NAME

       CREATE_INDEX - define a new index

SYNOPSIS

       CREATE [ UNIQUE ] INDEX [ CONCURRENTLY ] [ 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, and GIN. 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.

       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 and
           gin. 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 55.3.1, “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 a different parameter:

       FASTUPDATE
           This setting controls usage of the fast update technique described in Section 57.3.1,
           “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 18.1, “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 afterward to ensure the pending list is
               emptied.

   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. For these reasons, hash index use is presently discouraged.

       Currently, only the B-tree, GiST and GIN 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 35.14,
       “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))