Ubuntu Manpage: CREATE_INDEX - define a new index

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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 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.

For temporary tables, CREATE INDEX is always non-concurrent, as no other session can
access them, and non-concurrent index creation is cheaper.

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,
brin, or user-installed access methods like bloom. 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 62.4.1 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 64.4.1.
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.) 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 different parameters:

pages_per_range
Defines the number of table blocks that make up one block range for each entry of a
BRIN index (see Section 65.1 for more details). The default is 128.

autosummarize
Defines whether a summarization run is invoked for the previous page range whenever an
insertion is detected on the next one.

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) predating the second scan to terminate, including transactions
used by any phase of concurrent index builds on other tables. 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:

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 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, 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 and in Section 37.14.

       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.

       The system regularly collects statistics on all of a table's columns. Newly-created
       non-expression indexes can immediately use these statistics to determine an index's
       usefulness. For new expression indexes, it is necessary to run ANALYZE or wait for the
       autovacuum daemon to analyze the table to generate statistics for these indexes.

       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.