create_index.7

来自「PostgreSQL 8.2中增加了很多企业用户所需要的功能和性能上的提高,其开」· 7 代码 · 共 277 行

.\\" auto-generated by docbook2man-spec $Revision: 1.1.1.1 $
.TH "CREATE INDEX" "" "2008-01-03" "SQL - Language Statements" "SQL Commands"
.SH NAME
CREATE INDEX \- define a new index

.SH SYNOPSIS
.sp
.nf
CREATE [ UNIQUE ] INDEX [ CONCURRENTLY ] \fIname\fR ON \fItable\fR [ USING \fImethod\fR ]
    ( { \fIcolumn\fR | ( \fIexpression\fR ) } [ \fIopclass\fR ] [, ...] )
    [ WITH ( \fIstorage_parameter\fR = \fIvalue\fR [, ... ] ) ]
    [ TABLESPACE \fItablespace\fR ]
    [ WHERE \fIpredicate\fR ]
.sp
.fi
.SH "DESCRIPTION"
.PP
\fBCREATE INDEX\fR constructs an index \fIindex_name\fR on the specified table.
Indexes are primarily used to enhance database performance (though
inappropriate use can result in slower performance).
.PP
The key field(s) for the index are specified as column names,
or alternatively as expressions written in parentheses.
Multiple fields can be specified if the index method supports
multicolumn indexes.
.PP
An index field can be an expression computed from the values of
one or more columns of the table row. This feature can be used
to obtain fast access to data based on some transformation of
the basic data. For example, an index computed on
upper(col) would allow the clause
WHERE upper(col) = 'JIM' to use an index.
.PP
PostgreSQL provides the index methods
B-tree, hash, GiST, and GIN. Users can also define their own index
methods, but that is fairly complicated.
.PP
When the WHERE clause is present, a
\fIpartial index\fR is created.
A partial index is an index that contains entries for only a portion of
a table, usually a portion that is more useful for indexing than the
rest of the table. For example, 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.
Another possible application is to use WHERE with
UNIQUE to enforce uniqueness over a subset of a
table. See in the documentation for more discussion.
.PP
The expression used in the WHERE clause may refer
only to columns of the underlying table, but it can use all columns,
not just the ones being indexed. Presently, subqueries and
aggregate expressions are also forbidden in WHERE.
The same restrictions apply to index fields that are expressions.
.PP
All functions and operators used in an index definition must be
``immutable'', that is, their results must depend only on
their arguments and never on any outside influence (such as
the contents of another table or the current time). This restriction
ensures that the behavior of the index is well-defined. To use a
user-defined function in an index expression or WHERE
clause, remember to mark the function immutable when you create it.
.SH "PARAMETERS"
.TP
\fBUNIQUE\fR
Causes the system to check for
duplicate values in the table when the index is created (if data
already exist) and each time data is added. Attempts to
insert or update data which would result in duplicate entries
will generate an error.
.TP
\fBCONCURRENTLY\fR
When this option is used, PostgreSQL will build the
index without taking any locks that prevent concurrent inserts,
updates, or deletes on the table; whereas a standard index build
locks out writes (but not reads) on the table until it's done.
There are several caveats to be aware of when using this option
\(em see Building Indexes Concurrently [\fBcreate_index\fR(7)].
.TP
\fB\fIname\fB\fR
The name of the index to be created. No schema name can be included
here; the index is always created in the same schema as its parent
table.
.TP
\fB\fItable\fB\fR
The name (possibly schema-qualified) of the table to be indexed.
.TP
\fB\fImethod\fB\fR
The name of the index method to be used. Choices are
btree, hash,
gist, and gin. The
default method is btree.
.TP
\fB\fIcolumn\fB\fR
The name of a column of the table.
.TP
\fB\fIexpression\fB\fR
An expression based on one or more columns of the table. The
expression usually must be written with surrounding parentheses,
as shown in the syntax. However, the parentheses may be omitted
if the expression has the form of a function call.
.TP
\fB\fIopclass\fB\fR
The name of an operator class. See below for details.
.TP
\fB\fIstorage_parameter\fB\fR
The name of an index-method-specific storage parameter. See
below for details.
.TP
\fB\fItablespace\fB\fR
The tablespace in which to create the index. If not specified,
default_tablespace is used, or the database's
default tablespace if default_tablespace is an empty
string.
.TP
\fB\fIpredicate\fB\fR
The constraint expression for a partial index.
.SS "INDEX STORAGE PARAMETERS"
.PP
The WITH clause can specify \fIstorage parameters\fR
for indexes. Each index method can have its own set of allowed storage
parameters. The built-in index methods all accept a single parameter:
.TP
\fBFILLFACTOR\fR
The fillfactor for an index is a percentage that determines how full
the index method will try to pack index pages. For B-trees, leaf pages
are filled to this percentage during initial index build, and also
when extending the index at the right (largest key values). If pages
subsequently become completely full, they will be split, leading to
gradual degradation in the index's efficiency. B-trees use a default
fillfactor of 90, but any value from 10 to 100 can be selected.
If the table is static then fillfactor 100 is best to minimize the
index's physical size, but for heavily updated tables a smaller
fillfactor is better to minimize the need for page splits. The
other index methods use fillfactor in different but roughly analogous
ways; the default fillfactor varies between methods.
.SS "BUILDING INDEXES CONCURRENTLY"

