#29561: Window Function Lag/Lead supported on Filter (Not equal) for FloatField
-------------------------------------+-------------------------------------
Reporter: | Owner: nobody
gabriellima |
Type: | Status: new
Uncategorized |
Component: Database | Version: 2.0
layer (models, ORM) | Keywords: window functions
Severity: Normal | database
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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I'd like to be able to reproduce the following example in Django, but on
the Temperature column
(https://fle.github.io/detect-value-changes-between-successive-lines-with-
postgresql.html)
Given a table
db=> SELECT * FROM weather ORDER BY day DESC;
||= day ||= temperature ||= rainy =||
|| 2014-04-08 || 20.0 || f
||2014-04-07 || 20.0 || f
|| 2014-04-06 || 16.0 || t
|| 2014-04-05 || 16.0 || t
|| 2014-04-04 || 16.0 || t
|| 2014-04-03 || 22.0 || f
|| 2014-04-02 || 22.0 || f
|| 2014-04-01 || 22.0 || t
I'd like to show dates only if the temperature changed:
||= day ||= temperature =||
||2014-04-08 || 20.0||
||2014-04-06 || 16.0||
||2014-04-03 || 22.0||
On pure PostgreSQL, this translates to:
{{{#!sql
SELECT
w1.day, w1.temperature
FROM
(SELECT
w2.day,
w2.temperature,
lead(w2.temperature) OVER (ORDER BY w2.day DESC) as prev_temp
FROM
weather w2
ORDER BY
w2.day DESC) as w1
WHERE
w1.temp IS DISTINCT FROM w1.prev_temp
ORDER BY
w1.day DESC;
}}}
I could accomplish the inner query by using the new Window Functions:
{{{#!python
Weather.objects.annotate(prev_temp=Window(expression=Lead('temperature'),
order_by=F('day').desc()))
}}}
Now my problem is use this annotation to filter only when temperature
differs from prev_temp
(in order to accomplish something similar to the "temperature IS DISTINCT
FROM prev_temp")
When I try to use the available filters, the following errors occurs:
{{{#!python
Weather.objects.annotate(prev_temp=Window(expression=Lead('temperature'),
order_by=F('day').desc())).order_by('-day').filter(temperature__ne=F('prev_temp'))
}}}
Gives the error:
{{{#!python
FieldError: Unsupported lookup 'ne' for FloatField or join on the field
not permitted.
}}}
Another try:
{{{#!python
Weather.objects.annotate(prev_temp=Window(expression=Lead('temperature'),
order_by=F('day').desc())).order_by('-day').filter(~Q(temperature=F('prev_temp'))))
}}}
Gives the error:
{{{#!python
ProgrammingError: window functions are not allowed in WHERE
}}}
It's in fact a PostgreSQL error, because the generated SQL query tries to
pass the LAG function inside the where clause.
How may I accomplish that, even if I have to use the extra fields, or even
RawSQL class ?
--
Ticket URL: <https://code.djangoproject.com/ticket/29561>
Django <https://code.djangoproject.com/>
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