Multi-column comparisons

[Ryan Hiebert]

Multi-column comparisons

In order to implement keyset pagination, AKA the seek method, across multiple fields including ordering on non-unique fields, I'm wanting to write a PostgreSQL that compares multiple fields in an ordered fashion. To my knowledge this isn't possible with the current ORM syntax. My particular use-case further complicates this by also wanting a subquery in order to avoid serializing more than the unique key in the response. Here's a generic example where clause that mirrors my use-case:

(a,b,c,id) > (SELECT a,b,c,id FROM mytable WHERE id = ?)

What this does special is account for all of the columns when making the comparison. So it will only compare the id column if all of the a, b, and c columns are the same, to allow for exact indentification in an arbitrary ordering.

In the PostgreSQL docs this feature is called Row Constructor Comparison, and the documentation is here: ​https://www.postgresql.org/docs/9.6/static/functions-comparisons.html#ROW-WISE-COMPARISON.

[Ryan Hiebert]

With the help of a patient colleague (thanks Tim!) I've been able to accomplish what I needed to natively, without using .extra(). With one caveat, I needed to add the LHS as a calculated column using annotate, which is a bit of a kludge, which ends up returning annotated data that I didn't really need. For my case, that's not too big of a deal, but perhaps less than ideal.

It might be deserving of a way to remove annotations that don't need to be selected, or to allow filter expressions that allow a dynamic expression on both sides. I'm not sure if that deserves its own ticket, if it should be on this one, or if perhaps there's already a ticket open for that.

Here's what we came up with:

from django.db.models import Func, F, Subquery

def after(queryset, value):
    return (
        queryset
        .annotate(rank=Func(F('a'), F('b'), F('c'), F('id'), function='ROW'))
        .filter(rank__gt=Subquery(queryset.filter(id=value).values('a', 'b', 'c', 'id')))
        .order_by('a', 'b', 'c', 'id')
    )

One other thing to note is that this opts to use the explicit ROW function, rather than relying on that being the result of just using parenthesis, because parenthesis alone with a single value doesn't necessarily make a ROW.

[Tim Graham]

I'm not sure what's to be done to close this ticket. Can you offer a patch?

[Simon Charette]

With one caveat, I needed to add the LHS as a calculated column using annotate, which is a bit of a kludge, which ends up returning annotated data that I didn't really need.

FWIW this can now be avoided in two ways

1. Use QuerySet.alias instead of .annotate
2. Pass lookups directly to filter

from django.db.models.lookups import GreaterThan
from django.db.models import Func, F, Subquery

class Row(Func):
    function = 'ROW'

def after(queryset, value):
    return (
        queryset
        .filter(GreaterThan(
            Row('a', 'b', 'c', 'id'),
            Subquery(queryset.filter(id=value).values('a', 'b', 'c', 'id'))
        ))
        .order_by('a', 'b', 'c', 'id')
    )