CREATE INDEX() SQL Commands CREATE INDEX()
NAME
CREATE INDEX - define a new index
SYNOPSIS
CREATE [ UNIQUE ] INDEX [ CONCURRENTLY ] name ON table [ USING method ]
( { column | ( expression ) } [ opclass ] [ ASC | DESC ] [ NULLS { FIRST | LAST } ] [, ...] )
[ WITH ( storage_parameter = value [, ... ] ) ]
[ TABLESPACE tablespace ]
[ WHERE predicate ]
DESCRIPTION
CREATE INDEX constructs an index name on the specified table. Indexes
are primarily used to enhance database performance (though inappropri-
ate use can result in slower performance).
The key field(s) for the index are specified as column names, or alter-
natively as expressions written in parentheses. Multiple fields can be
specified if the index method supports multicolumn indexes.
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.
PostgreSQL provides the index methods B-tree, hash, GiST, and GIN.
Users can also define their own index methods, but that is fairly com-
plicated.
When the WHERE clause is present, a partial index is created. A par-
tial 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.
The expression used in the WHERE clause can 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.
All functions and operators used in an index definition must be
``immutable'', that is, their results must depend only on their argu-
ments 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.
PARAMETERS
UNIQUE 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.
CONCURRENTLY
When this option is used, PostgreSQL will build the index with-
out 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 -- see
Building Indexes Concurrently [create_index(l)].
name 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.
table The name (possibly schema-qualified) of the table to be indexed.
method The name of the index method to be used. Choices are btree,
hash, gist, and gin. The default method is btree.
column The name of a column of the table.
expression
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 can be omitted
if the expression has the form of a function call.
opclass
The name of an operator class. See below for details.
ASC Specifies ascending sort order (which is the default).
DESC Specifies descending sort order.
NULLS FIRST
Specifies that nulls sort before non-nulls. This is the default
when DESC is specified.
NULLS LAST
Specifies that nulls sort after non-nulls. This is the default
when DESC is not specified.
storage_parameter
The name of an index-method-specific storage parameter. See
below for details.
tablespace
The tablespace in which to create the index. If not specified,
default_tablespace is consulted, or temp_tablespaces for indexes
on temporary tables.
predicate
The constraint expression for a partial index.
INDEX STORAGE PARAMETERS
The WITH clause can specify storage parameters for indexes. Each index
method can have its own set of allowed storage parameters. The built-in
index methods all accept a single parameter:
FILLFACTOR
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.
BUILDING INDEXES CONCURRENTLY
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. Very large tables can take many hours to be
indexed, and even for smaller tables, an index build can lock out writ-
ers for periods that are unacceptably long for a production system.
PostgreSQL supports building indexes without locking out writes. This
method is invoked by specifying the CONCURRENTLY option of CREATE
INDEX. When this option is used, PostgreSQL must perform two scans of
the table, and in addition it must wait for all existing transactions
that could potentially use the index to terminate. Thus this method
requires more total work than a standard index build and takes signifi-
cantly 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 might slow other operations.
In a concurrent index build, the index is actually entered into the
system catalogs in one transaction, then the two table scans occur in a
second and third transaction. If a problem arises while scanning the
table, such as a uniqueness violation in a unique index, the CREATE
INDEX command will fail but leave behind an ``invalid'' index. This
index will be ignored for querying purposes because it might be incom-
plete; however it will still consume update overhead. The psql \d com-
mand will mark such an index as INVALID:
postgres=# \d tab
Table "public.tab"
Column | Type | Modifiers
--------+---------+-----------
col | integer |
Indexes:
"idx" btree (col) INVALID
The recommended recovery method in such cases is to drop the index and
try again to perform CREATE INDEX CONCURRENTLY. (Another possibility is
to rebuild the index with REINDEX. However, since REINDEX does not sup-
port concurrent builds, this option is unlikely to seem attractive.)
Another caveat when building a unique index concurrently is that the
uniqueness constraint is already being enforced against other transac-
tions when the second table scan begins. This means that constraint
violations could be reported in other queries prior to the index becom-
ing available for use, or even in cases where the index build eventu-
ally fails. Also, if a failure does occur in the second scan, the
``invalid'' index continues to enforce its uniqueness constraint after-
wards.
Concurrent builds of expression indexes and partial indexes are sup-
ported. Errors occurring in the evaluation of these expressions could
cause behavior similar to that described above for unique constraint
violations.
Regular index builds permit other regular index builds on the same ta-
ble 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 modifica-
tion on the table are allowed meanwhile. Another difference is that a
regular CREATE INDEX command can be performed within a transaction
block, but CREATE INDEX CONCURRENTLY cannot.
NOTES
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.
Currently, only the B-tree and GiST index methods support multicolumn
indexes. Up to 32 fields can be specified by default. (This limit can
be altered when building PostgreSQL.) Only B-tree currently supports
unique indexes.
An operator class 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 func-
tions 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 docu-
mentation.
For index methods that support ordered scans (currently, only B-tree),
the optional clauses ASC, DESC, NULLS FIRST, and/or NULLS LAST can be
specified to reverse the normal sort direction of the index. Since an
ordered index can be scanned either forward or backward, it is not nor-
mally useful to create a single-column DESC index -- that sort ordering
is already available with a regular index. The value of these options
is that multicolumn indexes can be created that match the sort ordering
requested by a mixed-ordering query, such as SELECT ... ORDER BY x ASC,
y DESC. The NULLS options are useful if you need to support ``nulls
sort low'' behavior, rather than the default ``nulls sort high'', in
queries that depend on indexes to avoid sorting steps.
Use DROP INDEX [drop_index(l)] to remove an index.
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, CREATE INDEX will inter-
pret it as USING gist, to simplify conversion of old databases to GiST.
EXAMPLES
To create a B-tree index on the column title in the table films:
CREATE UNIQUE INDEX title_idx ON films (title);
To create an index on the expression lower(title), allowing efficient
case-insensitive searches:
CREATE INDEX lower_title_idx ON films ((lower(title)));
To create an index with non-default sort ordering of nulls:
CREATE INDEX title_idx_nulls_low ON films (title NULLS FIRST);
To create an index with non-default fill factor:
CREATE UNIQUE INDEX title_idx ON films (title) WITH (fillfactor = 70);
To create an index on the column code in the table films and have the
index reside in the tablespace indexspace:
CREATE INDEX code_idx ON films(code) TABLESPACE indexspace;
To create an index without locking out writes to the table:
CREATE INDEX CONCURRENTLY sales_quantity_index ON sales_table (quantity);
COMPATIBILITY
CREATE INDEX is a PostgreSQL language extension. There are no provi-
sions for indexes in the SQL standard.
SEE ALSO
ALTER INDEX [alter_index(l)], DROP INDEX [drop_index(l)]
SQL - Language Statements 2008-09-19 CREATE INDEX()
postgresql 8.3.4 - Generated Thu Oct 2 10:09:09 CDT 2008