#27719: Add queryset.alias() to mimic .annotate() for aggregations without
loading
data
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Reporter: Marc Tamlyn | Owner: Alexandr
| Tatarinov
Type: New feature | Status: assigned
Component: Database layer | Version: master
(models, ORM) |
Severity: Normal | Resolution:
Keywords: | Triage Stage: Accepted
Has patch: 0 | Needs documentation: 0
Needs tests: 0 | Patch needs improvement: 0
Easy pickings: 0 | UI/UX: 0
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Changes (by Alexandr Tatarinov):
* owner: nobody => Alexandr Tatarinov
* status: new => assigned
Comment:
I am working on this one. I've had a lot of cases where unneeded
expressions ended up in a select clause.
When someone builds complex queries spanning multiple methods and modules,
this can save a lot of CPU (for ORM to build the query) and query
execution time.
My suggestion is to support only the usages below, values(), distinct()
and aggregate() will require to use .annotate() to add the expression into
the select list.
{{{
.alias(alias=SomeExpression()).annotate(SomeOtherExpression('alias'))
.alias(alias=SomeExpression()).filter(alias__lookup=value)
.alias(alias=SomeExpression()).order_by('alias')
.alias(alias=SomeExpression()).update(field=F('alias'))
}}}
Also automatically supported: earliest/latest and dates/datetimes.
And if you want to use values() or distinct(), promote alias to annotation
{{{
.alias(alias=SomeExpression()).annotate(alias=F('alias')).distinct('alias')
}}}
The implementation relies on annotation_mask to remove an expression from
the select list, all other logic remains the same: despite the expression
is not selected, the joins and group by are the same as for annotation.
--
Ticket URL: <https://code.djangoproject.com/ticket/27719#comment:10>
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