my-first-blog/myvenv/Lib/site-packages/django/db/backends/postgresql/schema.py

import psycopg2

from django.db.backends.base.schema import BaseDatabaseSchemaEditor
from django.db.backends.ddl_references import IndexColumns


class DatabaseSchemaEditor(BaseDatabaseSchemaEditor):

    sql_alter_column_type = "ALTER COLUMN %(column)s TYPE %(type)s USING %(column)s::%(type)s"

    sql_create_sequence = "CREATE SEQUENCE %(sequence)s"
    sql_delete_sequence = "DROP SEQUENCE IF EXISTS %(sequence)s CASCADE"
    sql_set_sequence_max = "SELECT setval('%(sequence)s', MAX(%(column)s)) FROM %(table)s"

    sql_create_index = "CREATE INDEX %(name)s ON %(table)s%(using)s (%(columns)s)%(extra)s%(condition)s"
    sql_delete_index = "DROP INDEX IF EXISTS %(name)s"

    # Setting the constraint to IMMEDIATE runs any deferred checks to allow
    # dropping it in the same transaction.
    sql_delete_fk = "SET CONSTRAINTS %(name)s IMMEDIATE; ALTER TABLE %(table)s DROP CONSTRAINT %(name)s"

    sql_delete_procedure = 'DROP FUNCTION %(procedure)s(%(param_types)s)'

    def quote_value(self, value):
        if isinstance(value, str):
            value = value.replace('%', '%%')
        # getquoted() returns a quoted bytestring of the adapted value.
        return psycopg2.extensions.adapt(value).getquoted().decode()

    def _field_indexes_sql(self, model, field):
        output = super()._field_indexes_sql(model, field)
        like_index_statement = self._create_like_index_sql(model, field)
        if like_index_statement is not None:
            output.append(like_index_statement)
        return output

    def _create_like_index_sql(self, model, field):
        """
        Return the statement to create an index with varchar operator pattern
        when the column type is 'varchar' or 'text', otherwise return None.
        """
        db_type = field.db_type(connection=self.connection)
        if db_type is not None and (field.db_index or field.unique):
            # Fields with database column types of `varchar` and `text` need
            # a second index that specifies their operator class, which is
            # needed when performing correct LIKE queries outside the
            # C locale. See #12234.
            #
            # The same doesn't apply to array fields such as varchar[size]
            # and text[size], so skip them.
            if '[' in db_type:
                return None
            if db_type.startswith('varchar'):
                return self._create_index_sql(model, [field], suffix='_like', opclasses=['varchar_pattern_ops'])
            elif db_type.startswith('text'):
                return self._create_index_sql(model, [field], suffix='_like', opclasses=['text_pattern_ops'])
        return None

    def _alter_column_type_sql(self, model, old_field, new_field, new_type):
        """Make ALTER TYPE with SERIAL make sense."""
        table = model._meta.db_table
        if new_type.lower() in ("serial", "bigserial"):
            column = new_field.column
            sequence_name = "%s_%s_seq" % (table, column)
            col_type = "integer" if new_type.lower() == "serial" else "bigint"
            return (
                (
                    self.sql_alter_column_type % {
                        "column": self.quote_name(column),
                        "type": col_type,
                    },
                    [],
                ),
                [
                    (
                        self.sql_delete_sequence % {
                            "sequence": self.quote_name(sequence_name),
                        },
                        [],
                    ),
                    (
                        self.sql_create_sequence % {
                            "sequence": self.quote_name(sequence_name),
                        },
                        [],
                    ),
                    (
                        self.sql_alter_column % {
                            "table": self.quote_name(table),
                            "changes": self.sql_alter_column_default % {
                                "column": self.quote_name(column),
                                "default": "nextval('%s')" % self.quote_name(sequence_name),
                            }
                        },
                        [],
                    ),
                    (
                        self.sql_set_sequence_max % {
                            "table": self.quote_name(table),
                            "column": self.quote_name(column),
                            "sequence": self.quote_name(sequence_name),
                        },
                        [],
                    ),
                ],
            )
        else:
            return super()._alter_column_type_sql(model, old_field, new_field, new_type)

    def _alter_field(self, model, old_field, new_field, old_type, new_type,
                     old_db_params, new_db_params, strict=False):
        # Drop indexes on varchar/text/citext columns that are changing to a
        # different type.
        if (old_field.db_index or old_field.unique) and (
            (old_type.startswith('varchar') and not new_type.startswith('varchar')) or
            (old_type.startswith('text') and not new_type.startswith('text')) or
            (old_type.startswith('citext') and not new_type.startswith('citext'))
        ):
            index_name = self._create_index_name(model._meta.db_table, [old_field.column], suffix='_like')
            self.execute(self._delete_index_sql(model, index_name))

        super()._alter_field(
            model, old_field, new_field, old_type, new_type, old_db_params,
            new_db_params, strict,
        )
        # Added an index? Create any PostgreSQL-specific indexes.
        if ((not (old_field.db_index or old_field.unique) and new_field.db_index) or
                (not old_field.unique and new_field.unique)):
            like_index_statement = self._create_like_index_sql(model, new_field)
            if like_index_statement is not None:
                self.execute(like_index_statement)

        # Removed an index? Drop any PostgreSQL-specific indexes.
        if old_field.unique and not (new_field.db_index or new_field.unique):
            index_to_remove = self._create_index_name(model._meta.db_table, [old_field.column], suffix='_like')
            self.execute(self._delete_index_sql(model, index_to_remove))

    def _index_columns(self, table, columns, col_suffixes, opclasses):
        if opclasses:
            return IndexColumns(table, columns, self.quote_name, col_suffixes=col_suffixes, opclasses=opclasses)
        return super()._index_columns(table, columns, col_suffixes, opclasses)