Opened 6 years ago

Last modified 6 years ago

#29561 closed Uncategorized

Window Function Lag/Lead supported on Filter (Not equal) for FloatField — at Version 1

Reported by: Gabriel Oliveira Owned by: nobody
Component: Database layer (models, ORM) Version: 2.0
Severity: Normal Keywords: window functions database
Cc: Triage Stage: Unreviewed
Has patch: no Needs documentation: no
Needs tests: no Patch needs improvement: no
Easy pickings: no UI/UX: no

Description (last modified by Gabriel Oliveira)

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:

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:

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:

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:

FieldError: Unsupported lookup 'ne' for FloatField or join on the field not permitted.

Another try:

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:

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 ?

Right now I'm doing:

with connection.cursor() as c:
    c.execute('SELECT q.day, q.temperature FROM (%s) AS q WHERE q.temperature IS DISTINCT FROM q.prev_temp' % str(Weather.objects.annotate(prev_temp=Window(expression=Lead('temperature'), order_by=F('day').desc())).order_by('-day').values('day','temperature', 'prev_temp').query))
    result = c.fetchall()

Important to note that I couldn't use the str(queryset.query) as parameter to the c.execute. I had to do a python replace.
I.e, had to use ('...%s...' % parameter) instead of c.execute ('...%s...', [parameter])

Change History (1)

comment:1 by Gabriel Oliveira, 6 years ago

Description: modified (diff)
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