# Django 2.2.9.dev20191202104323 文档

# Model index reference

Index classes ease creating database indexes. They can be added using the `Meta.indexes` option. This document explains the API references of `Index` which includes the index options.

Referencing built-in indexes

Indexes are defined in `django.db.models.indexes`, but for convenience they're imported into `django.db.models`. The standard convention is to use `from django.db import models` and refer to the indexes as `models.<IndexClass>`.

## `Index` options

class `Index`(fields=(), name=None, db_tablespace=None, opclasses=(), condition=None)

  Creates an index (B-Tree) in the database.

### `fields`

`Index.``fields`

A list or tuple of the name of the fields on which the index is desired.

By default, indexes are created with an ascending order for each column. To define an index with a descending order for a column, add a hyphen before the field's name.

For example `Index(fields=['headline', '-pub_date'])` would create SQL with `(headline, pub_date DESC)`. Index ordering isn't supported on MySQL. In that case, a descending index is created as a normal index.

Changed in Django 2.1:

Older versions don't accept a tuple.

### `name`

`Index.``name`

The name of the index. If `name` isn't provided Django will auto-generate a name. For compatibility with different databases, index names cannot be longer than 30 characters and shouldn't start with a number (0-9) or underscore (_).

Partial indexes in abstract base classes

You must always specify a unique name for an index. As such, you cannot normally specify a partial index on an abstract base class, since the `Meta.indexes` option is inherited by subclasses, with exactly the same values for the attributes (including `name`) each time. Instead, specify the `indexes` option on subclasses directly, providing a unique name for each index.

### `db_tablespace`

`Index.``db_tablespace`

The name of the database tablespace to use for this index. For single field indexes, if `db_tablespace` isn't provided, the index is created in the `db_tablespace` of the field.

If `Field.db_tablespace` isn't specified (or if the index uses multiple fields), the index is created in tablespace specified in the `db_tablespace` option inside the model's `class Meta`. If neither of those tablespaces are set, the index is created in the same tablespace as the table.

参见

For a list of PostgreSQL-specific indexes, see `django.contrib.postgres.indexes`.

### `opclasses`

`Index.``opclasses`

New in Django 2.2.

The names of the PostgreSQL operator classes to use for this index. If you require a custom operator class, you must provide one for each field in the index.

For example, `GinIndex(name='json_index', fields=['jsonfield'], opclasses=['jsonb_path_ops'])` creates a gin index on `jsonfield` using `jsonb_path_ops`.

`opclasses` are ignored for databases besides PostgreSQL.

`Index.name` is required when using `opclasses`.

### `condition`

`Index.``condition`

New in Django 2.2.

If the table is very large and your queries mostly target a subset of rows, it may be useful to restrict an index to that subset. Specify a condition as a `Q`. For example, `condition=Q(pages__gt=400)` indexes records with more than 400 pages.

`Index.name` is required when using `condition`.

Restrictions on PostgreSQL

PostgreSQL requires functions referenced in the condition to be marked as IMMUTABLE. Django doesn't validate this but PostgreSQL will error. This means that functions such as Date functions and `Concat` aren't accepted. If you store dates in `DateTimeField`, comparison to `datetime` objects may require the `tzinfo` argument to be provided because otherwise the comparison could result in a mutable function due to the casting Django does for lookups.

Restrictions on SQLite

SQLite imposes restrictions on how a partial index can be constructed.

Oracle

Oracle does not support partial indexes. Instead, partial indexes can be emulated using functional indexes. Use a migration to add the index using `RunSQL`.

MySQL and MariaDB

The `condition` argument is ignored with MySQL and MariaDB as neither supports conditional indexes.