ freely, but you should pick something that enables you to remember
+ later what the index was for.
+ </para>
- <itemizedlist>
- <listitem>
- <para>
- For a <firstterm>value index</firstterm>,
- the key fields for the
- index are specified as column names; multiple columns
- can be specified if the index access method supports
- multi-column indexes.
- </para>
- </listitem>
+ <para>
+ To remove an index, use the <command>DROP INDEX</command> command.
+ Indices can be added and removed from tables at any time.
+ </para>
- <listitem>
- <para>
- For a <firstterm>functional index</firstterm>, an index is defined
- on the result of a function applied
- to one or more columns of a single table.
- This is a single-column index (namely, the function result)
- even if the function uses more than one input field.
- Functional indices can be used to obtain fast access to data
- based on operators that would normally require some
- transformation to apply them to the base data.
- </para>
- </listitem>
- </itemizedlist>
+ <para>
+ Once the index is created, no further intervention is required: the
+ system will use the index when it thinks it would be more efficient
+ than a sequential table scan. But you may have to run the
+ <command>VACUUM ANALYZE</command> command regularly to update
+ statistics to allow the query planner to make educated decisions.
+ Also read <xref linkend="performance-tips"> for information about
+ how to find out whether an index is used and when and why the
+ planner may choose to <emphasis>not</emphasis> use an index.
</para>
<para>
- Postgres provides btree, rtree and hash access methods for
- indices. The btree access method is an implementation of
- Lehman-Yao high-concurrency btrees. The rtree access method
- implements standard rtrees using Guttman's quadratic split algorithm.
- The hash access method is an implementation of Litwin's linear
- hashing. We mention the algorithms used solely to indicate that all
- of these access methods are fully dynamic and do not have to be
- optimized periodically (as is the case with, for example, static hash
- access methods).
+ Indices can also benefit <command>UPDATE</command>s and
+ <command>DELETE</command>s with search conditions. Note that a
+ query or data manipulation commands can only use at most one index
+ per table. Indices can also be used in table join methods. Thus,
+ an index defined on a column that is part of a join condition can
+ significantly speed up queries with joins.
</para>
<para>
- The <productname>Postgres</productname>
- query optimizer will consider using a btree index whenever
- an indexed attribute is involved in a comparison using one of:
+ When an index is created, it has to be kept synchronized with the
+ table. This adds overhead to data manipulation operations.
+ Therefore indices that are non-essential or do not get used at all
+ should be removed.
+ </para>
+ </sect1>
+
+
+ <sect1 id="indices-types">
+ <title>Index Types</title>
+
+ <para>
+ <productname>Postgres</productname> provides several index types:
+ B-tree, R-tree, and Hash. Each index type is more appropriate for
+ a particular query type because of the algorithm it uses. By
+ default, the <command>CREATE INDEX</command> command will create a
+ B-tree index, which fits the most common situations. In
+ particular, the <productname>Postgres</productname> query optimizer
+ will consider using a B-tree index whenever an indexed column is
+ involved in a comparison using one of these operators:
<simplelist type="inline">
- <member><</member>
- <member><=</member>
- <member>=</member>
- <member>>=</member>
- <member>></member>
+ <member><literal><</literal></member>
+ <member><literal><=</literal></member>
+ <member><literal>=</literal></member>
+ <member><literal>>=</literal></member>
+ <member><literal>></literal></member>
</simplelist>
</para>
<para>
- The <productname>Postgres</productname>
- query optimizer will consider using an rtree index whenever
- an indexed attribute is involved in a comparison using one of:
+ R-tree indices are especially suited for spacial data. To create
+ an R-tree index, use a command of the form
+<synopsis>
+CREATE INDEX <replaceable>name</replaceable> ON <replaceable>table</replaceable> USING RTREE (<replaceable>column</replaceable>);
+</synopsis>
+ The <productname>Postgres</productname> query optimizer will
+ consider using an R-tree index whenever an indexed column is
+ involved in a comparison using one of these operators:
<simplelist type="inline">
- <member><<</member>
- <member>&<</member>
- <member>&></member>
- <member>>></member>
- <member>@</member>
- <member>~=</member>
- <member>&&</member>
+ <member><literal><<</literal></member>
+ <member><literal>&<</literal></member>
+ <member><literal>&></literal></member>
+ <member><literal>>></literal></member>
+ <member><literal>@</literal></member>
+ <member><literal>~=</literal></member>
+ <member><literal>&&</literal></member>
</simplelist>
+ (Refer to <xref linkend="functions-geometry"> about the meaning of
+ these operators.)