.PP
Creating an index can interfere with regular operation of a database.
Normally PostgreSQL locks the table to be indexed against
writes and performs the entire index build with a single scan of the
table. Other transactions can still read the table, but if they try to
insert, update, or delete rows in the table they will block until the
index build is finished. This could have a severe effect if the system is
a live production database. Large tables can take many hours to be
indexed, and even for smaller tables, an index build can lock out writers
for periods that are unacceptably long for a production system.
.PP
PostgreSQL supports building indexes without locking
out writes. This method is invoked by specifying the
CONCURRENTLY option of \fBCREATE INDEX\fR.
When this option is used,
PostgreSQL must perform two scans of the table, and in
addition it must wait for all existing transactions to terminate. Thus
this method requires more total work than a standard index build and takes
significantly longer to complete. However, since it allows normal
operations to continue while the index is built, this method is useful for
adding new indexes in a production environment. Of course, the extra CPU
and I/O load imposed by the index creation may slow other operations.
.PP
If a problem arises during the second scan of the table, such as a
uniqueness violation in a unique index, the \fBCREATE INDEX\fR
command will fail but leave behind an ``invalid'' index. This index
will be ignored for querying purposes because it may be incomplete;
however it will still consume update overhead. The recommended recovery
method in such cases is to drop the index and try again to perform
\fBCREATE INDEX CONCURRENTLY\fR. (Another possibility is to rebuild
the index with \fBREINDEX\fR. However, since \fBREINDEX\fR
does not support concurrent builds, this option is unlikely to seem
attractive.)
.PP
Another caveat when building a unique index concurrently is that the
uniqueness constraint is already being enforced against other transactions
when the second table scan begins. This means that constraint violations
could be reported in other queries prior to the index becoming available
for use, or even in cases where the index build eventually fails. Also,
if a failure does occur in the second scan, the ``invalid'' index
continues to enforce its uniqueness constraint afterwards.
.PP
Concurrent builds of expression indexes and partial indexes are supported.
Errors occurring in the evaluation of these expressions could cause
behavior similar to that described above for unique constraint violations.
.PP
Regular index builds permit other regular index builds on the
same table to occur in parallel, but only one concurrent index build
can occur on a table at a time. In both cases, no other types of schema
modification on the table are allowed meanwhile. Another difference
is that a regular \fBCREATE INDEX\fR command can be performed within
a transaction block, but \fBCREATE INDEX CONCURRENTLY\fR cannot.
.SH "NOTES"
.PP
See in the documentation for information about when indexes can
be used, when they are not used, and in which particular situations
they can be useful.
.PP
Currently, only the B-tree and GiST index methods support
multicolumn indexes. Up to 32 fields may be specified by default.
(This limit can be altered when building
PostgreSQL.) Only B-tree currently
supports unique indexes.
.PP
An \fIoperator class\fR 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 B-tree index on
four-byte integers would use the int4_ops 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. More information about
operator classes is in in the documentation and in in the documentation.
.PP
Use DROP INDEX [\fBdrop_index\fR(7)]
to remove an index.
.PP
Indexes are not used for IS NULL clauses by default.
The best way to use indexes in such cases is to create a partial index
using an IS NULL predicate.
.PP
Prior releases of PostgreSQL also had an
R-tree index method. This method has been removed because
it had no significant advantages over the GiST method.
If USING rtree is specified, \fBCREATE INDEX\fR
will interpret it as USING gist, to simplify conversion
of old databases to GiST.
.SH "EXAMPLES"
.PP
To create a B-tree index on the column title in
the table films:
.sp
.nf
CREATE UNIQUE INDEX title_idx ON films (title);
.sp
.fi
.PP
To create an index on the expression lower(title),
allowing efficient case-insensitive searches:
.sp
.nf
CREATE INDEX lower_title_idx ON films ((lower(title)));
.sp
.fi
.PP
To create an index with non-default fill factor:
.sp
.nf
CREATE UNIQUE INDEX title_idx ON films (title) WITH (fillfactor = 70);
.sp
.fi
.PP
To create an index on the column code in the table
films and have the index reside in the tablespace
indexspace:
.sp
.nf
CREATE INDEX code_idx ON films(code) TABLESPACE indexspace;
.sp
.fi
.PP
To create an index without locking out writes to the table:
.sp
.nf
CREATE INDEX CONCURRENTLY sales_quantity_index ON sales_table (quantity);
.sp
.fi
.SH "COMPATIBILITY"
.PP
\fBCREATE INDEX\fR is a
PostgreSQL language extension. There
are no provisions for indexes in the SQL standard.
.SH "SEE ALSO"
ALTER INDEX [\fBalter_index\fR(7)], DROP INDEX [\fBdrop_index\fR(l)]