Fixed #26974 -- Added HashIndex to django.contrib.postgres.

Thanks Akshesh Doshi for the initial implementation.

django/contrib/postgres/indexes.py

 from django.db.models import Index
 from django.utils.functional import cached_property
 
-__all__ = ['BrinIndex', 'GinIndex', 'GistIndex']
+__all__ = ['BrinIndex', 'GinIndex', 'GistIndex', 'HashIndex']
 
 
 class PostgresIndex(Index):
@@ -98,3 +98,23 @@ class GistIndex(PostgresIndex):
         if self.fillfactor is not None:
             with_params.append('fillfactor = %d' % self.fillfactor)
         return with_params
+
+
+class HashIndex(PostgresIndex):
+    suffix = 'hash'
+
+    def __init__(self, *, fillfactor=None, **kwargs):
+        self.fillfactor = fillfactor
+        super().__init__(**kwargs)
+
+    def deconstruct(self):
+        path, args, kwargs = super().deconstruct()
+        if self.fillfactor is not None:
+            kwargs['fillfactor'] = self.fillfactor
+        return path, args, kwargs
+
+    def get_with_params(self):
+        with_params = []
+        if self.fillfactor is not None:
+            with_params.append('fillfactor = %d' % self.fillfactor)
+        return with_params

docs/ref/contrib/postgres/indexes.txt

@@ -72,3 +72,22 @@ available from the ``django.contrib.postgres.indexes`` module.
 
     .. _buffering build: https://www.postgresql.org/docs/current/static/gist-implementation.html#GIST-BUFFERING-BUILD
     .. _fillfactor: https://www.postgresql.org/docs/current/static/sql-createindex.html#SQL-CREATEINDEX-STORAGE-PARAMETERS
+
+``HashIndex``
+=============
+
+.. class:: HashIndex(fillfactor=None, **options)
+
+    .. versionadded:: 2.2
+
+    Creates a hash index.
+
+    Provide an integer value from 10 to 100 to the fillfactor_ parameter to
+    tune how packed the index pages will be. PostgreSQL's default is 90.
+
+    .. admonition:: Use this index only on PostgreSQL 10 and later
+
+        Hash indexes have been available in PostgreSQL for a long time, but
+        they suffer from a number of data integrity issues in older versions.
+
+    .. _fillfactor: https://www.postgresql.org/docs/current/static/sql-createindex.html#SQL-CREATEINDEX-STORAGE-PARAMETERS

docs/releases/2.2.txt

@@ -82,6 +82,9 @@ Minor features
   :class:`~django.contrib.postgres.aggregates.StringAgg` determines the
   ordering of the aggregated elements.
 
+* The new :class:`~django.contrib.postgres.indexes.HashIndex` class
+  allows creating ``hash`` indexes in the database.
+
 :mod:`django.contrib.redirects`
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 

tests/postgres_tests/test_indexes.py

-from django.contrib.postgres.indexes import BrinIndex, GinIndex, GistIndex
+from django.contrib.postgres.indexes import (
+    BrinIndex, GinIndex, GistIndex, HashIndex,
+)
 from django.db import connection
 from django.test import skipUnlessDBFeature
 
@@ -83,6 +85,20 @@ class GistIndexTests(IndexTestMixin, PostgreSQLTestCase):
         })
 
 
+class HashIndexTests(IndexTestMixin, PostgreSQLTestCase):
+    index_class = HashIndex
+
+    def test_suffix(self):
+        self.assertEqual(HashIndex.suffix, 'hash')
+
+    def test_deconstruction(self):
+        index = HashIndex(fields=['title'], name='test_title_hash', fillfactor=80)
+        path, args, kwargs = index.deconstruct()
+        self.assertEqual(path, 'django.contrib.postgres.indexes.HashIndex')
+        self.assertEqual(args, ())
+        self.assertEqual(kwargs, {'fields': ['title'], 'name': 'test_title_hash', 'fillfactor': 80})
+
+
 class SchemaTests(PostgreSQLTestCase):
 
     def get_constraints(self, table):
@@ -173,3 +189,31 @@ class SchemaTests(PostgreSQLTestCase):
         with connection.schema_editor() as editor:
             editor.remove_index(CharFieldModel, index)
         self.assertNotIn(index_name, self.get_constraints(CharFieldModel._meta.db_table))
+
+    def test_hash_index(self):
+        # Ensure the table is there and doesn't have an index.
+        self.assertNotIn('field', self.get_constraints(CharFieldModel._meta.db_table))
+        # Add the index.
+        index_name = 'char_field_model_field_hash'
+        index = HashIndex(fields=['field'], name=index_name)
+        with connection.schema_editor() as editor:
+            editor.add_index(CharFieldModel, index)
+        constraints = self.get_constraints(CharFieldModel._meta.db_table)
+        # The index was added.
+        self.assertEqual(constraints[index_name]['type'], HashIndex.suffix)
+        # Drop the index.
+        with connection.schema_editor() as editor:
+            editor.remove_index(CharFieldModel, index)
+        self.assertNotIn(index_name, self.get_constraints(CharFieldModel._meta.db_table))
+
+    def test_hash_parameters(self):
+        index_name = 'integer_array_hash_fillfactor'
+        index = HashIndex(fields=['field'], name=index_name, fillfactor=80)
+        with connection.schema_editor() as editor:
+            editor.add_index(CharFieldModel, index)
+        constraints = self.get_constraints(CharFieldModel._meta.db_table)
+        self.assertEqual(constraints[index_name]['type'], HashIndex.suffix)
+        self.assertEqual(constraints[index_name]['options'], ['fillfactor=80'])
+        with connection.schema_editor() as editor:
+            editor.remove_index(CharFieldModel, index)
+        self.assertNotIn(index_name, self.get_constraints(CharFieldModel._meta.db_table))