+ </para>
+
+ <para>
+ The query optimizer will consider using a hash index whenever an
+ indexed column is involved in a comparison using the
+ <literal>=</literal> operator. The following command is used to
+ create a hash index:
+<synopsis>
+CREATE INDEX <replaceable>name</replaceable> ON <replaceable>table</replaceable> USING HASH (<replaceable>column</replaceable>);
+</synopsis>
+ <note>
+ <para>
+ Because of the limited utility of hash indices, a B-tree index
+ should generally be preferred over a hash index. We do not have
+ sufficient evidence that hash indices are actually faster than
+ B-trees even for <literal>=</literal> comparisons. Moreover,
+ hash indices require coarser locks; see <xref
+ linkend="locking-indices">.
+ </para>
+ </note>
+ </para>
+
+ <para>
+ The B-tree index is an implementation of Lehman-Yao
+ high-concurrency B-trees. The R-tree index method implements
+ standard R-trees using Guttman's quadratic split algorithm. The
+ hash index is an implementation of Litwin's linear hashing. We
+ mention the algorithms used solely to indicate that all of these
+ access methods are fully dynamic and do not have to be optimized
+ periodically (as is the case with, for example, static hash access
+ methods).
+ </para>
+ </sect1>
+
+
+ <sect1 id="indices-multicolumn">
+ <title>Multi-Column Indices</title>
+
+ <para>
+ An index can be defined on more than one column. For example, if
+ you have a table of this form:
+<programlisting>
+CREATE TABLE test2 (
+ major int,
+ minor int,
+ name varchar
+);
+</programlisting>
+ (Say, you keep you your <filename class="directory">/dev</filename>
+ directory in a database...) and you frequently make queries like
+<programlisting>
+SELECT name FROM test2 WHERE major = <replaceable>constant</replaceable> AND minor = <replaceable>constant</replaceable>;
+</programlisting>
+ then it may be appropriate to define an index on the columns
+ <structfield>major</structfield> and
+ <structfield>minor</structfield> together, e.g.,
+<programlisting>
+CREATE INDEX test2_mm_idx ON test2 (major, minor);
+</programlisting>
</para>
<para>
- The <productname>Postgres</productname>
- query optimizer will consider using a hash index whenever
- an indexed attribute is involved in a comparison using
- the <literal>=</literal> operator.
+ Currently, only the B-tree implementation supports multi-column
+ indices. Up to 16 columns may be specified. (This limit can be
+ altered when building <productname>Postgres</productname>; see the
+ file <filename>config.h</filename>.)
</para>
<para>
- Currently, only the btree access method supports multi-column
- indexes. Up to 16 keys may be specified by default (this limit
- can be altered when building Postgres).
+ The query optimizer can use a multi-column 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,
+ an index on <literal>(a, b, c)</literal> can be used in queries
+ involving all of <literal>a</literal>, <literal>b</literal>, and
+ <literal>c</literal>, or in queries involving both
+ <literal>a</literal> and <literal>b</literal>, or in queries
+ involving only <literal>a</literal>, but not in other combinations.
+ (In a query involving <literal>a</literal> and <literal>c</literal>
+ the optimizer might choose to use the index for
+ <literal>a</literal> only and treat <literal>c</literal> like an
+ ordinary unindexed column.)
