Fixed #35334 -- Removed supports_sequence_reset feature flag

[shangxiao]

https://code.djangoproject.com/ticket/35334

Draft PR

There are a few issues with sequences resetting:

• In some cases sequences are restarted, some cases they're set to the max value
• The MySQL backend doesn't do the max value one, even though it could (providing it's not within a tx?)
• The Oracle backend reset doesn't restart (comment incorrectly states it does?)

Notes about how sequences currently work in Django

sequence_reset_by_name_sql()

If supported explicitly sets the sequence to 1

Called by:

• SQLite's sql_flush()
• Oracle's sql_flush() see note below
• TransactionTestCase._fixture_setup() if reset_sequences = True

Note: Despite comments in oracle.operations.sql_flush():

   # Since we've just deleted all the rows, running our sequence ALTER
   # code will reset the sequence to 0.

there is no ALTER command issued, neither do the sequences get reset.

sequence_reset_sql()

If supported by the backend re-aligns the sequence with the table's contents

Called by:

• contrib.sites' create_default_site() on post_migrate
• loadddata mgt command if there's at least 1 object loaded
• in turn is called by any test case to load fixtures
• sqlsequencereset mgt command

SQLite

• ✓ supports_sequence_reset set
• ✓ sql_flush() defined - uses DELETE FROM <table>
  • calls sequence_reset_by_name_sql()
• ✓ sequence_reset_by_name_sql() defined - using UPDATE sqlite_sequence …
• ✗ sequence_reset_sql() not defined

PostgreSQL

• ✓ supports_sequence_reset set
• ✓ sql_flush() defined - uses TRUCATE <table1>, <table2>, … , adds RESTART IDENTITY if resetting sequences
• ✓ sequence_reset_by_name_sql() defined - using SELECT setval(pg_get_serial_sequence(<table>, <column>), 1, false)
• ✓ sequence_reset_sql() defined with SELECT setval(pg_get_serial_sequence(<table>, <column>), coalesce(max(<column>), 1), max(<column>) IS NOT NULL) FROM <table>;

MySQL

• ✓ supports_sequence_reset set
• ✓ sql_flush() defined - resets sequences by using TRUNCATE instead of DELETE
• ✓ sequence_reset_by_name_sql() defined using ALTER TABLE <table> AUTO_INCREMENT = 1
• ✗ sequence_reset_sql() not defined

Oracle

• ✗ supports_sequence_reset not set
• ✓ sql_flush() defined - iterates over tables, disables constraints then TRUNCATE, then re-enabling constraints
  • calls sequence_reset_by_name_sql()
• ✓ sequence_reset_by_name_sql() defined using custom PL/SQL
• ✓ sequence_reset_sql() defined using custom PL/SQL

Some notes about resetting sequences on Oracle

• DDL statements implicitly commit transactions
• Implicitly generated sequences cannot be restarted with ALTER SEQUENCE, one needs to specify the start by ALTER TABLE … MODIFY id GENERATED BY DEFAULT ON NULL AS IDENTITY START WITH …
• Identity columns can use LIMIT VALUE to specify the max value
• Regular sequences don't have a LIMIT VALUE equivalent so we need to retain the existing SELECT MAX(…) statement to retrieve the max value

Additional notes

• The previous sequence reset code only provided the ability to "reset" to the current max value by selecting nextvalue until it reached the max
• The ALTER TABLE … MODIFY statement should probably get the proper type info from schema?

[timgraham]

Please don't remove DatabaseFeatures.supports_sequence_reset as third-party database backends (at least CockroachDB and Snowflake) set it to False.

[shangxiao]

buildbot, test on oracle.

[shangxiao]

buildbot, test on oracle.