Sometimes, if your apps have cyclic dependencies or complicated FK structure,
django migrations creates multiple initial migration files, which can very
easily fail linting because they are adding foreign keys or constraints etc.
This adds an option to automatically ignore all initial migrations.
While it's always a dangerous *pattern* to create an index while a
transaction is open & holds an exclusive lock, in practice it's not
always a big deal (for example, indexing a table with very few rows can
be fast). Accordingly, downgrade to a warning.
Briefly, this check helps prevent a long-running index creation from
preventing all reads and writes to a table.
This supplements an existing check
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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.
Mikuláš Poul
Chris Conn
Kevin Wu
Artem Streltsov
David Wobrock
jason.lee
David Cain
David Cain
Sören Wegener
David Wobrock
Ben Limmer
Daniel Hahler
Raju Mazumder
Łukasz Skarżyński