</para>
<para>
- An <firstterm>operator class</firstterm> can be specified for each
- column of an index. The operator class identifies the operators to
- be used by the index for that column. For example, a btree index on
- four-byte integers would use the <literal>int4_ops</literal> class;
- this operator class includes comparison functions for four-byte
- integers. In practice the default operator class for the field's
- datatype is usually sufficient. The main point of having operator classes
- is that for some datatypes, there could be more than one meaningful
- ordering. For example, we might want to sort a complex-number datatype
- either by absolute value or by real part. We could do this by defining
- two operator classes for the datatype and then selecting the proper
- class when making an index. There are also some operator classes with
- special purposes:
+ Multi-column 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>;
+</programlisting>
+ cannot make use of the index <structname>test2_mm_idx</structname>
+ defined above to look up both columns. (It can be used to look up
+ only the <structfield>major</structfield> column, however.)
+ </para>
+
+ <para>
+ Multi-column indices 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.
+ </para>
+ </sect1>
+
+
+ <sect1 id="indices-unique">
+ <title>Unique Indices</title>
+
+ <para>
+ Indexes may also be used to enforce uniqueness of a column's value,
+ or the uniqueness of the combined values of more than one column.
+<synopsis>
+CREATE UNIQUE INDEX <replaceable>name</replaceable> ON <replaceable>table</replaceable> (<replaceable>column</replaceable> <optional>, ...</optional>);
+</synopsis>
+ Only B-tree indices can be declared unique.
+ </para>
+
+ <para>
+ When an index is declared unique, multiple table rows with equal
+ indexed values will not be allowed. NULL values are not considered
+ equal.
+ </para>
+
+ <para>
+ <productname>PostgreSQL</productname> automatically creates unique
+ indices 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
+ 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.)
+ </para>
+ </sect1>
+
+
+ <sect1 id="indices-functional">
+ <title>Functional Indices</title>
+
+ <para>
+ For a <firstterm>functional index</firstterm>, an index is defined
+ on the result of a function applied to one or more columns of a
+ single table. Functional indices can be used to obtain fast access
+ to data based on the result of function calls.
+ </para>
+
+ <para>
+ For example, a common way to do case-insensitive comparisons is to
+ use the <function>lower</function>:
+<programlisting>
+SELECT * FROM test1 WHERE lower(col1) = 'value';
+</programlisting>
+ In order for that query to be able to use an index, it has to be
+ defined on the result of the <literal>lower(column)</literal>
+ operation:
+<programlisting>
+CREATE INDEX test1_lower_col1_idx ON test1 (lower(col1));
+</programlisting>
+ </para>
+
+ <para>
+ The function in the index definition can take more than one
+ argument, but they must be table columns, not constants.
+ Functional indices are always single-column (namely, the function
+ result) even if the function uses more than one input field; there
+ cannot be multi-column indices that contain function calls.
+ </para>
+
+ <tip>
+ <para>
+ The restrictions mentioned in the previous paragraph can easily be
+ worked around by defining custom functions to use in the index
+ definition that call the desired function(s) internally.
+ </para>
+ </tip>
+ </sect1>
+
+
+ <sect1 id="indices-opclass">
+ <title>Operator Classes</title>
+
+ <para>
+ An index definition may specify an <firstterm>operator
+ class</firstterm> for each column of an index.
+<synopsis>
+CREATE INDEX <replaceable>name</replaceable> ON <replaceable>table</replaceable> (<replaceable>column</replaceable> <replaceable>opclass</replaceable> <optional>, ...</optional>);
+</synopsis>
+ The operator class identifies the operators to be used by the index
+ for that column. For example, a B-tree index on four-byte integers
+ would use the <literal>int4_ops</literal> class; this operator
+ class includes comparison functions for four-byte integers. In
+ practice the default operator class for the column's data type is
+ usually sufficient. The main point of having operator classes is
+ that for some data types, there could be more than one meaningful
+ ordering. For example, we might want to sort a complex-number data
+ type either by absolute value or by real part. We could do this by
+ defining two operator classes for the data type and then selecting
+ the proper class when making an index. There are also some
+ operator classes with special purposes:
<itemizedlist>
<listitem>
<para>
The operator classes <literal>box_ops</literal> and
- <literal>bigbox_ops</literal> both support rtree indices on the
- <literal>box</literal> datatype.
