CREATE INDEX(7) PostgreSQL 9.6.7 Documentation CREATE INDEX(7)
NAME
CREATE_INDEX - define a new index
SYNOPSIS
CREATE [ UNIQUE ] INDEX [ CONCURRENTLY ] [ [ IF NOT EXISTS ] name ] ON table_name [ USING method ]
( { column_name | ( expression ) } [ COLLATE collation ] [ opclass ] [ ASC | DESC ] [ NULLS { FIRST | LAST } ] [, ...] )
[ WITH ( storage_parameter = value [, ... ] ) ]
[ TABLESPACE tablespace_name ]
[ WHERE predicate ]
DESCRIPTION
CREATE INDEX constructs an index on the specified column(s) of the
specified relation, which can be a table or a materialized view.
Indexes are primarily used to enhance database performance (though
inappropriate use can result in slower performance).
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.
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, SP-GiST, GIN,
and BRIN. Users can also define their own index methods, but that is
fairly complicated.
When the WHERE clause is present, a partial index 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 Section 11.8, "Partial Indexes", 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 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.
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 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 -- see
Building Indexes Concurrently.
IF NOT EXISTS
Do not throw an error if a relation with the same name already
exists. A notice is issued in this case. Note that there is no
guarantee that the existing index is anything like the one that
would have been created. Index name is required when IF NOT EXISTS
is specified.
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. If the name is omitted, PostgreSQL chooses a suitable name
based on the parent table's name and the indexed column name(s).
table_name
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, spgist, gin, and brin. The default method is btree.
column_name
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.
collation
The name of the collation to use for the index. By default, the
index uses the collation declared for the column to be indexed or
the result collation of the expression to be indexed. Indexes with
non-default collations can be useful for queries that involve
expressions using non-default collations.
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 Index
Storage Parameters for details.
tablespace_name
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 optional WITH clause specifies storage parameters for the index.
Each index method has its own set of allowed storage parameters. The
B-tree, hash, GiST and SP-GiST index methods all accept this 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 (adding new
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 integer
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.
GiST indexes additionally accept this parameter:
buffering
Determines whether the buffering build technique described in
Section 61.4.1, "GiST buffering build", in the documentation is
used to build the index. With OFF it is disabled, with ON it is
enabled, and with AUTO it is initially disabled, but turned on
on-the-fly once the index size reaches effective_cache_size. The
default is AUTO.
GIN indexes accept different parameters:
fastupdate
This setting controls usage of the fast update technique described
in Section 63.4.1, "GIN Fast Update Technique", in the
documentation. It is a Boolean parameter: ON enables fast update,
OFF disables it. (Alternative spellings of ON and OFF are allowed
as described in Section 19.1, "Setting Parameters", in the
documentation.) The default is ON.
Note
Turning fastupdate off via ALTER INDEX prevents future
insertions from going into the list of pending index entries,
but does not in itself flush previous entries. You might want
to VACUUM the table or call gin_clean_pending_list function
afterward to ensure the pending list is emptied.
gin_pending_list_limit
Custom gin_pending_list_limit parameter. This value is specified in
kilobytes.
BRIN indexes accept a different parameter:
pages_per_range
Defines the number of table blocks that make up one block range for
each entry of a BRIN index (see Section 64.1, "Introduction", in
the documentation for more details). The default is 128.
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
writers 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 modify or use the index 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 might slow other
operations.
In a concurrent index build, the index is actually entered into the
system catalogs in one transaction, then two table scans occur in two
more transactions. Before each table scan, the index build must wait
for existing transactions that have modified the table to terminate.
After the second scan, the index build must wait for any transactions
that have a snapshot (see Chapter 13, Concurrency Control, in the
documentation) predating the second scan to terminate. Then finally the
index can be marked ready for use, and the CREATE INDEX command
terminates. Even then, however, the index may not be immediately usable
for queries: in the worst case, it cannot be used as long as
transactions exist that predate the start of the index build.
If a problem arises while scanning the table, such as a deadlock or 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 incomplete; however it will
still consume update overhead. The psql \d command will report 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
support 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
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.
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.
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 CREATE INDEX command can be performed within a
transaction block, but CREATE INDEX CONCURRENTLY cannot.
NOTES
See Chapter 11, Indexes, 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.
Caution
Hash index operations are not presently WAL-logged, so hash indexes
might need to be rebuilt with REINDEX after a database crash if
there were unwritten changes. Also, changes to hash indexes are not
replicated over streaming or file-based replication after the
initial base backup, so they give wrong answers to queries that
subsequently use them. Hash indexes are also not properly restored
during point-in-time recovery. For these reasons, hash index use is
presently discouraged.
Currently, only the B-tree, GiST, GIN, and BRIN 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
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 Section 11.9, "Operator Classes and
Operator Families", in the documentation and in Section 36.14,
"Interfacing Extensions To Indexes", in the documentation.
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 modify the sort ordering of the index. Since an ordered
index can be scanned either forward or backward, it is not normally
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.
For most index methods, the speed of creating an index is dependent on
the setting of maintenance_work_mem. Larger values will reduce the time
needed for index creation, so long as you don't make it larger than the
amount of memory really available, which would drive the machine into
swapping.
Use DROP INDEX (DROP_INDEX(7)) 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
interpret 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 ON films ((lower(title)));
(In this example we have chosen to omit the index name, so the system
will choose a name, typically films_lower_idx.)
To create an index with non-default collation:
CREATE INDEX title_idx_german ON films (title COLLATE "de_DE");
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 a GIN index with fast updates disabled:
CREATE INDEX gin_idx ON documents_table USING GIN (locations) WITH (fastupdate = off);
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 a GiST index on a point attribute so that we can efficiently
use box operators on the result of the conversion function:
CREATE INDEX pointloc
ON points USING gist (box(location,location));
SELECT * FROM points
WHERE box(location,location) && '(0,0),(1,1)'::box;
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
provisions for indexes in the SQL standard.
SEE ALSO
ALTER INDEX (ALTER_INDEX(7)), DROP INDEX (DROP_INDEX(7))
PostgreSQL 9.6.7 2018 CREATE INDEX(7)
postgresql 9.6.7 - Generated Fri Feb 9 08:18:44 CST 2018