How to remove index varchar_pattern_ops in a django (1.8) migration?

Question

When creating a model with a models.varchar(...) field, a varchar_pattern_ops index is being created.

This is the table generated in postgresql

              Table "public.logger_btilog"
      Column      |           Type           | Modifiers 
------------------+--------------------------+-----------
 md5hash          | text                     | 
 id               | integer                  | not null
Indexes:
    "logger_btilog_pkey" PRIMARY KEY, btree (id)
    "logger_btilog_md5hash_6454d7bb20588b61_like" btree (md5hash varchar_pattern_ops)

I want to remove that varchar_pattern_ops index in a migration, and add a hash index in that field.

I tried doing this:

# models.py
class Btilog(models.Model):
    md5hash = models.TextField(db_index=False)
    [...]

And in migration also force adding db_field=False

# 0013_migration.py
# -*- coding: utf-8 -*-
from __future__ import unicode_literals

from django.db import models, migrations


class Migration(migrations.Migration):

    dependencies = [
        ('logger', '0014_btilog_id'),
    ]

    operations = [
        # this should remove all indexes for md5hash, but it does not work
        migrations.AlterField(
            model_name='btilog',
            name='md5hash',
            field=models.TextField(null=True, blank=True, db_index=False),
        ),
        migrations.RunSQL(
            "create index logger_btilog_md5hash_hashindex on logger_btilog using hash(md5hash);",
            "drop index logger_btilog_md5hash_hashindex;"
        ),
]

After running the migrations, this are the indexes in the database

                              relation                              |  size   
--------------------------------------------------------------------+---------
 public.logger_btilog                                               | 7185 MB
 public.logger_btilog_md5hash_6454d7bb20588b61_like                 | 1442 MB
 public.logger_btilog_md5hash_hashindex                             | 1024 MB
 public.logger_btilog_pkey                                          | 548 MB

Note that public.logger_btilog_md5hash_6454d7bb20588b61_like is the index I want to delete. This index is being added automatically by django, see this

More info on that index

vtfx=# \d logger_btilog_md5hash_6454d7bb20588b61_like
Index "public.logger_btilog_md5hash_6454d7bb20588b61_like"
 Column  | Type | Definition 
---------+------+------------
 md5hash | text | md5hash
btree, for table "public.logger_btilog"

Footnote: I'm not confused about the usage of a hash index, I only want to do = (strictrly equal) where searches in md5hash field, then (casually) a hash index would be the fastest and will occupy less space than a btree index (django's default)

Answer 1

Answer update notice:

• django < 1.11: Use this answer

• django >= 1.11: Use @Cesar Canassa's answer

Ok, I found some info here https://docs.djangoproject.com/en/1.8/_modules/django/db/backends/base/schema/#BaseDatabaseSchemaEditor.alter_field

And made a manual RunPython migration to delete the varchar_pattern_ops index using the SchemaEditor

# -*- coding: utf-8 -*-
from __future__ import unicode_literals

from django.db import models, migrations


import re
def drop_md5hash_varchar_pattern_ops_index(apps, schemaEditor):
    # code based on https://docs.djangoproject.com/en/1.8/_modules/django/db/backends/base/schema/#BaseDatabaseSchemaEditor.alter_field
    model = apps.get_model("logger", "Btilog")
    index_names = schemaEditor._constraint_names(model, index=True)
    for index_name in index_names:
        if re.search('logger_btilog_md5hash_.+_like', index_name):
            print 'dropping index {}'.format(index_name)
            schemaEditor.execute(schemaEditor._delete_constraint_sql(schemaEditor.sql_delete_index, model, index_name))


class Migration(migrations.Migration):
    dependencies = [
        ('logger', '0012_auto_20150529_1745'),
    ]

    operations = [
        # Remove the annoying index using a hack
        migrations.RunPython(
            drop_md5hash_varchar_pattern_ops_index
        ),
    ]

Answer 2

You can avoid the creation of the varchar_pattern_ops "LIKE" index altogether by using the new Model Meta indexes option that was added in Django 1.11. For example, instead of writing your Model like this:

class MyModel(models.Model):
    my_field = models.CharField(max_length=64, db_index=True)

You need to set the index with the Model Meta option:

class MyModel(models.Model):
    my_field = models.CharField(max_length=64)

    class Meta:
        indexes = [
            models.Index(fields=['my_field'])
        ]

By doing so, Django will not create the duplicated index.

Answer 3

I'm using Django 3.0.11 and I had same problem. I don't want _like index for primary key since it's used only for joins and exact matching.

Defining index manually won't help if field is primary_key. So I came up with hacky solution similar to first answer.

from django.db import migrations, models


def drop_pattern_ops_index(schema_editor, model, field):
    indexes = schema_editor._model_indexes_sql(model)
    for index in indexes:
        columns = index.parts["columns"].columns
        opclasses = index.parts["columns"].opclasses
        if field in columns and len(columns) == 1:
            # Django uses varchar_pattern_ops or text_pattern_ops
            if "pattern_ops" in opclasses[0]:
                drop_sql = schema_editor._delete_index_sql(
                    model, str(index.parts["name"]).strip('"')
                )
                # Indexes creation is always deferred in migration 
                # so we have to execute remove at the end
                schema_editor.deferred_sql.extend([drop_sql])
                break


def drop_extra_indexes(apps, schema_editor):
    model = apps.get_model("com_erp", "TestModel")
    drop_pattern_ops_index(schema_editor, model, "code")


class Migration(migrations.Migration):
    operations = [
        migrations.CreateModel(
            name="TestModel",
            fields=[
                ("code", models.TextField(primary_key=True, serialize=False)),
                ("name", models.TextField()),
            ],
            options={"db_table": "test_table",},
        ),
        migrations.RunPython(drop_extra_indexes, migrations.RunPython.noop),
    ]