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# django.db.models.indexes.Index
class Index:
    def create_sql(self, model, schema_editor, using='', **kwargs):
        fields = [model._meta.get_field(field_name) for field_name, _ in self.fields_orders]
        col_suffixes = [order[1] for order in self.fields_orders]
        condition = self._get_condition_sql(model, schema_editor)
        return schema_editor._create_index_sql(
            model, fields, name=self.name, using=using, db_tablespace=self.db_tablespace,
            col_suffixes=col_suffixes, opclasses=self.opclasses, condition=condition,
            **kwargs,
        )

# django.db.backends.base.schema.BaseDatabaseSchemaEditor
class BaseDatabaseSchemaEditor:
    def _create_index_sql(self, model, fields, *, name=None, suffix='', using='',
                          db_tablespace=None, col_suffixes=(), sql=None, opclasses=(),
                          condition=None):
        """
        Return the SQL statement to create the index for one or several fields.
        `sql` can be specified if the syntax differs from the standard (GIS
        indexes, ...).
        """
        tablespace_sql = self._get_index_tablespace_sql(model, fields, db_tablespace=db_tablespace)
        columns = [field.column for field in fields]
        sql_create_index = sql or self.sql_create_index
        table = model._meta.db_table

        def create_index_name(*args, **kwargs):
            nonlocal name
            if name is None:
                name = self._create_index_name(*args, **kwargs)
            return self.quote_name(name)

        return Statement(
            sql_create_index,
            table=Table(table, self.quote_name),
            name=IndexName(table, columns, suffix, create_index_name),
            using=using,
            columns=self._index_columns(table, columns, col_suffixes, opclasses),
            extra=tablespace_sql,
            condition=self._index_condition_sql(condition),
        )

# migrations/0002_add_index_runsql.py

from django.db import migrations, models

class Migration(migrations.Migration):
    atomic = False

    dependencies = [
        ('app', '0001_initial'),
    ]

    operations = [
        migrations.RunSQL(
            'CREATE INDEX app_sale_sold_at_b9438ae4 ON app_sale (sold_at);',
            reverse_sql='DROP INDEX "app_sale_sold_at_b9438ae4";',
        ),
    ]

class BTreeIdx(models.Model):
    content = models.CharField(max_length=200, db_index=True)

class JsonIdx(models.Model):
    content = models.JSONField(db_index=True)