CREATE INDEX — define a new index
CREATE [ UNIQUE ] INDEX [ CONCURRENTLY ] [ [ IF NOT EXISTS ]name
] ONtable_name
[ USINGmethod
] ( {column_name
| (expression
) } [ COLLATEcollation
] [opclass
] [ ASC | DESC ] [ NULLS { FIRST | LAST } ] [, ...] ) [ WITH (storage_parameter
=value
[, ... ] ) ] [ TABLESPACEtablespace_name
] [ WHEREpredicate
]
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.
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.
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
.
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.
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. 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 | Collation | Nullable | Default --------+---------+-----------+----------+--------- 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.
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.
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 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.
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);
CREATE INDEX
is a
PostgreSQL language extension. There
are no provisions for indexes in the SQL standard.