- The difference between them is that <literal>bigbox_ops</literal>
- scales box coordinates down, to avoid floating point exceptions from
- doing multiplication, addition, and subtraction on very large
- floating-point coordinates. If the field on which your rectangles lie
- is about 20,000 units square or larger, you should use
+ <literal>bigbox_ops</literal> both support R-tree indices on the
+ <literal>box</literal> data type. The difference between them is
+ that <literal>bigbox_ops</literal> scales box coordinates down,
+ to avoid floating point exceptions from doing multiplication,
+ addition, and subtraction on very large floating point
+ coordinates. If the field on which your rectangles lie is about
+ 20 000 units square or larger, you should use
<literal>bigbox_ops</literal>.
</para>
</listitem>
<para>
The following query shows all defined operator classes:
-
- <programlisting>
+<programlisting>
SELECT am.amname AS acc_name,
opc.opcname AS ops_name,
opr.oprname AS ops_comp
WHERE amop.amopid = am.oid AND
amop.amopclaid = opc.oid AND
amop.amopopr = opr.oid
- ORDER BY acc_name, ops_name, ops_comp
- </programlisting>
+ ORDER BY acc_name, ops_name, ops_comp;
+</programlisting>
</para>
+ </sect1>
- <para>
- Use <command>DROP INDEX</command> to remove an index.
- </para>
<sect1 id="keys">
<title id="keys-title">Keys</title>
</para>
<para>
- <programlisting>
+<literallayout>
Subject: Re: [QUESTIONS] PRIMARY KEY | UNIQUE
What's the difference between:
- Is this an alias?
- If PRIMARY KEY is already unique, then why
is there another kind of key named UNIQUE?
- </programlisting>
+</literallayout>
</para>
<para>
However, my application requires that each collection will also have a
unique name. Why? So that a human being who wants to modify a collection
will be able to identify it. It's much harder to know, if you have two
- collections named "Life Science", the the one tagged 24433 is the one you
+ collections named <quote>Life Science</quote>, the the one tagged 24433 is the one you
need, and the one tagged 29882 is not.
</para>
<para>
Moreover, despite being unique, the collection name does not actually
define the collection! For example, if somebody decided to change the name
- of the collection from "Life Science" to "Biology", it will still be the
+ of the collection from <quote>Life Science</quote> to <quote>Biology</quote>, it will still be the
same collection, only with a different name. As long as the name is unique,
that's OK.
</para>
</listitem>
<listitem>
<para>
- Are updateable, so long as they are kept unique.
+ Are updatable, so long as they are kept unique.
</para>
</listitem>
<listitem>
<para>
Thus, you may query a table by any combination of its columns, despite the
fact that you don't have an index on these columns. The indexes are merely
- an implementation
al aid that each <acronym>RDBMS</acronym> offers
+ an implementation aid that each <acronym>RDBMS</acronym> offers
you, in order to cause
commonly used queries to be done more efficiently.
Some <acronym>RDBMS</acronym> may give you
additional measures, such as keeping a key stored in main memory. They will
have a special command, for example
- <programlisting>
-CREATE MEMSTORE ON <table> COLUMNS <cols>
- </programlisting>
- (this is not an existing command, just an example).
+<synopsis>
+CREATE MEMSTORE ON <replaceable>table</replaceable> COLUMNS <replaceable>cols</replaceable>
+</synopsis>
+ (This is not an existing command, just an example.)
</para>
<para>
</para>
</sect1>
+
<sect1 id="partial-index">
<title id="partial-index-title">Partial Indices</title>
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