#31276: Add 'flush' option to only clear non-empty tables
-------------------------------------+-------------------------------------
Reporter: Adam | Owner: nobody
(Chainz) Johnson |
Type: | Status: new
Cleanup/optimization |
Component: Database | Version: master
layer (models, ORM) |
Severity: Normal | Keywords:
Triage Stage: | Has patch: 0
Unreviewed |
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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This was suggested as an extra feature for Django-MySQL by @jonatron in
https://github.com/adamchainz/django-mysql/pull/611/files , but it would
make more sense to make it in Django core.
The `sql_flush` database operation, used by the 'flush' management command
and, thus through `TransactionTestCase`, creates one `TRUNCATE TABLE`
statement for every table - regardless of whether they are empty already.
This means unnecessary database operations are run, which will be most
noticeable in test runs.
Instead of flushing *every* table, we can flush only the non-empty ones.
A minimal, hopefully cross-database way of quickly finding out which
tables in a list are non-empty is to make existence queries on them
combined with union:
{{{
chainz@localhost [14]> (select 't' as has_rows from t limit 1) union
(select 't2' as has_rows from t2 limit 1);
+----------+
| has_rows |
+----------+
| t2 |
+----------+
1 row in set (0.001 sec)
}}}
We could use logic like this to batch the tables into such union queries
in `django.core.management.sql.sql_flush`. We could then reduce the list
of tables we wish to flush down to the non-empty ones for the `sql_flush`
operation. Perhaps it would be best to put this behind a flag that is set
only in tests, for backwards compatibility.
--
Ticket URL: <https://code.djangoproject.com/ticket/31276>
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