David Cain
0c5519d9dc
Briefly, this check helps prevent a long-running index creation from preventing all reads and writes to a table. This supplements an existing check ================================== There is an existing check, `CREATE_INDEX` which warns about creating indices without the `CONCURRENTLY` argument. It's a helpful check! However, there are some valid reasons to prefer non-concurrent index creation. For example, concurrent index creation may struggle to complete when applied to write-heavy tables (since Postgres will have to make repeated passes on the table to accommodate the new reads since the last round). The danger of `EXCLUSIVE` locks =============================== If you *do* want to create indices nonconcurrently, there is still a **huge footgun** to be aware of -- locks which are held in a transaction are not released until the transaction closes. Dangerous Django defaults ========================= By default, any time you add a new Django field with `db_index=True` or `unique=True`, the automatically generated `Migration` has: - implicit setting of `atomic=True` (transactions are on) - an `ALTER TABLE...ADD COLUMN` call (exclusively locks the table) - a `CREATE INDEX` invocation (*hopefully* quick, due to nullable columns) It's even more dangerous if you combine an `ALTER TABLE` call with `CREATE INDEX` of an existing, non-nullable column. Favor `atomic=False` if you must index ====================================== Index-creating migrations should have few `operations`. Since index creation does not modify row-level data (and only requires locking against writes), there's no reason to use transactions. It's prudent to bypass the default `atomic=True` for index creation.
19 lines
853 B
Python
19 lines
853 B
Python
from __future__ import annotations
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CREATE_TABLE_WITH_NOT_NULL_COLUMN = "app_create_table_with_not_null_column"
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ADD_NOT_NULL_COLUMN = "app_add_not_null_column"
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DROP_COLUMN = "app_drop_column"
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DROP_TABLE = "app_drop_table"
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RENAME_COLUMN = "app_rename_column"
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RENAME_TABLE = "app_rename_table"
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IGNORE_MIGRATION = "app_ignore_migration"
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ADD_NOT_NULL_COLUMN_FOLLOWED_BY_DEFAULT = "app_add_not_null_column_followed_by_default"
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ALTER_COLUMN = "app_alter_column"
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ALTER_COLUMN_DROP_NOT_NULL = "app_alter_column_drop_not_null"
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DROP_UNIQUE_TOGETHER = "app_unique_together"
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ADD_MANYTOMANY_FIELD = "app_add_manytomany_field"
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DATA_MIGRATIONS = "app_data_migrations"
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MAKE_NOT_NULL_WITH_DJANGO_DEFAULT = "app_make_not_null_with_django_default"
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MAKE_NOT_NULL_WITH_LIB_DEFAULT = "app_make_not_null_with_lib_default"
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CREATE_INDEX_EXCLUSIVE = "app_create_index_exclusive"
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