#36869: Optimise migration graph planning
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Reporter: James Fysh | Type:
| Cleanup/optimization
Status: new | Component: Database
| layer (models, ORM)
Version: dev | Severity: Normal
Keywords: migration digraph | Triage Stage:
loader | Unreviewed
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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When django needs to work with migrations - making & applying migrations,
running checks - it loads available migrations from disk and builds up an
in-memory digraph.
The django project that I work on has managed to accumulate a large number
of django apps and associated migrations. I'm going to leave the obvious
discussion around migration squashing out of this - let's just assume it
is not practical to do in the near term.
Anecdotally, starting the django application, eg. when running management
commands or simply launching the server is noticeably slow. Profiling has
revealed that a significant portion of startup time is spent running
migration checks.
To cut a long story short, MigrationGraph._generate_plan() currently runs
in O(n*n), and with a ~3 line patch we can reduce this to O(n).
For small django projects I believe this will not result in any change in
performance. For large projects such as ours, this can result in a very
significant reduction in startup time. Given the nature of the change
(simple and arguably low-risk) I think it would be a reasonable change to
make.
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Ticket URL: <https://code.djangoproject.com/ticket/36869>
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