#36170: Running no-op migrations (verbose_name, ...) slow on sqlite
-------------------------------------+-------------------------------------
Reporter: Klaas van Schelven | Type:
| Uncategorized
Status: new | Component:
| Uncategorized
Version: 5.1 | Severity: Normal
Keywords: | Triage Stage:
| Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Running no-op migrations (verbose_name, help_text, choices) triggers DB-
wide check_constraints on sqlite
This is because
[https://github.com/django/django/blob/5da3ad7bf90fba7321f4c2834db44aa920c70bc7/django/db/backends/sqlite3/schema.py#L39
this is done as part of the DatabaseSchemaEditor's exit] which is
[https://github.com/django/django/blob/5da3ad7bf90fba7321f4c2834db44aa920c70bc7/django/db/migrations/executor.py#L252
unconditionally called as part of apply_migration].
Because [https://code.djangoproject.com/ticket/21498 no-op migrations
cannot be ignored by design] (also
[https://code.djangoproject.com/ticket/30048 here]) this means that
changing your help-text will lead to an unnecessary check of all
constraints in the database (if no workarounds are employed, e.g.
squashing with something else).
If we accept the principle of "no ignoring of fields on-detect" as per the
issues mentioned above, we might still try to detect the fact that for any
given field and DB nothing happened in practice (and hence no
check_constraints is needed)
In particular: the call to `check_constraints` was introduced as part of
[https://github.com/django/django/commit/7289874adceec46b5367ec3157cdd10c711253a0
this commit], in which it is directly tied to a 5-step process.
Without claiming to have fully thought through all consequences: can't we
pull the check closer to those 5 steps, or make it conditional to
`_remake_table` having been actually called between enter & exit?
**Refs / background:**
I'm doing this in the context of [https://www.bugsink.com/ Bugsink, a
Self-hosted Error Tracker]; SQLite is a first level citizen (for
production) in that tool, which means large databases can be expected. On
a not all too big ~100K-row database I timed the above unnecessary check
to take ~20 seconds. Scaling that up by 1 or 2 orders of magnitude would
be my goal. Charitably assuming linear growth in time-complexity, you can
see how this can quickly become ''really annoying.''
[https://stackoverflow.com/a/39801321/339144 this SO post] provides tricks
for avoiding the creation of such migrations in the first place (may be
relevant context as long as the above isn't fixed yet).
--
Ticket URL: <https://code.djangoproject.com/ticket/36170>
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